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Oklahoma Soil Fertility Handbook

Oklahoma Cooperative Extension Service

Division of Agricultural Sciences and Natural Resources

Oklahoma State University


Table of Contents Chapter 1 Soil and Soil Productivity ...........................................................

Jason Warren


Chapter 2 Essential Plant Nutrients ............................................................. Bill Raun


Chapter 3 Problem Soils ............................................................................... Hailin Zhang


Chapter 4 Determining Fertilizer Need ........................................................ Hailin Zhang


Chapter 5 Fertilizer Use in Oklahoma........................................................... Brian Arnall


Chapter 6 Manure ........................................................................................... Hailin Zhang


Chapter 7 Environmental ............................................................................. Chad Penn


Chapter 8 Drilling Mud .................................................................................. Chad Penn


Chapter 9 Long Term Soil Fertility Research ............................................ Josh Bushong


Chapt 10. Precision Nutrient Management ................................................ Brian Arnall


Chapter 11. N-Rich Strips, GreenSeeker Sensor and SBNRC .................. Joy Abit


Chapter 12. Laws and Acts .......................................................................... Bill Raun Editors: Brian Arnall and Gayle Hiner


Project supported by Oklahoma Fertilizer Check-off Program.



Chapter 1. Soil and Soil Productivity

Soil and Soil ProductivitySoil is perhaps the most important natural resource in Oklahoma. It is important to all, for without soil there would be no life on Earth. Our food and much of our clothing and shelter come from soil. Soil supports the gigantic agricultural system, which is the major contributor to the state’s development and continued prosperity.


Oklahoma has a land area of more than 44 million acres, part of which is covered by water. The majority, about 41 million acres, is used for production of food and fiber. This land has an average value of more than $400 per acre or a total value in excess of $16.4 billion. It is an asset well worth protecting.


Many different kinds of soil occupy this land area. Some soils are extremely productive, while others are not as productive. Each soil has a set of unique characteristics that distinguishes it from other soils. These characteristics determine the potential productivity of the soil.


Soil productivity is a result of how well the soil is able to receive and store moisture and nutrients, as well as providing a desirable environment for all plant root functions.


What is Soil?

Soil is the unconsolidated mineral and organic material on the immediate surface of the Earth which provides nutrients, moisture and anchorage for land plants.


The principal components of soil are mineral material, organic matter, water and air. These are combined in widely varying amounts in different soils. In a typical loam soil, solid material and pore space are equally divided on a volume basis, with the pore space containing nearly equal amounts of water and air. The approximate proportions are illustrated in Figure 1.1.


How Soils are Formed

The development of soils from parent rock is a long-term process involving physical and chemical weathering along with biological activity. The wide variety of soils and their properties are a function of the soil forming factors including parent material, climate, living organisms, topography and time.


The initial action on the parent rock is largely mechanical-cracking and chipping due to temperature changes. As the rock is broken, the total surface area exposed to the atmosphere increases. Chemical action of water, oxygen, carbon dioxide and various acids further reduce the size of rock fragments and change the chemical composition of many resulting particles. Finally, the microorganism activity and higher plant and animal life contribute organic matter to the weathered rock material, and a true soil begins to form.


Volume composition of a desirable surface soil.


Figure 1.1: Volume composition of a desirable surface soil.

Since all of these soil-forming agents are in operation constantly, the process of soil formation is continual. Evidence indicates the soils we depend on today to produce our crops required hundreds or even thousands of years to develop. In this regard, consider soil as a nonrenewable resource measured in terms of man’s life span. Thus, it is very important to protect soils from destructive erosive forces and nutrient depletion, which can rapidly destroy the product of hundreds of years of nature’s work, as well as greatly reduce soil productivity.


Soil Profile

A vertical cross-section through a soil typically represents a layered pattern. This section is called a “profile” and the individual layers are called “horizons.” A typical soil profile is illustrated in Figure 1.2.


A typical soil profile.


Figure 1.2: A typical soil profile.


The uppermost layer includes the surface soil or topsoil and is designated the ‘A’ horizon. This is the layer which is most subject to climatic and biological influence. It usually IS the layer of maximum organic accumulation, has a darker color, and has less clay than subsoil. The majority of plant roots and most of the soil’s fertility are contained in this horizon.


The next successive horizon is called the subsoil or ‘B’ horizon. It is a layer that commonly accumulates materials that have migrated downward from the surface. Much of the deposition is clay particles, iron and aluminum oxides, calcium carbonate, calcium sulfate and possibly other salts. The accumulation of these materials creates a layer that is normally more compact and has more clay than the surface. This often leads to restricted movement of moisture and reduced crop yields.


The parent material, or ‘C’ horizon is the least affected by physical, chemical and biological weathering agents. It is very similar in chemical composition to the original has formed in its original position by weathering of bedrock is termed “residual;” or transported if it has been moved to a new location by natural forces. This latter type is further characterized on the basis of the kind of natural force responsible for its transportation and deposition. When water is the transporting agent, the parent materials are referred to as alluvial (stream deposited). This type is especially important in Oklahoma. These are often the most productive soils for agricultural crops. Wind-deposited materials are called aeolian.


Climate has a strong influence on soil profile development. Certain characteristics of soils formed in areas of different climates can be described. For example, soils in western Oklahoma are drier and tend to be coarser textured, less well developed and contain more calcium, phosphorus, potassium and other nutrients than do soils in the humid eastern part of the state. The soil profile is an important consideration in terms of plant growth. The depth of the soil, its texture and structure, its chemical nature as well as the soil position on the landscape and slope of the land largely determine crop production potential. The potential productivity is vitally important in determining the level of fertilization.


Soil Texture

Soils are composed of particles with an infinite variety of sizes. The individual particles are divided by size into the categories of sand, silt and clay. Soil texture refers to the relative proportion of sand, silt and clay in the soil. Textural class is the name given to soil, based on the relative amounts of sand, silt and clay present, as indicated by the textural triangle shown in Figure 1.3. Such divisions are very meaningful in terms of relative plant growth. Many of the important chemical and physical reactions are associated with the surface of the particles, and hence are more active in fine than coarse-texture soils.


A textural class description of soils can tell a lot about soil-plant interactions, since the physical and chemical properties of soils are determined largely by texture. In mineral soils, exchange capacity (ability to hold plant nutrient elements) is related closely to the amount and kind of clay in soils. Texture is a major determining factor for water-holding capacity. Fine-textured soils (high percentage of silt and clay) hold more water than coarse-textured soils (sandy). Water and air movement through the finer-textured soils is reduced, making it more difficult to work.


Triangle for determining soil textural classes.  (Percent Sand;  Percent Silt; Percent  Clay)

Figure 1.3: Triangle for determining soil textural classes.  Percent Sand 0- 100 ;  Percent Silt  0-100 ; Percent Clay 0-100



From the standpoint of plant growth, medium-textured soils, such as loams, sandy loams and silt loams, are the most ideal. Nevertheless, the relationships between soil textural class and soil productivity cannot be generally applied to all soils, since texture is one of the many factors that influence crop production. Check the texture of the surface and subsoil. Normally, the surface includes the top foot of soil, but it may be shallower or deeper in certain situations. Soil below the tillage zone is called subsoil. It is also necessary to consider the subsoil texture when determining productivity potentials.


Soil Structure

Soil structure refers to the presence of aggregates of soil particles that have been bound together to form distinct shapes. Sometimes the binding or cementing is weak, however the aggregates are much larger than individual soil particles. Soil organic matter contributes significantly as a cementing agent. Air and water movement and root penetration in the soil is related to the soil structure. The better the structure, the higher the productivity of the soil.


Size and shape of the structure units is important. When height of the structure unit is approximately equal to its width (blocky structure) we expect good air and water movement. Structure units that have greater height than width (prismatic structure) often are associated with subsoils that swell when wet and shrink when dry, resulting in poor air and water movement. When particles have greater width than height (platy structure), water and air movement and root development in the soil is restricted, compared to a soil with desirable structure. Granular structure, particularly in fine-textured soils, is ideal for water penetration and air movement. Water and air move more freely through subsoils that have blocky structure than those with platy structure. Good air and water movement is conducive to plant root development. Types of soil structure are illustrated in Figure 1.4.


Types of soil structure.   prismatic; colunbar; angular blocky ; Subangular blocky; platy; granular

Figure 1.4: Types of soil structure. ( prismatic; colunbar; angular blocky ; Subangular blocky; platy; granular)


The productivity of the soil is influenced by both surface and subsoil texture and structure. An approximate rating for soils considering texture and structure is shown in Table 1.1. Raise or lower the rating 10 to 20 percent, according to whether the soil structure is more, or less, favorable than the average. If gravel occurs in the soil, lower the rating according to its effect on the productive capacity.


Table 1.1: Soil productivity rating as affected by texture.*
(Surface Soil Texture: Percent of Maximum Productivity )


Loam Clay
Silty Clay
Sandy 50% 55% 65% 60% 55%
Sandy Loam 60% 70% 80% 75% 65%
Loam 70% 80% 95% 90% 75%
Clay Loam 70% 80% 90% 90% 75%
Clay; Silty Clay 65% 70% 80% 80% 70%

* Numbers represent average soil conditions.


Soil Depth

Soil depth generally is used to describe how deep roots can favorably penetrate. Soils that are deep, well drained and have desirable texture and structure are suitable for production of most crops. For satisfactory production, most plants require considerable soil depth for root development to secure nutrients and water. Plants growing on shallow soils have little soil volume from which to secure water and nutrients. Depth of soil and its capacity to hold nutrients and water frequently determines crop yield, particularly for summer crops.


Roots of most crops extend 3 feet or more into favorable soil. Soils should be at least six feet deep to give maximum production. Look for materials or conditions that limit soil depth, such as hardpans, shale, coarse gravelly layers and tight impervious layers. These are almost impossible to change. A high water table may limit root growth, but it usually can be corrected by drainage. Soil productivity estimates on the basis of soil depth can be made using Table 1.2.


Table 1.2. Soil productivity rating as affected by depth.

Soil Depth Usable by Crop
Roots Relative Productivity
1 5
2 60
3 75
4 85
5 95
6 100


Soil Slope

Topography of the land largely determines potential for runoff and erosion, method of irrigation and management practices needed to conserve soil and water. Higher-sloping land requires more management, labor and equipment expenditures.


Table 1.3 can be used to rate land productivity based on slope. If slope varies, use steeper slopes for the rating.


Table 1.3. Soil productivity ratings as affected by slope.

( % )

Relative Productivity
Stable Soil

( % )

Relative Productivity
Unstable, Easily Eroded soil
( % )
0-1 100 95
1-3 90 75
3-5 80 50
5-8 60 30
8-12 40 10



Principal reasons for soil erosion in Oklahoma are 1) insufficient vegetative cover, which usually is a result of inadequate fertility to support a good plant cover, 2) growing cultivated crops on soils not suited to cultivation and 3) improper tillage of the soil. Soil erosion can be held to a minimum by 1) using the soil to produce crops for which it is suitable, 2) using adequate fertilizer and lime to promote vigorous plant growth and 3) using proven soil preparation and tillage methods.


 Soils that have lost part or all their surfaces are usually harder to till and have lower productivity than non-eroded soils. To compensate for surface soil loss, more fertilization, liming and other management practices should be used.


Soil and Available Water

Plants are totally dependent on water for growth and production. Even with well-fertilized soils, limited water can greatly reduce yields. Rainfall is not always dependable in Oklahoma. Therefore, crops are dependent on the moisture stored in the soil profile for growth and production.


Soils differ in their ability to supply water to plants. Limited root zones caused by shallow soils, high water table or claypans or extremely porous subsoils cause drought stress in plants faster than more desirable soils. Table 1.4 illustrates the differences in available water in selected soil profiles. Soils with silt loam or fine sandy loam surface textures have high available water-holding capacities. Differences in available water-holding capacity between the soils caused by widely varying textures of the subsoil and soil depth point out the need for knowing what is below the surface. (This kind of information is available in county soil survey manuals). During a drought, differences of 2 inches of available water can keep plants growing for an extra 10 days during peak plant use and could be the difference between success and crop failure.


Table 1.4. Effect of depth and texture on available water for crop use.

 Soil Name Texture



Available Water


Dennis Silt loam 0-11 1.98
  Silty clay loam 11-23 2.52
  Clay 23-60 5.55
  TOTAL 60 10.05
Sallisaw Silt loam 0-10 1.8
  Silt loam 10-20 1.8
  Gravelly clay loam '20-40 2.8
  Very gravelly clay loam 40-60 1.6
  TOTAL 60 8
Shellabarger Fine sandy loam 0-16 1.92
  Sandy clay loam 16-52 5.86
  Fine sandy loam 52-60 0.88
  TOTAL 60 8.66
Stephenville Fine sandy loam 0-14 1.82
  Sandy clay loam 14-38 3.84
  Sandstone 38+ -----
  TOTAL 38+ 5.66


Soil Fertility

Soil fertility is the soil’s ability to provide essential plant nutrients in adequate amounts and proper proportions to sustain plant growth. These nutrients and their functions are covered in details in the next chapter. Soil fertility is a component of soil productivity that is quite variable and strongly influenced by management. Other components of soil productivity, especially soil slope and soil depth, will be the same year after year. Together with climate, these components set the soil productivity limits, above which yields cannot be obtained even with ideal use of fertilizer. It is important to understand added fertilizer cannot compensate for an unproductive soil due to it being excessively stony or has a subsoil layer that restricts normal root growth and development. This point is illustrated in Figure 1.5. 


 Influence of soil productivity on yield response to fertility.

 Figure 1.5: Influence of soil productivity on yield response to fertility.


Soil Management

There are numerous other soil characteristics that can be important to soil productivity in specific areas. These include: soil drainage, soli salinity, presence of stone and/or rocks and organic matter content. They are not major limiting factors over wide areas and will not be discussed here.


One additional factor on which soil productivity is highly dependent is soil management. This implies using the best available knowledge, techniques, materials and equipment in crop production. The use of minimum tillage is an important management practice used to reduce the potential damage to soil structure from overworking, and for economic and fuel conservation purposes, as well as to allow farming of more acres per unit of labor.


Soil conservation is a concept integrating important management practices that deserves close attention. In the U.S., it is estimated that four billion tons of sediment are lost annually from the land in runoff waters, and with it much of the natural and applied fertility. That is equivalent to the total loss of topsoil (6 inches deep) from four million acres. Wind erosion is also a problem in certain areas. Management practices such as contouring, strip planting, covercropping, reduced tillage, terracing and crop residue management help eliminate or minimize the loss of soil from water and wind erosion.


Proper utilization of crop residues can be a key management practice. Crop residues returned to the soil improve soil productivity through the addition of organic matter and plant nutrients. The organic matter also contributes to an improved physical condition of the soil, which increases water infiltration and storage and aids aeration. This is vital to crop growth.



Limitations of soil, water or climate reduce the soil’s ability to produce. These limitations increase the need for better management practices. Poor management, or the presence of weeds, compact soils, soil erosion, etc., will result in low yields even on the most productive soils. On the other hand, good management on moderately productive soils can give high yields. By considering the factors discussed in this chapter, one can make a better determination of the soil’s overall crop productivity and make better decisions about nutrient management including use of fertilizers.



Chapter 2. Essential Plant Nutrient Functions, Soil Reactions and Availability

More than 100 chemical elements are known to man today. However, only 16 have been proven to be essential for plant growth. For a nutrient to be classified as essential, certain rigid criteria must be met. The criteria are as follows:


  1. The element is essential if a deficiency prevents the plant from completing its vegetative or reproductive cycle.
  2.   The element is essential if the deficiency in question can be prevented or corrected only by supplying the element.
  3.   The element is essential if it is directly involved in the nutrition of the plant and is not a result of correcting some microbiological or chemical condition in the soil or culture media.


The essential elements and their chemical symbols are listed in Table 2.1. Three of the 16 essential elements – carbon, hydrogen and oxygen – are supplied mostly by air and water. These elements are used in relatively large amounts by plants and are considered to be non-mineral, since they are supplied to plants by carbon dioxide and water. The non-mineral elements are not considered fertilizer elements. The other 13 essential elements are mineral elements and must be supplied by the soil and/or fertilizers.


Table 2.1. Essential plant nutrients, chemical symbols and sources.


Mostly from air and water



Mostly from air and water



From soil and/or fertilizers


From soil and/or fertilizers



From soil and/or fertilizers


From soil and/or fertilizers





























Magnesium Mg














The essential plant nutrients may be grouped into three categories. They are as follows:


  1. Primary nutrients - nitrogen, phosphorus and potassium
  2. Secondary nutrients - calcium, magnesium and sulfur
  3. Micronutrients - iron, manganese, zinc, copper, boron, molybdenum and chlorine

This grouping separates the elements based on relative amounts required for plant growth, and is not meant to imply any element is more essential than another.


Primary Non-Mineral Nutrients

Carbon, Hydrogen and Oxygen

Carbon is the backbone of all organic molecules in the plant and is the basic building block for growth. After absorption of carbon dioxide (CO2 ) by the leaves of the plant, carbon is transformed into carbohydrates by combining with hydrogen and oxygen through the process of photosynthesis.


Metabolic processes within the plant transform carbohydrates into amino acids and proteins and other essential components.


Primary Mineral Nutrients


Nitrogen is an integral component of amino acids, which are the building blocks for proteins. Proteins are present in the plant as enzymes that are responsible for metabolic reactions in the plant. Because nitrogen is so important, plants often respond dramatically to available nitrogen.


Soil Nitrogen Reactions and Availability

Most of the nitrogen in Oklahoma soil is present as organic nitrogen in the soil organic matter. There are about 1,000 pounds per acre of nitrogen in this form for every 1 percent organic matter in the soil. However, since the soil organic matter is resistant to further decay, most of this nitrogen is unavailable during any given growing season. Normally, about 2 percent of the nitrogen from soil organic matter will be released each year to mineral forms when soils are cultivated. This 20 to 40 pounds per acre of nitrogen is typical of the amount present in unfertilized soils after cultivation and seed bed preparation.


Nitrogen Mineralization and Immobilization

Because nitrogen release from organic matter is dependent upon decay by microorganisms, which themselves require nitrogen, the amount of nitrogen available for a crop is in constant flux. Unlike crops, which get their carbon as carbon dioxide from the air, many microorganisms get their carbon by decaying organic matter. Nitrogen availability depends upon the relative amount of carbon and nitrogen in the organic matter, its resistance to decay, and environmental conditions to support microbial activity. Figure 2.1 illustrates how nitrogen becomes more concentrated as soil organic matter decays.


 Narrowing of carbon to nitrogen ratio as residue decay until mineral nitrogen finally becomes available.

Figure 2.1:  Narrowing of carbon to nitrogen ratio as residue decay until mineral nitrogen finally becomes available.


Note that nitrogen is not released during the first stages of decay. This is because nitrogen that is released is immediately consumed by active microorganisms. With time, remaining organic material becomes more resistant to decay, microorganisms die off, and there is more mineral nitrogen present than can be consumed by the few active microorganisms. This results in a final release of measurable mineral nitrogen in the form of ammonia (NH3 ). The ammonia readily reacts with soil moisture to form ammonium (NH4+). These two reactions can be stated simply as:



organic nitrogen

NH3 (gas)



NH3 + H2 O

ammonia + water


NH4+ + OH-

ammonium + hydroxide



The process of converting or transforming nitrogen from organic compounds to inorganic compounds is called mineralization. This results in increasing nitrogen available for crops. When the reverse happens, and available nitrogen is absorbed by crops or microorganisms, the process is called immobilization and results in a decrease in the amount of nitrogen immediately available for crops. These processes and their interacting nature with soil nitrogen for a typical field situation are illustrated in Figure 2.2.


Approximately 98 percent of the soil nitrogen is unavailable for plant uptake. This large reservoir of organic nitrogen provides an important buffer against rapid changes in available nitrogen and plant stress. The small reservoir of mineral nitrogen can often be slowly replenished by mineralization (Figure 2.2) when crops need additional nitrogen.



Figure 2.2: Interacting pools of soil nitrogen.


Supplemental nitrogen as fertilizer usually must be added to support high, economic production levels. Immediately following fertilization with 120 pounds nitrogen, the system may be illustrated by Figure 2.3a. Addition of fertilizer nitrogen will stimulate microorganism activity, resulting in consumption of nitrogen and breakdown of some crop residues (immobilization) as illustrated in (Figure 2.3b). The immobilized nitrogen will be present as microbial tissue and other new material in the organic pool. As indicated by the two arrows pointing in opposite pathways, mineralization and immobilization are both taking place simultaneously. Immobilized fertilizer nitrogen will again become available in a few weeks if conditions favor crop uptake.


 Relative amounts of organic and mineral nitrogen in soil immediately after fertilizing (a) and several days after active immobilization (b).

Figure 2.3: Relative amounts of organic and mineral nitrogen in soil immediately after fertilizing (a) and several days after active immobilization (b).



In addition to the general mineralization and immobilization reactions, other reactions also are responsible for nitrogen changes (transformations) in the soil. Nitrification is one of the first reactions to occur after organic nitrogen has been converted to ammonium-N. This change is also a result of aerobic microorganism activity as depicted in the following reaction.



 2NH4+ + 3O2

ammonium + oxygen

 2NO2- + 2H2O + 4H+

nitrite + water + hydrogen ion 


This reaction produces nitrite-N and hydrogen ions. Since hydrogen ions are generated, it is easy to see this step will at least temporarily contribute to soil acidity. However, this production of acidity is partially compensated for by the hydroxide (OH- ) produced from reaction [2]. The hydrogen and hydroxide will combine to form water, so the net effect on acidity when organic nitrogen is mineralized will be 1 pound of hydrogen produced for every 14 pounds of nitrogen mineralized. The same reactions and acidity will occur when fertilizer nitrogen is added in the ammonia form (anhydrous ammonia or urea). Ammonium sulfate will be twice as acidifying because equation [2] will be avoided by adding the ammonium (NH4+) form of nitrogen.


Almost immediately after nitrite (NO2- ) nitrogen is produced (reaction [3]), a companion reaction occurs that is also carried out by soil microorganisms resulting in nitrate-N (NO3 - N) being produced from nitrite. 2NO2- + O2 → 2NO3 - [4]  Because this change is quite rapid compared to the change from ammonium to nitrite [3] there is seldom any nitrite (NO2- ) present in soils. Ammonium and nitrate are common and will increase or decrease depending on microbial activity that will both generate and consume ammonium and nitrate. This cyclic interaction of nitrogen transformations is shown in Figure 2.4.


Primary forms of nitrogen in soils and the transformations among them.

Figure 2.4:  Primary forms of nitrogen in soils and the transformations among them. (1) Decay of soil organic matter releasing ammonia; (2) reaction of ammonia with water to form ammonium; (3) transformation of ammonium to nitrate by microorganisms; (4) uptake of ammonium and/ or nitrate by plants and microorganisms; (5) plants eaten by animals; (6) animal manures, nitrogen fixation and plant residue return to soil; (7) residues decay to resistant organic matter, ammonia produced from nitrogen rich materials; (8) soil organic matter produced as decay continues.


Whenever nitrate and/or ammonium nitrogen are measured in the soil, these measurements provide a view of two components of the nitrogen cycle at a single point in time. If the measurement is made when the system is likely to be in balance, or equilibrium, such as when wheatland soils are tested for nitrate in July or August, the value can be a useful guide for determining nitrogen fertilizer needs. Figure 2.5 illustrates the changes that took place for ammonium and nitrate nitrogen in soil during wheat production under different rates of fertilizer use. Because ammonium and nitrate nitrogen are the two forms of nitrogen that higher plants utilize, these two forms have received the greatest attention.


Surface soil (0-6”) ammonium and nitrate nitrogen following fertilization at different rates from OSU Soil Fertility Research.

Figure 2.5. Surface soil (0-6”) ammonium and nitrate nitrogen following fertilization at different rates from OSU Soil Fertility Research.


Soil fertility research at OSU has documented the change of ammonium and nitrate nitrogen following fertilization (Figure 2.5). Only about 60 percent of the fertilizer nitrogen could be accounted for at the first sampling after fertilization. This was mostly present as nitrate although the fertilizer (ammonium nitrate) was an equal mixture of the two nitrogen forms measured. In the short period after application, some transformations had taken place. These continued, resulting in a gradual increase in ammonium nitrogen (probably from some mineralization) and a rapid decline in nitrate, likely from immobilization caused by microbial activity and uptake by the wheat crop.


When crop production is added to the cycle in Figure 2.4, it becomes obvious that the cycle is not self sustaining. Harvesting removes significant amounts of nitrogen each year and eventually the system becomes depleted in organic matter and available nitrogen to support normal crop yields. A common response is to add nitrogen back using legumes and commercial fertilizers. When additions are balanced with removals, soil organic matter and productivity can potentially be sustained. However, excessive tillage, residue removal (straw and chaff in wheat production) and residue burning often result in continued soil organic matter decline. This loss in soil organic matter can lead to more pronounced surface crusting following rain.


Nitrogen Fixation

Additions to soil nitrogen are made as a result of either atmospheric, biological or industrial fixation of atmospheric nitrogen (N2 ). These processes are responsible for transforming nitrogen from the atmosphere to either ammonium or nitrate nitrogen that can be used by plants. The atmosphere contains an inexhaustible amount (78 percent) of nitrogen. Approximately 35,000 tons of nitrogen are present in the atmosphere above each acre of the earth’s surface.


Atmospheric nitrogen fixation occurs when there is electrical discharge or lightning during thunderstorms. This causes elemental nitrogen (N2 ) to combine with elemental oxygen (O2 ) to form nitrate (NO3 - ). The nitrate is added to the soil with rainwater and accounts for about 3 to 5 pounds of nitrogen per acre per year.


Biological nitrogen fixation can be either symbiotic or non-symbiotic. Symbiotic nitrogen fixation occurs within legumes. Bacteria (rhizobium sp.) infect the root of the legume and cause a nodule to form. The rhizobium obtain their energy from the legume and convert free nitrogen to ammonia (NH3 ), which the host plant utilizes to make amino acids and proteins. Legumes may fix as much as 500 pounds of nitrogen per acre per year (alfalfa) by this process. However, only a small fraction of the nitrogen fixed by legumes will be available for subsequent crops unless the legume is “plowed down” when a significant amount of top growth is present. Normally, most of the fixed nitrogen is removed in the harvest. Typical amounts of nitrogen added from legumes are shown in Table 2.2.


Legume N-credit (lb N/acre) Legume N-credit (lb N/acre)
Alfalfa 80 Cowpeas 30
Ladino clover 60 Lespedeza (annual) 20
Sweet clover 60 Vetch 40
Red clover 40 Peas 40
Kudzu 40 Winter peas 40
White clover 20 Peanuts 20
Soybeans 20 Beans 20

Table 2.2: Average nitrogen remaining (N-credit) in the soil after legume crops.


Biological nitrogen fixation is an extremely important source of adding nitrogen to soils when fertilizer nitrogen is unavailable. In Oklahoma, the addition of nitrogen to soils as a result of growing legumes is significant and should always be accounted for when determining nitrogen needs for non-legume crops in the subsequent season. However, the cost of establishing and growing legumes for this purpose alone, precludes their use as a sole substitute for nitrogen fertilizers.


Non-symbiotic nitrogen fixation is accomplished by certain “free-living” microorganisms (cyanobacteria or blue-green algae), which live independently of other living tissue. The total contribution of nitrogen from these microorganisms can actually be significant. Recent studies from the Magruder Plots started in 1892 found cyanobacteria in the check plot where no nitrogen has ever been applied. This helps to explain why wheat yields in these plots continue to be around 20 bushels per acre, more than 120 years later with no nitrogen additions.


Industrial fixation of nitrogen involves reacting atmospheric nitrogen (N2) with hydrogen (H), usually in the form of natural gas, under high temperature and pressure to form ammonia (NH3 ). The ammonia may be used directly as anhydrous ammonia gas or converted to other nitrogen fertilizers such as urea, ammonium nitrate, urea-ammonium nitrate solution, ammonium sulfate or ammonium phosphates. Industrial fixation in Oklahoma is responsible for additions of about 300,000 tons of nitrogen per year. This amount of nitrogen is roughly equal to nitrogen removed in harvested crops.


Nitrogen fixation results in addition of nitrogen to the soil through utilization by plants and their residues subsequently added back to the soil (Figure 2.6). In order for soil organic matter to be maintained it is necessary for these additions to be at least equal to the amount of nitrogen removed from the field by harvest. Figure 2.6 illustrates how nitrogen fixation interacts with other forms of nitrogen and their transformations.


Addition of nitrogen to the nitrogen cycle from fixation of atmospheric nitrogen by: (9) lightning; (10) symbiosis with legumes; (11) industrial fertilizer plants.

Figure 2.6: Addition of nitrogen to the nitrogen cycle from fixation of atmospheric nitrogen by: (9) lightning; (10) symbiosis with legumes; (11) industrial fertilizer plants.


Nitrogen Losses

The major nitrogen loss from soils is the removal of nitrogen by harvest of non-legume crops. Other significant nitrogen losses include:


  1. Volatilization of ammonia.
  2. Volatilization of nitrous oxide (N2 O) and nitric oxide (NO) from nitrate in poorly aerated soils (denitrification).
  3. Leaching of nitrate out of the root zone in permeable soils receiving heavy rainfall or irrigation.
  4. Plant nitrogen loss as ammonia from plants containing nitrogen in excess of what the plant can use in seed production, just after flowering.


Each of these processes is responsible only for very small amounts of nitrogen loss over the course of a crop growing season. However, when considered over a generation of farming, or even just a few years, the amount of nitrogen lost can be significant. Nitrogen losses by these processes are responsible for the fact only 30 to 40 percent of fertilizer nitrogen applied can be found in the crop at harvest. Research at OSU and other institutions continues to examine practices that will improve fertilizer-nitrogen-use efficiency. Figure 2.7 illustrates the interaction of these nitrogen losses with other forms of nitrogen and their transformations.


 Losses of nitrogen from the nitrogen cycle as a result of: (12) ammonia volatilization; (13) transformation of nitrate to gaseous oxides (denitrification); (14) leaching below the root zone; (15) volatilization from crops; and (16) harvest removal.

Figure 2.7: Losses of nitrogen from the nitrogen cycle as a result of: (12) ammonia volatilization; (13) transformation of nitrate to gaseous oxides (denitrification); (14) leaching below the root zone; (15) volatilization from crops; and (16) harvest removal.



Most of the total phosphorus in soils is tied up chemically in compounds with low solubility. In neutral- to alkaline-pH soils, calcium phosphates are formed, while in acid soils, iron and aluminum phosphates are produced.


Soil Phosphorus Reactions and Availability

Available soil phosphorus, or that fraction which the plant can use, makes up about one percent or less of the total phosphorus in soils. The availability of inorganic phosphorus in soils is related to the solubility of specific phosphorus compounds present. Phosphorus solubility in particular is controlled by a number of factors – most importantly soil pH.


The amount of precipitated phosphorus is one factor. The greater the total amount present in soil, the greater the chance to have more phosphorus in solution. Another important factor is the extent of contact between precipitated phosphorus forms and the soil solution. Greater exposure of phosphate to soil solution and plant roots increases its ability to maintain replacement supplies. During periods of rapid growth, phosphorus in the soil solution may be replaced 10 times or more per day from the precipitated or solid phase. The rate of dissolution and diffusion of soluble phosphorus determines soil phosphate availability. As phosphate ions (mainly H2 PO4 - and HPO4 2-) are taken up by the plant, more must come from the solid phase.


Soil pH can be a controlling factor that determines phosphorus solubility. Maximum phosphorus availability occurs in a pH range of 5.5 to 7.2. At soil pH levels below 5.5, iron (Fe), aluminum (Al) and manganese (Mn) react with phosphorus to form insoluble compounds. When soil pH exceeds 7.2, phosphorus will complex with calcium (Ca) to form plant-unavailable phosphorus forms. However, it should be noted the solubility of calcium phosphates is much greater than aluminum and iron phosphates.


The proportion of total soil phosphorus relatively available is dependent upon time of reaction, type of clay present in the soil, organic matter content and temperature. The solubility of phosphate compounds formed from added phosphorus due to time of reaction can be broken down in three major groups (Figure 2.8). Fertilizer phosphates are generally in the readily available phosphate group but are quickly converted to slowly available forms. These can be utilized by plants at first, but upon aging are rendered less available and are then classified as being very slowly available. At any one time, 80 to 90 percent of the soil phosphorus is in very slowly available forms. Most of the remainder is in the slowly available form since less than 1 percent would be expected to be readily available.


 The formation of insoluble phosphorus containing compounds in soils that renders phosphorus unavailable for plant use is called phosphorus fixation. Each soil has an inherent fixation capacity that must be satisfied in order to build available phosphorus levels. In Oklahoma, a large portion of the clays have a lower fixation capacity than the highly weathered soils found in high rainfall areas. It is important to understand the actual amount of phosphorus in the soil and the amount available to crops will not necessarily be reflected in a soil test. These soil tests simply provide an index of sufficiency and not an index of build-up or accumulation. Because different soils will have differing fixation capacities, the importance of annual soil testing becomes clear, since this practice is the only method used to estimate future crop fertilizer needs. In addition, these tests should reflect past management (farmers applying more than enough or not enough on an annual basis), and farmers thus can compensate accordingly.

Very slowly available phosphates Apatites, aged Fe, Mn and Al phosphates, stable organic phosphates

⇑                   ⇓

Slowly available phosphates Ca3 (PO4 ) 2 , freshly formed Fe, Al, Mn phosphates (small crystals) and mineralized organic phosphates

⇑                   ⇓

Readily available phosphates Water-soluble ammonium phosphates NH4 H2 PO4 (MAP 11-52-0) (NH4 ) 2 HPO4 (DAP 18-46-0) monocalcium phosphate Ca(H2 PO4 ) 2 (0-46-0) Water-insoluble dicalcium phosphate CaHPO4

Figure 2.8: Relative availability of different phosphate forms and their transformations.


Organic matter and microbial activity affect available soil phosphorus levels. As was the case with nitrogen, the rapid decomposition of organic matter and consequent high microbial population results in temporary tying up of inorganic phosphorus (immobilization) in microbial tissue, which later is rendered available through release (mineralization) processes. This is one of the reasons why broadcasting phosphorus in zero/minimum tillage systems can be beneficial, especially where soil phosphorus fixation capacities are high.


Less than 30 percent of phosphorus fertilizers applied is recovered in plants. Therefore, due to fixation reactions, more phosphorus must be added than is actually removed by crops. Legumes, in general, require much larger amounts of phosphorus than many of the common grain crops grown in Oklahoma.


Because phosphorus is immobile in the soil, roots must come in direct contact with this element before the plant can take it up. However, phosphorus is mobile within the plant and if deficient, lower leaves generally will demonstrate purple coloration on the outer edge of the leaf and/or the leaf margins.


Over a wide range of soils and cropping conditions, phosphorus has proven to be one of the more deficient elements in Oklahoma production agriculture. Soil testing on an annual basis should assist in determining crop needs.



Plants take up potassium as the potassium ion (K+). Potassium within plants is not synthesized into compounds and tends to remain in ionic form in cells and plant tissue. Potassium is essential for photosynthesis, starch formation and translocation of sugars within plants. It is necessary for the development of chlorophyll, although it is not part of its molecular structure.


The main functions of potassium in plants are in the translocation of sugars and its involvement in photosynthesis.


Soil Potassium Reactions and Availability

In most soils (except extremely sandy soils in high rainfall regions), total potassium contents are high. Similar to nitrogen and phosphorus, not all of the total potassium is available for plant growth. The relationship of unavailable, slowly available and readily available forms of potassium is illustrated in Figure 2.9. Only 1 to 2 percent of the total potassium in soils is readily available. Of this, approximately 90 percent is exchangeable or attached to the outside edge of clays, and the remaining 10 percent is in the soil solution. Equilibrium exists between the nonexchangeable, exchangeable and water soluble forms. When the plant removes potassium from the water soluble form, the concentration is readjusted by the exchangeable and nonexchangeable forms. In the case of added potassium, some of the available forms will move toward nonexchangeable forms. The nonexchangeable form also may be referred to as fixed. Certain 2:1 type clay minerals have pore space large enough for the potassium ions (K+) to become trapped, rendering the ions unavailable for immediate plant use. Potassium is positively charged and clays are negatively charged and this makes the potassium ion relatively immobile in the soil. Except in extremely sandy soils, leaching losses under normal Oklahoma conditions are minimal. The largest loss comes from crop removal, particularly where hay crops are produced. Most of western Oklahoma soils have adequate plant available potassium, however, this can best be determined for individual fields by soil testing.

 Relatively Unavailable Potassium (Feldspars, Micas, etc.) 90 to 98% of total potassium

⇒  Slowly Available Potassium (Nonexchangeable (fixed)) 1 to 10% of total potassium

                          ⇑                       ⇓

⇒  Readily Available Potassium (Exchangeable and solution) 1 to 2% of total potassium

 Figure 2.9: Relative amounts of soil potassium present in different levels of availability to plan


Secondary Mineral Elements

Nutrients that are used in relatively moderate amounts by most plants have been categorized as secondary nutrients. These nutrients are calcium (Ca), magnesium (Mg) and sulfur (S).



Calcium is taken up by plants as the cation, Ca2+. Calcium functions in the plant in cell wall development and formation. Calcium is not translocated in plants and consequently, the deficiency of calcium will be observed first in the new, developing plant tissue. Calcium deficient tissue fails to develop normal morphological features and will appear to be an undifferentiated gelatinous mass in the region of new leaf development.


The calcium ion is referred to as a basic ion because the element reacts with water to form the strong base calcium hydroxide, Ca(OH)2 . Calcium is held tightly on the negatively charged clay and organic particles in soils and is abundant in soils that have developed in arid and semi-arid climates. Because of this, it is primarily responsible for maintaining these soils at or near a neutral pH. In addition to unweathered primary and secondary minerals, soils often contain calcium in the form of impure lime (calcium carbonate, CaCO3 ) and gypsum (calcium sulfate, CaSO4 ). Except in the production of peanuts on sandy, acid soils, calcium deficiency in Oklahoma crops has not been substantiated by research. However, because calcium absorption by the developing peanut pod is not very effective from soils with a marginal supply of calcium, peanut producers often apply gypsum over the pegging zone just before the plant begins to peg to assure the crop will be adequately supplied with calcium. For most soils, before the available calcium level reaches a critically low point, the soil pH will become so low that soil acidity will be a major limitation to crop production. Since the common correction of acid soils is to add lime in amounts of tons per acre, this practice will incidentally maintain a high level of available calcium for crops.




Magnesium is absorbed as the divalent cation, Mg2 +, and functions in many enzymatic reactions as a co-factor or in a co-enzyme. The most noteworthy function of magnesium in plants is as the central cation in the chlorophyll molecule. Without magnesium, plants cannot produce adequate chlorophyll and will lose their green color and ability to carry out photosynthesis, the process responsible for capturing energy from sunlight and converting it into chemical energy within the plant. Magnesium deficiency will result in yellow, stunted plants.


Magnesium reactions in soils are similar to calcium in many respects. Magnesium, like calcium, is a basic ion that generally is abundant in arid and semi-arid soils with near neutral pH. Deficiencies most often occur in deep sandy soils with a history of high forage production (8 to 10 tons per acre annually), where forage has been removed as hay. In Oklahoma, deficiencies have occasionally been noted under these conditions in the eastern half of the state. Like calcium, deficiencies are likely to occur on acid soils, and since most lime will contain a small amount (2 to 5 percent) of magnesium carbonate, liming acid soils on a regular basis usually will assure an abundant supply of plant available magnesium. If magnesium deficiency is a reoccurring problem, dolomitic lime (primarily magnesium carbonate) should be sought as a liming source.



Sulfur is absorbed by plants as the sulfate anion, SO42- . Sulfur is a component of three of the 21 essential amino acids and thus, is critical to the formation and function of proteins. Sulfur deficiency causes plants to become light green and stunted. Most crops require about 1/20 the amount of sulfur that they do of nitrogen. Bumper yields of most crops can be supported by 5 to 15 pounds per acre of sulfur.


Sulfur is found in soil in the form of soil organic matter (like nitrogen), dissolved in the soil solution as the sulfate ion and as a part of the solid mineral matter of soils. Sulfur compounds, such as gypsum, are slightly soluble in water. Like nitrate nitrogen, the negatively charged sulfate ion is not readily adsorbed to clay and humus particles and may be leached into the subsoil with a porous surface soil layer. Sulfur deficiencies most often occur in deep sandy soils, low in organic matter, with a history of high crop production and removal. Soils that have a well developed B horizon seldom will be deficient in sulfur because sulfur will not leach out of the root zone and the accumulated sulfur in the subsoil will adequately satisfy crop needs. This is one of the reasons why early sulpher deficiencies often disappear at late stages of growth, at which time roots have penetrated subsoil horizons rich in sulfur. Plant deficiencies in general show up on the younger leaves, with light yellow discoloration. Soils that contain normal amounts of organic matter will release sulfur by mineralization, much like nitrogen, and this will contribute significantly to meeting crop needs. Sulfur deficiencies in Oklahoma are very rare because on the average there is about 6 pounds per acre of sulfur added to soils annually in the form of rainfall. Sulfur is still added incidentally as a component of phosphate fertilizers and other agricultural chemicals which contribute significantly to the requirement of crops. Also, Oklahoma irrigation waters are usually high in sulfate, and add significant amounts each year (for every ppm of sulfate-S, 2.7 pounds per acre of sulfur is added for each acre-foot of irrigation).




The micronutrients are grouped together because they are all required by plants in very small amounts. Some, like molybdenum (Mo), are required in such small amounts that deficiencies can be corrected by applying the element at only a fraction of a pound per acre. Similarly, chlorine is needed in such small quantities that when researchers at the University of California were attempting to document its necessity, they found that touching plant leaves with their fingers transferred enough chlorine from the perspiration on their skin to meet the plant’s requirements. These elements do not function in plants as a component of structural tissues like primary and secondary nutrients. Instead, micronutrients are mainly involved in metabolic reactions as a part of enzymes where they are used over and over without being consumed. Nevertheless, their functions are very specific and cannot be substituted for by some other element. Deficiencies of any of the elements can be corrected by foliar application of solutions containing the element.


Manganese, Chlorine, Copper and Molybdenum

Deficiencies of these nutrients have yet to be documented in Oklahoma, except for chlorine in wheat on a deep sandy soil near Perkins. Each of the elements is adsorbed by plants in the ionic form, manganese and copper as the divalent cations Mn2 + and Cu2 +, molybdenum as the oxyanion MoO42 - , and chlorine as the simple Cl- anion. Of these four nutrients, molybdenum and chlorine are probably the most likely to receive attention. Molybdenum functions in plants in the enzyme nitrate reductase, which is very important in nitrogen metabolism in legumes. Availability is reduced in acid soils and often if molybdenum availability is marginal it can be increased to adequate levels by simply liming the soil. Where large seeded legumes are grown, like soybeans or peanuts, obtaining seed that was grown with a good supply of molybdenum will avoid the deficiency because normal levels of molybdenum in the seed will be enough to meet the plant needs.


Soil fertility research in the Great Plains has occasionally shown small grain response to fertilizers containing chlorine. Often the response has been the result of disease suppression (take-all disease) rather than correction of an actual nutrient deficiency in the plant, and usually it has been in areas that do not commonly apply potassium fertilizers containing chloride (such as muriate of potash or potassium chloride, 0-0-62).



Boron is absorbed by plants as uncharged boric acid, B(OH)3 , the chemical form also present in soil solution. Boron is believed to function in plants in the translocation of sugars. Because B is uncharged in soil solution and it forms slightly soluble compounds, it also is relatively mobile in soils and can be leached out of the surface soil. This is sometimes critical in peanut production because of the very sandy, porous soils peanuts are produced in. As a result, boron deficiency has been reported in peanuts. The deficiency manifests itself as a condition known as “hollow heart” whereby the center of the nut is not completely filled and an inferior crop is harvested. Although alfalfa has an annual requirement twice that of peanuts, the deficiency of boron has never been documented in alfalfa. The reason for this is likely because alfalfa is usually grown in deep, medium textured soils and because alfalfa has an extensive root system even at lower depths in the soil profile. Whenever boron deficiencies are suspected, and if boron fertilizer is applied, care should be exercised as toxicities can be created by simply doubling the recommended rate.


Iron and Zinc

Iron and zinc deficiencies both occur in Oklahoma and are associated with unique soil and crop situations. Zinc is absorbed as the divalent cation Zn2 +, while iron is absorbed as a “plant provided” chelated Fe3 + complex by grass type plants and as the “plant-reduced” divalent cation Fe2 + by broad-leaved plants.


Corn is sensitive to moderately low soil zinc levels and deficiencies may occur at DTPA soil test values below 0.8 parts per million. Winter wheat, on the other hand, has been grown in research experiments near Woodward, Oklahoma where the soil test zinc value was less than 0.15 parts per million without showing any deficiency or responding to zinc fertilizer. Obviously winter wheat is very effective in utilizing small amounts of soil zinc. Zinc deficiencies in corn are most common where fields have been leveled or for some other reason the topsoil has been removed and the surface soil has very little organic matter and where the subsoil pH is high. Deficiencies are easily corrected by broadcast application of about 4 to 6 pounds per acre of zinc preplant. An application of this rate should remove the deficiency for 3 to 4 years. The most sensitive plant to zinc deficiency in Oklahoma is pecans. Deficiencies may occur whenever DTPA soil test values are less than 2.0 parts per million. Foliar sprays are very effective in preventing and/or correcting the deficiency. A single application usually lasting the entire growing season.


 Iron deficiency is most common in sorghum and sorghum-sudan crops in Oklahoma. The occurrence is limited to the western half of the state in soils that are slightly alkaline (pH above 7.5). All soils in Oklahoma contain large amounts of iron, usually in excess of 50,000 pounds per acre. However, almost all of this iron is in a form that is not available to crops, like rust. Iron availability is increased greatly in acid soils, consequently the deficiency is seldom observed in any crops in eastern and central Oklahoma, where soil pH is usually less than 7.0. Iron deficiency cannot be corrected by soil application of iron-containing fertilizers because the iron from the fertilizer is quickly converted to unavailable iron just like that already present in the soil. The exception to this general rule is the use of chelated iron. However, these fertilizer materials can be cost prohibitive for field scale use. Foliar application of iron sulfate solutions is effective for supplying iron to deficient plants. Unfortunately, iron is not translocated in the plant and subsequent new leaves will again exhibit the interveinal chlorosis (yellow between the veins) characteristic of iron deficiency. Repeated spraying will provide iron for normal growth but often will be cost prohibitive. The most effective long-term corrective measure for dealing with iron chlorosis is to increase soil organic matter since iron deficiency usually is limited to small areas of a field. Organic matter can be effectively increased by annual additions of animal manure or rotted hay. This results in additional chelating of iron and also has a tendency to acidify the soil. Broadleaf plants have what is called an “adaptive response mechanism” that allows them to make iron more available if they experience iron stress. The strength of this mechanism is a genetic trait and some varieties, such as ‘forest’ soybeans, do not possess this ability and will often become chlorotic if grown in neutral or alkaline soils.


The Mobility Concept

 The nutrient mobility concept as it relates to soil fertility was first proposed in 1954 by Roger H. Bray at the University of Illinois. Much research since then has supported his mobility concept and it is now considered basic to the understanding of soil fertility. Bray simplified all the soil chemistry surrounding the essential nutrients to the fact that some are quite mobile in soils and others are relatively immobile.


Mobile Nutrients

Plants are able to extract mobile nutrients from a large volume of soil, even soil beyond the furthest extension of their roots because as the plants extract water from around their roots, water from further away moves toward the root and carries the mobile nutrient with it. Figure 2.10 illustrates this point. Plants obtain mobile nutrients from a “root system sorption zone” and are capable of using nearly all of the mobile nutrient (or mobile form of the nutrient) if the supply is limited. According to Bray, the mobile nutrients are: nitrogen, sulfur, boron and chlorine.


The large volume of soil from which plants extract mobile nutrients (root system sorption zone).

Figure 2.10. The large volume of soil from which plants extract mobile nutrients (root system sorption zone).


 In a field situation, where there is more than one plant, root system sorption zones overlap if plants are close enough together as illustrated in Figure 2.11. As a result there is a volume of soil between plants where the nutrient is in demand by both plants. As plants are placed closer and closer together (e.g. increasing plant population to increase potential yield) the competition for nutrients increases. Unless the competition among plants in a field for a mobile nutrient is satisfied by supplying more of the nutrient, the growth and yield of plants will be restricted. From this simple illustration we learn the supply of mobile nutrients like nitrogen must be provided in direct proportion to the number of plants, or potential yield of the crop. This “supply” can be easily determined by calculating the amount of nutrient that will be taken up by the crop. To do this, we only need to know the average concentration of the nutrient in the crop and what the crop yield will be. Average nutrient concentrations are commonly known, however yields vary from field to field and year to year. For this reason it is critical to have in mind a “yield goal” or expected yield in order to determine fertilizer needs for mobile nutrients like nitrogen. For example, in Oklahoma the rule “2 pounds nitrogen per acre for every bushel of wheat” is commonly used to estimate the nitrogen requirements of winter wheat. This rule takes into account that soil test and fertilizer nitrogen will only be about 70 percent utilized by the plant. Because mobile nutrients are almost completely extracted from the root system zone, soil test values like nitrate nitrogen will change drastically from one year to the next in relation to how much nitrogen was available and the crop yield.


Competition among plants brought about by increasing yield goal.

Figure 2.11. Competition among plants brought about by increasing yield goal.


Immobile Nutrients

Nutrients that are immobile in the soil are: phosphorus, potassium, calcium, magnesium, iron, zinc, manganese, copper and molybdenum. These nutrients are not transported to plant roots as soil water moves to and is absorbed by the root. These nutrients are absorbed from the soil and soil water that is right next to the root surface. Because of this there is only a small volume of soil next to the root surface that is involved in supplying immobile nutrients to plants. Figure 2.12 identifies this soil volume as the root surface sorption zone. This figure illustrates that only a small fraction of the soil in the total rooting zone is actually involved in supplying immobile nutrients. The total amount of immobile nutrient in the whole soil volume is not as important as the concentration right next to the root surface. Because only the thin layer of soil surrounding the roots is involved in supplying immobile nutrients, when more plants are considered as in Figure 2.13, there is still little or no competition among the plants for immobile nutrients. Competition would occur only at points where roots from adjacent plants actually came in contact with one another. This illustration indicates that the supply of immobile nutrients like phosphorus does not have to be adjusted (increased) in relation to an increase in yield goal or yield potential. If soil availability is adequate for a 25-bushel wheat yield, then in the event that conditions are favorable (better moisture supply) for 50-plus-bushel yield, the more extensive root system that develops for the higher yield will explore new soil and extract the required phosphorus.



Small volume of soil from which plants extract immobile nutrients (root surface sorption zone).

Figure 2.12. Small volume of soil from which plants extract immobile nutrients (root surface sorption zone).



 Limited competition among plants for immobile nutrients.

Figure 2.13. Limited competition among plants for immobile nutrients.


 The mobility concept and these simple illustrations can help one understand the basis for some common practices and observations. For example, immobile nutrient fertilizers usually are more effective if they can be incorporated, but especially should be placed where roots have a high probability of coming in contact with the fertilizer. This is why band applying phosphate fertilizers is generally more effective than the same rate broadcast and incorporated. Mobile nutrients like nitrogen can be broadcast during the growing season (topdressing wheat) because they are moved easily to the roots with rain or irrigation. The phosphorus soil test does not change much from year to year regardless of the previous year’s yield or fertilizer rate because much of the soil was not in contact with the roots or fertilizer and its available phosphorus status was therefore unchanged. Continued broadcast application of high rates of phosphorus will cause a build up and an increase in the soil test phosphorus because only a fraction (15 to 20 percent) of the fertilizer comes in contact with the roots (fertilizes the crop) and the rest reacts only with the soil (fertilizes the soil).


It sometimes is useful to compare mobile and immobile nutrients and their management to fuel and oil for a tractor or pickup. Fuel is required in relation to the amount of work expected from the tractor in much the same way nitrogen is required in relation to the amount of yield expected from the crop. Oil is required more in relation to the level in the crankcase identified by the dipstick than by what or how much work is expected from the tractor (oil burners excepted). Similarly, phosphorus and potassium requirements are determined from the soil test and the amount of fertilizer recommended does not depend on the yield goal. Like the dipstick that is calibrated with a mark showing full and 1-quart low, the soil test for phosphorus (and any immobile nutrient) must be calibrated by field research. Just as the dipstick is uniquely calibrated for each kind of tractor, soil test calibrations vary slightly for different crops and soils and may be somewhat unique for states and regions.


 Nitrogen cycle

 Figure 2.14. Nitrogen cycle. 


Advanced Considerations

The students and faculty at OSU developed a nitrogen cycle (Figure 2.14) that includes various components interlinked with what has been presented here. In addition, this cycle includes the relationships of temperature, pH and oxygen with nitrogen dynamics in plant-soil systems. Note that this cycle is more complex than that illustrated in Figures 2.4, 2.6 or 2.7.


Chapter 3. Problem Soils

Most soils in Oklahoma have developed under conditions that have resulted in them being naturally productive. Because of how they have been managed for agricultural production and otherwise changed by man’s activities, some of these soils are now less productive. Two of the most common causes of productivity losses are the development of acidic and saline (including saline-alkali and alkali) conditions. They are often considered as problem soils because they do not respond to normal management. Therefore, their treatment and management should be different.


Acid Soils

Soil acidity is a crop production problem of increasing concern in central and western Oklahoma. Although acid soil conditions are more widespread in eastern Oklahoma, their more natural occurrence has resulted in farm operators being better able to manage soil acidity in that part of the state. However, in central and western Oklahoma this problem is increasing with time.


The median soil pH of all agricultural samples tested by the Soil, Water and Forage Analytical Laboratory from 2009 to 2013 was 6.1. This means 50 percent of the sample had a pH less than 6.1 and 50 percent higher than 6.1 statewide. Some counties had more than 35 percent of fields with pH lower than 5.5, which is critically low for most field crops. The median soil pH for all counties is shown in Figure 3.1. More acidic soils frequently are found in the central part of the state, which likely is due to intensive crop production.


Median soil pH for all Oklahoma counties tested between 2009 and 2013.

Figure 3.1: Median soil pH for all Oklahoma counties tested between 2009 and 2013.


Why Soils are Acidic

The four major causes for soils to become acidic are listed below:

  1. Rainfall and leaching
  2.   Acidic parent material
  3.   Organic matter decay
  4. Harvest of high yielding crops
  5. Nitrification of ammonium

The above causes of soil acidity are most easily understood when we consider a soil is acidic when there is an abundance of acidic cations, like hydrogen (H+) and aluminum (Al3+) present compared to the alkaline cations like calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+).


Rainfall and Leaching

Excessive rainfall is an effective agent for removing basic cations. In Oklahoma, for example, we generally can conclude soils are naturally acidic if the rainfall is above about 30 inches per year. Therefore, soils east of I-35 tend to be acidic and those west of I-35, alkaline. There are many exceptions to this rule though, mostly as a result of item 4, intensive crop production and application of nitrogen fertilizers. Rainfall is most effective in causing soils to become acidic if a lot of water moves through the soil rapidly. Sandy soils are often the first to become acidic because water percolates rapidly, and sandy soils contain only a small reservoir (buffer capacity) of bases due to low clay and organic matter contents. Since the effect of rainfall on acid soil development is very slow, it may take hundreds of years for new parent material to become acidic even under high rainfall.


Parent Material

Due to differences in chemical composition of parent materials, soils will become acidic after different lengths of time. Thus, soils that developed from granite material are likely to be more acidic than soils developed from calcareous shale or limestone.


Crop Production

Harvesting of crops has its effect on soil acidity development because crops absorb lime-like elements, as cations, for their nutrition. When these crops are harvested and the yield is removed from the field, some of the basic material responsible for counteracting the acidity developed by other processes is lost, and the net effect is increased soil acidity. Increasing crop yields will cause greater amounts of basic material to be removed. Grain contains less basic materials than leaves or stems. For this reason, soil acidity will develop faster under continuous wheat pasture than when only grain is harvested. High yielding forages, such as Bermudagrass or alfalfa, can cause soil acidity to develop faster than with other crops.


Table 3.1 identifies the approximate amount of lime-like elements removed from the soil by a 30-bushel wheat crop. Note there is almost four times as much lime material removed in the forage as the grain. This explains why wheat pasture that is grazed will become acidic much faster than when grain alone is produced. Using 50 percent Effective calcium carbonate equivalent lime, it would take about one ton every 10 years to maintain soil pH when straw (or forage) and grain are harvested annually at the 30-bushels-per-acre level.



The use of fertilizers, especially those supplying nitrogen, often is a cause of soil acidity. Acidity is produced when ammonium containing materials are transformed to nitrate in the soil. The more ammoniacal nitrogen fertilizer is applied, the more acidic the soil gets.


Table 3.1: Bases removed by a 30-bushel wheat crop.


























**One ton of alfalfa will remove slightly more than this amount.


What Happens in Acid Soils

Knowing the soil pH helps identify the kinds of chemical reactions likely to occur in soils. Generally, the most important reactions, from the standpoint of crop production are those dealing with solubilities of compounds or materials in soils. In this regard, we are most concerned with the effects of pH on the availability of toxic elements and nutrient elements.


Toxic elements like aluminum (Al) and manganese (Mn) are the major causes for crop failure in acid soils. These elements are a problem in acid soils because they are more soluble (available for plant uptake) at low pH. In other words, more of the solid form of these elements will dissolve in water when the pH is very low. There is always a lot of solid aluminum present in soils because it is a part of most clay particles.


Element Toxicities

When soil pH is above 5.5, aluminum in soils remains in a solid combination with other elements and is not harmful to plants. As pH drops below 5.5, aluminum containing materials begin to dissolve. Because of its nature as a trivalent cation (Al3+), the amount of dissolved aluminum is 1,000 times greater at pH 4.5 than at 5.5 and 1,000 times greater at 3.5 than at 4.5. For this reason, some crops may seem to do very well, but then fail completely with just a small change in soil pH. Wheat, for example, may do well even at pH 5.0, but usually will fail completely at a pH of 4.0.


The relationship between pH and dissolved manganese in the soil is similar to that described for aluminum, except that manganese (Mn2+) only increases 100 fold when the pH drops from 5.0 to 4.0.

Toxic levels of aluminum harm the crop by root pruning. That is, a small amount of aluminum in the soil solution in excess of what is normal causes the roots of most plants to either deteriorate or stop growing. As a result, the plants are unable to normally absorb water and nutrients, appear stunted and exhibit nutrient deficiency symptoms, especially those for phosphorus. The final effect is either complete crop failure or significant yield loss. Often, the field will appear to be under greater stress from pests, such as weeds, because of the poor crop conditions.


Toxic levels of manganese interfere with normal growth processes in above ground plant parts. This usually results in stunted, discolored growth and poor yields.


Desirable pH

The adverse effect of these toxic elements is most easily (and economically) eliminated by liming the soil. Liming raises soil pH and causes aluminum and manganese to go from the soil solution back into solid (non-toxic) chemical forms. For grasses, raising soil pH to 5.5 will generally restore normal yields. Legumes, on the other hand, do best in a calcium-rich environment and often need a soil pH between 6.5 and 7.0 for maximum yields.


Soil pH in the range of 6.0 to 7.0 also is desirable from the stand point of optimum nutrient availability. However, the most common nutrient deficiencies in Oklahoma are for nitrogen, potassium and phosphorus, and availability of these elements will not be greatly changed by liming. Nutrients most affected by soil pH are iron and molybdenum. Iron deficiency is more likely to occur in non-acid (high pH) soils. Molybdenum deficiency is not common in Oklahoma, but would be most apt to occur in acid soils and could be corrected by liming.


Soil Buffer Capacity and Buffer Index

Although crops remove large quantities of lime-like materials that are harvested each year, the soil pH usually does not change noticeably from one season to the next. Because soil pH does not change quickly, it is said to be buffered. Buffer means the resistance to the change of pH.


There are several reasons why soils have this buffer ability or capacity. For example, in the Oklahoma Panhandle, soils commonly contain free calcium carbonate (lime). The term caliche is used to describe layers of soil material cemented by accumulated calcium carbonate. These accumulations provide a huge reserve of lime that will maintain soil pH in the alkaline range (above pH 7.0 for generations, perhaps centuries, even with the most productive agricultural systems.


A second contribution to the buffering capacity of soils is the release of basic chemical elements from normal chemical weathering of soil minerals. This is a very slow process that occurs whenever water is added to soil. The effect is influenced by the type of minerals in the soil, the amount and frequency of water addition, and soil temperature.


The most important source of buffer capacity in acid soils (no free lime present) is exchangeable cations. These are the lime-like chemical elements (mostly calcium) that are adsorbed on the surface of soil particles. These adsorbed basic materials act like a large reservoir that continually replenishes basic materials in the soil solution when they are removed by a crop or neutralized by acid. Figure 3.2 illustrates this and the relationship between soil pH and buffer capacity.


As crops remove bases from soil water in the reservoir on the right (Figure 3.2), bases from the large reservoir of soil solids (clay and humus) on the left move to the soil solution and replenish the supply. Because of this relationship and the large reserve of bases from soil solids, the pH does not change much from month to month or even year to year. Also since the large reservoir on the left is shaped like a pyramid, pH can often be changed more easily by liming at pH near 6 than in the very acid pH 4.5 to 5.5 range.


Figure 3.3 shows the influence of soil organic matter and texture on buffer capacity. Both soils have a pH of 4.3, and are too acidic for efficient crop production. In order to provide a more favorable pH, the soils must each be limed. The amount of lime required will depend on the size of the large reservoirs and how base depleted they may be.


From these diagrams it is easy to understand why it takes much more lime to raise the pH of a clay soil with its large reservoir than it does for a sandy soil and its small reservoir. Also, because the reservoir of sandy soil is small, if acidifying conditions are equal, sandy soil will tend to become acidic more rapidly and need to be limed more frequently than a clayey soil.


 The relationship of basic materials in soil solids to pH of the soil solution.

Figure 3.2: The relationship of basic materials in soil solids to pH of the soil solution.


Reservoirs of soil solids in clayey vs. sandy soil.

Figure 3.3: Reservoirs of soil solids in clayey vs. sandy soil.


The Soil Test

Buffer Index, measured in the laboratory as a part of the OSU routine soil test, is an indirect estimate of the soil reservoir size for storing basic material. Because the test involves adding basic (lime-like) material to soils of pH less than 6.3 and then measuring pH again, the BI pH is larger when the reservoir is small. The two soils illustrated in Figure 3.3 need to be limed. The Pond Creek Silt Loam soil would have a Buffer Index value of about 6.2. About 4.2 tons of effective calcium carbonate equivalent lime would be required to raise the soil pH to 6.5. The sandy soil, having the same soil pH, would have a BI value of about 6.5 and require only 2.5 tons of effective calcium carbonate equivalent lime to reach the same pH. The field calibration for BI and lime requirement is provided in Table 3.2.


How to Interpret pH and Buffer Index

Considering a soil test result of pH 5.8 and Buffer Index 6.8, where establishment of alfalfa is intended, the following steps are taken to determine the lime requirement.


First, the soil test pH of 5.8 is compared to the preferred pH for alfalfa in Table 3.3. Since the soil pH 5.8 is below the lowest pH in the preferred range, lime must be added to raise the pH to the desired level.


The amount of lime required is determined from Table 3.2 by locating the Buffer Index value of 6.8 in the left hand column and matching it to the number directly across from it (bold) under the middle column of numbers. In this case, 1.2 tons of effective calcium carbonate equivalent lime would be required.


If the intended crop is wheat instead of alfalfa, no lime is required because Table 3.3 shows that pH 5.8 is satisfactory for wheat production. Since the pH is satisfactory for wheat, the lime requirement would not be reported, even though the Buffer Index was measured. It would be important to regularly test this soil, especially if it were sandy, so lime could be applied before the soil became seriously acid (below pH 5.0) for wheat production.


Remember, the Buffer Index is used only as a guide for how much lime should be added to an acid soil when it is necessary to raise soil pH.


Table 3.2: Tons of effective calcium carbonate equivalent* lime required to raise soil pH of a 6-7 inch furrow slice to pH 6.5 or 6.4.


Buffer Index All other crops Continuous wheat


































 *Effective calcium carbonate equivalent guaranteed by lime vendor.


Table 3.3: Common pH preference of field crops.

Crops   pH Range
Cowpeas, Crimson Clover,    
 Mungbeans and Vetch   5.5-7.0
Soybeans, Peanuts    
Alsike, Red and White,   5.8-7.0
 (Ladino) Clovers,and    
 Arrowleaf Clover   6.0-7.0
Alfalfa and Sweet Clover   6.3-7.5
Fescue and Weeping Lovegrass   4.5-7.0
Buckwheat    5.0-6.5
Sorghum, Sudan, Corn and Wheat   5.5-7.0
Bermuda, Canola   5.7-7.0
Barley   6.3-7.0

Correcting Soil Acidity

Lime Reactions

Soil acidity can be corrected only by neutralizing the acid present, which is done by adding a basic material. While there are many basic materials that can neutralize acids, most are too costly or difficult to manage. The most commonly used material is agricultural limestone (aglime). It is used because it is relatively inexpensive and easy to manage.


The reason limestone is easy to manage is because it is not very soluble, meaning it does not dissolve easily in water. For this reason, it is not very corrosive to equipment, and more importantly, its pH at equilibrium (after it has dissolved as much as it can and there is still some lime left in the water) is only about 8.3. This latter aspect is very important because even if an excessive amount of lime is applied, a harmful effect on crop yields would generally not take place.


The reaction of lime, or calcium carbonate (CaCO3 ), with an acid soil is illustrated by Figure 3.4.


This diagram shows that the acidity is on the surface of soil particles. As lime dissolves in the soil, calcium from the lime moves to the surface of soil particles and replaces the acidity (H+ and Al3+). The acidity reacts with carbonate (CO3 ) to form carbon dioxide (CO2 ), water (H2O) and insoluble Al. The end result is a soil that is less acid.


 Illustration of how aglime neutralizes soil acidity.

Figure 3.4: Illustration of how aglime neutralizes soil acidity.


Lime Research

Several field research experiments have been conducted on wheat in the past to examine suitable liming materials and application rates. A common feature of all effective commercially available liming materials is that they contain a basic lime-like material such as calcium or magnesium carbonate. Since it is ultimately the material from which other basic materials are derived, aglime is usually the lowest cost per ton of active ingredient (effective calcium carbonate equivalent, finely ground pure CaCO3 is defined to have an effective calcium carbonate equivalent of 100).


A long-term liming study on wheat was conducted during a nine-year period on a Pond Creek silt loam soil near Carrier, Oklahoma. Results of the study are illustrated in Figure 3.5 and show through nine harvests, the yield of wheat was greatly improved by a single application of lime. It is important to note although 4.8 tons of effective calcium carbonate equivalent lime were recommended from the soil test in order to raise the pH to 6.8, one-fourth that rate (only 1.2 ton effective calcium carbonate equivalent) was sufficient for eight years to restore yields to almost 100 percent of the yield obtained when 4.8 tons effective calcium carbonate equivalent were applied. The 2.4 tons effective calcium carbonate equivalent rate, one-half the normally recommended rate, was still effective at the end of the experiment.


 Long-term effect of lime on wheat yields.

Figure 3.5: Long-term effect of lime on wheat yields.


Using information from field studies, such as the Carrier site, a relationship between OSU soil test pH values and expected wheat yield has been developed (Figure 3.6). The yield at a given pH is expressed as relative yield. This term means the expected yield as a percentage of that possible if soil acidity was not a limiting factor. For example, if a 40-bushel yield is expected with no acidity problems then at a soil pH of 5.0 a relative yield of 85 percent or 34 bushels, would be expected.


The effect of soil pH on wheat yields.

Figure 3.6: The effect of soil pH on wheat yields.



Lime Rates

(Minimum Amounts)

The amount of lime to apply for wheat production depends on whether you are growing continuous wheat or will rotate wheat with a legume. If wheat alone is grown year after year, it is necessary to only apply a rate of lime to raise the pH to above 5.5 because higher pH may favor some root rot diseases. If legumes are sometimes grown, then soil pH should be raised to 6.5 or above. Thus, for continuous wheat the following recommendation is made: The minimum amount of lime to apply is 0.5 ton effective calcium carbonate equivalent lime or 50 percent of the soil test deficiency amount required to raise the pH to 6.5, whichever is greater. An OSU soil test will identify these lime rates for wheat whenever the soil pH is below 5.5.


Calculating Rates

Lime requirements are expressed in terms of Effective calcium carbonate equivalent. The Effective calcium carbonate equivalent is provided as a guarantee from lime vendors who are registered to sell aglime in Oklahoma. The guarantee is obtained by an analysis of the lime by the Oklahoma State Department of Agriculture, Food and Forestry. There are two components to the determination by their lab. First, the purity of the lime is determined chemically (purity factor). In this test they analyze for the fraction of CaCO3 , or its equivalent, in the lime material. The second measurement is a determination of how finely the lime particles are ground (fineness factor). The fineness factor is determined by weighing sieved portions of a lime sample. The factor is then calculated by taking one-half times the fraction (e.g. 0.90) of sample passing an 8 mesh sieve plus one-half times the fraction (e.g. 0.70) of sample passing a 60 mesh sieve. The fineness factor for these example values would be:


.5 x 0.90 + .5 x 0.70 = 0.80


The purity factor (a fraction) and the fineness factor (a fraction) are multiplied by 100 to obtain the effective calcium carbonate equivalent value. If the purity factor was 0.90 (90 percent pure or equivalent calcium carbonate) then the effective calcium carbonate equivalent would be (0.90 x 0.80) x 100, or 72 percent. The more CaCO3 in the material and the finer the particle size, the greater the effective calcium carbonate equivalent. Good quality lime will have an effective calcium carbonate equivalent value above 60 percent. Because aglime does not always have an effective calcium carbonate equivalent of 100 percent, the amount required to provide a given amount of 100 percent effective calcium carbonate equivalent must be calculated. The calculations to use are shown below:



Effective calcium carbonate equivalent lime required x 100 = aglime required

Divided by

% Effective calcium carbonate equivalent


For example, let us assume the available aglime was 72 percent effective calcium carbonate equivalent and the soil test indicated a need for 1.5 tons effective calcium carbonate equivalent to raise the soil pH to the desired level.

The calculations would be:


(1.5 x 100) / 72  = 2.1 tons of aglime


So, 2.1 tons per acre of the 72 percent effective calcium carbonate equivalent lime would have to be applied in order to get the 1.5 tons of 100 percent effective calcium carbonate equivalent lime required to do the job.


Lime Applications

Because lime does not dissolve easily in water, it must be treated similarly to fertilizers that supply the soil with immobile nutrients like phosphorus. Thus, for lime to be most effective in neutralizing soil acidity it must be thoroughly mixed with the soil. Since neutralization involves a reaction between soil particles and lime particles, the better lime is mixed with the soil, the more efficiently the acidity is neutralized. For this reason, wet materials (like that from water treatment plants) which cannot be thoroughly mixed with the soil are often less effective. Similarly, pelleted lime particles are too large to mix well with small soil particles. Attempts to mix these materials with soil often result in soil acidity being neutralized only near the lime aggregates (or pellets), whereas acidity between aggregates remains unaffected. Once the proper rate has been determined and the lime has been spread to give a uniform application over the field, it is best to incorporate it with a light tillage operation such as disking. Disking can be followed by plowing, but care should be taken not to plow too deeply or the lime will be diluted by subsoil and be less effective. Lime rates are calculated on the basis of neutralizing the top six inches of soil.


Since the lime reaction involves water, the effect of lime will be very slow in dry soil. Even when everything is done correctly and the soil is moist, it often takes a year or more for a measurable change in soil pH to occur. For this reason, liming for wheat production should be done as soon after harvest as possible. However, when the soil pH is extremely low, sufficient change may occur in just a few weeks and make the difference between being able to establish a wheat crop and having a failure.


A similar approach should be used for annual planting of other grasses. When continuous production of perennial grasses is planned, the full rate identified by the soil test buffer index should be applied pre-plant. This practice allows incorporation of the lime to maximize its reaction with soil and will maintain a desirable pH for several years after establishment. Careful monitoring of high producing forage grasses, such as Bermudagrasses, by periodic soil testing will identify lime needs early enough to maintain desirable soil pH by unincorporated broadcast application.


Liming Materials

The most common and most effective liming material continues to be ground aglime. It is marketed by the ton, should generally be powdery with only a small percentage of coarse (sand size) particles, and have an effective calcium carbonate equivalent of 50 percent or greater. Variations and different formulations of ground aglime have been developed and marketed. These materials often are promoted on the basis of being more effective or less expensive. The merits of these products should be considered carefully.


“Liquid Lime” is a formulation of high-quality aglime (effective calcium carbonate equivalent above 90 percent) with water and enough clay to keep the lime in suspension. The amount of water added may range from 35 to 50 percent. Care should be taken to make sure that the added water is not being charged for, as if it were high quality lime. When 90 percent effective calcium carbonate equivalent lime is mixed 50 percent (weight to weight) with water, the resulting product is only 45 percent effective calcium carbonate equivalent lime (90 percent x .50 = 45 percent). The fact that it is suspended in water does not increase its effectiveness. On the contrary, wet lime will not mix as easily with soil and therefore, its neutralizing effectiveness may be less than an equal amount of dry effective calcium carbonate equivalent aglime.


Similarly, “water treatment lime” may not be as effective as an equal rate of aglime. This material is a waste product from water treatment plants. Although it has a high Effective calcium carbonate equivalent, it often is wet when applied and a good mixture with soil is difficult to obtain. Too often, large chunks or globs remain mixed with the soil and only the acid soil next to the chunk of lime is neutralized, leaving large areas of soil between chunks that remain acid.


Pelleted lime is finely ground lime pressed into pellets. Until the pellets physically break up and the fragments of powder size lime become thoroughly mixed with soil, these too are limited in neutralizing soil acidity. Pellets, liquid lime, and water treatment lime can be spread or applied without dust common to good aglime. Although easily visible, airborn dust associated with aglime application represents only a small fraction of the total applied, and loss from the field should not be significant.


Finally, sometimes coarse road grade lime is in abundance and can be purchased at a very low cost. This cheap lime is too coarse to have a reasonable effective calcium carbonate equivalent and will not be sold as aglime. Because of the existing aglime law in Oklahoma, whenever a material is marketed and sold in Oklahoma as aglime it must be accompanied by a guaranteed effective calcium carbonate equivalent. The guaranteed effective calcium carbonate equivalent must be of the formulated product and not its ingredients.


Reducing Metal Toxicity

Fertilizer Reactions

Phosphate in the soil has long been known to be less available to crops in some extremely acid soils because it reacts with aluminum and/or manganese, which are more available in acid soils. When phosphate reacts with these metals, the compound formed is a very insoluble solid (such as aluminum phosphate). As a result, not only is the phosphate unavailable, but also the aluminum and manganese are unavailable. For these reasons, when phosphate fertilizers are banded with the seed at planting time, the harmful effects of toxic aluminum and manganese are greatly reduced, and near-normal yields may be obtained. Figure 3.7 illustrates the benefit of this practice for both grain and forage production.



a) Grain yield in acud soils


b) Forage yield in acud soils

Figure 3.7. Responses of wheat grain and forage yields to seed-applied phosphate fertilizers (APP: ammonium polyphosphate; DAP: diammonium phosphate in a strongly acidic soil.


Phosphate Materials and Rates

Figure 3.7 also shows a higher rate of phosphate may be needed in order to get maximum benefits for fall forage production. It is especially important to use the higher rate for forage production on soil that has a pH below 4.5. The use of phosphate fertilizer in this way does not change soil pH. Also, within a few months after all the phosphate has been used up, more aluminum and manganese may become available. While this may not affect the developed crop, it will affect the next crop in the seedling stage. As a result, phosphate fertilizer must be applied each year whereas lime only needs to be applied every five to eight years. On the other hand, buildup of soil test phosphorus above crop needs may lead to increased phosphorus in the runoff.


When to Use Phosphate

As stated earlier, acid soil is best neutralized by adding aglime. However, seed-applied phosphate (either ammonium polyphosphate or diammonium phosphate) should be considered for acid wheatland soils when:


  1. the land is owned by someone who will not provide a long-term lease or pay some of the cost for liming,
  2. the soil acidity problem is discovered too late for lime application in a given season or
  3. the soil has a low soil test value for phosphorus.

It is important to remember this use of phosphate fertilizer is very different from normal. Banding phosphate on acid soils can increase yields even when the phosphate soil test value is high (more than 65), not because more phosphate is provided to the plant, but because metal toxicity is reduced. Also, it is important to remember the soil continues to become more acidic with time. Eventually, lime must be added to the soil to neutralize acidity


Saline and Alkali Soil

Two other problem soils are salty (saline) soils and slick-spot (alkali or sodic) soils. A third problem soil often develops from slick spots when they are poorly managed. This is the saline-alkali soil which results when slick-spot soils become salty.


Although all problem soils may be identified by poor crop production, these soils have other similarities and differences that are important to know before attempting to improve or reclaim them.


Saline soils are soils that contain at least 2600 parts per million dissolved salts in the solution from a soil saturated with water. The salt content is estimated by laboratory measurement of how well the soil water conducts electricity, and saline soils are those with an electrical conductivity (EC) of 4,000 micromohs/cm (about 2,600 parts per million total dissolved salt). This level of salts is great enough to reduce production of salt-sensitive crops. Normal, productive agricultural soils commonly have electrical conductivity values below 1,000.


Alkali soils are soils which contain enough sodium to cause 15 percent of the cation exchange sites to be occupied by sodium. Sodium in the soil prevents clay particles (and other very small, colloidal sized particles such as humus) from coming together and forming large soil aggregates. When soils contain 15 percent or more of exchangeable sodium most of the clay and humus particles are unattached or dispersed. These soils commonly have a pH of 8.5 or above (alkali). Some Oklahoma soils become dispersed when the exchangeable sodium is as low as 7 percent. Productive agricultural soils often have less than 1 percent exchangeable sodium. Soils can be classified into 4 groups based on the EC and ESP of saturated paste extract. They are illustrated in Figure 3.8.


General classification of salt affected soils.

Figure 3.8. General classification of salt affected soils.


Characteristics of Saline Soils

(Small, Growing Areas Affected)

Naturally developed saline soils usually represent only small areas of a field. Often these are low-lying parts of the field that may have poor internal soil drainage. Other small areas occur on slopes where erosion has exposed saline or alkali subsoil. Because low areas frequently are wet when the rest of the field is dry enough for cultivation, these small areas frequently are cultivated when the soil is too wet. This results in the soil becoming compacted in and around the area. Water does not move easily through the compacted soil so more water evaporates, leaving salts from the water to accumulate. As a result, the affected area increases with time.


Poor Yield

Crop production usually is less than normal in salt affected areas. Yield reduction is greatest in years of less than normal rainfall or when water stress has been a yield limiting factor. Salts tie up much of the water in the soil and prevent plants from absorbing it. Seedlings are the most sensitive to water stress and crop stand is reduced because of seedling death and poor yield results.


White Surface Crust

As water evaporates from saline soils, salts in the water are left behind to accumulate on the soil surface. Salts are light colored and when accumulation has continued for several days they form a very thin white film on the soil surface. During hot, dry weather, the light film will show up first along edges of the salt problem areas. The center of these areas usually has the most salt and will dry out last.


Good Soil Tilth

Saline soils generally have excellent physical conditions throughout the tillage depth. This is caused by salts effectively neutralizing the negative charge of clay particles, allowing them to attach to one another. When these soils are not too wet, the soil is friable, mellow and easily tilled.


High Soil Fertility

Soil that has been saline for several years usually will be very fertile, and high nitrogen, phosphorus and potassium soil test values are often a clue of a problem salty soil. These nutrients build up in salty areas when there is little crop nutrient removal and the area is fertilized each year. Soil pH does not change in relation to salt content and it cannot be used as an indicator.


Characteristics of Alkali Soils

Except as noted, alkali soils have characteristics similar to saline soils. For this reason, one problem soil may be confused with another. Their differences, however, are important to note as they relate to correcting the problem soils.


Poor Soil Tilth

The excess sodium in alkali soils does not allow soil particles to easily attach to one another. As a result, alkali soil dispersed and not friable or mellow like saline soil. Instead, alkali soil is slick-spot soil that is greasy when wet, especially if it is fine textured, and often very hard when dry. This poor physical condition makes these soils difficult to manage. They often are either too wet or too dry for tillage. Poor seed germination and stand establishment are common because good seedbed preparation is seldom accomplished. As a result, yields usually are lower than the rest of the field and fertility may build up.


Dark- or Light-Colored Surface

Soil colloids floating in the soil water are left as a thin film on the surface after water evaporates. The surface color will be darker than the rest of the field (black-alkali) when the particles are mainly humus since humic acid dissolves in alkali solution and lighter (white-alkali) when the particles are mainly clay and salts. The salts show up as a film when the surface dries.


Droughty Water

Large pores or channels in the soil which allow water entry and penetration become plugged with dispersed clay and humus. As a result, the subsoil may be very dry even though water is ponded on the surface. Plants that become established often suffer water stress and may eventually die from lack of water and/or oxygen.



In many instances, saline soils and alkali soils can be reclaimed by following a definite series of management steps designed to leach or wash out the salts or sodium. The order and description of these steps follows.


Verify Problem

The first step to solving the problem is clearly identifying it. This is best done by having the soil tested. Suspected areas should be sampled separate from the rest of the field. It is best to sample during a dry period of the growing season when affected areas of the field can easily be identified by poor crop growth. Samples should be taken at least one week from the last rain or irrigation and only the top three inches of soil should be sampled. Several small samples of the affected area should be combined in a plastic bucket and mixed to get a good sample.


About one pint of soil is required for the test which is done by the OSU Soil, Water and Forage Analytical Laboratory. Samples should be submitted through your County Extension Office requesting a salinity management test. Testing takes about a week and a small fee is charged to cover costs. This test will identify the type and severity of the problem.


Identify Cause

Whenever possible, it is important to find out what has caused the problem soil to develop. Knowing the cause can help in modifying the remaining reclamation practices and sometimes provide a clue as to how long it may take to complete the reclamation. The four most common causes of saline and alkali soils in Oklahoma are


  1. naturally poor drainage,
  2. poor irrigation water,
  3. brine spills and
  4. exposure of saline or alkali subsoil due to erosion.

Poorly drained soils are simply soils which water does not easily penetrate. This condition may be a result of the soil having a high clay content, having a water table near the surface (within 10 feet) or existing in a low-lying area of the field. In the last situation, normally adequate internal drainage may not be able to handle runoff from the surrounding area. In some instances, internal soil drainage is reduced greatly as a result of compacting the surface soil.


Use of poor-quality irrigation water may cause problem soils to develop if special precautions are not taken. The problem develops most rapidly during extremely dry years when evaporation and the amount of irrigation are high. Internal soil drainage also may be a contributing factor.


Problem soils sometimes develop seemingly overnight when brine solutions associated with oil- and gas-well activities spill onto the soil. Depending on the amount of brine solution spilled and the size of the area, the problem may be slight or very severe. Whenever the source of salt or sodium causing the problem is the result of addition from runoff, seeps, irrigation water or spilled brine, it is important to eliminate that source as soon as possible.


Improve Internal Soil Drainage

There are no chemicals or soil amendments that can be added to the soil to tie up or somehow inactivate soluble salts or sodium. Hence, the only way of lowering their concentration in the soil is to remove them. This can only be done by leaching (washing out) the salt or sodium downward out of the root zone. In order for this to happen, internal drainage must be good so water can easily pass through the soil.


There are a number of ways internal drainage can be improved. Most are expensive, but when the problem is severe many will pay for themselves with time. Tile drains and open ditches are effective for removing subsoil water that accumulates due to a restrictive layer such as compacted clay or bed rock. Compacted soil layers near the surface can be broken up by subsoiling. This is effective only if done when the soil is dry enough to have a shattering effect and at best provides only temporary benefit.


Problem soils which have developed from use of poor irrigation water or brine spills may already have good internal soil drainage.


Add Organic Matter

Once internal drainage has been assured, the next important step is to improve water movement into the soil. Incorporating 20-30 tons per acre of organic matter into the top six inches of soil creates large pores or channels for water to enter. Even rainfall from intense storms is more effective because there is less runoff. In addition to improving water movement into the soil, the large pores lessen the capillary or wick-like upward water movement during dry periods. Any coarse organic material such as barnyard manure, straw, rotted hay or crop residue is suitable.


Add Gypsum to Slick Spots

Up to this point the reclamation practices are the same for both saline and alkali soils. In either situation, leaching is critical to remove salt or sodium. However, since high amounts of sodium absorbed to the soil are the cause of alkali problems, sodium must be loosened from the soil before it can be leached out. Gypsum is the most effective soil amendment for removing sodium from the soil particles. Gypsum is a slightly soluble salt of calcium sulfate. This means gypsum will slowly react in the soil, but for a long time. The reaction is illustrated in Figure 3.9.



Alkali soil reacting with gypsum to form normal soil.

Figure 3.9: Alkali soil reacting with gypsum to form normal soil.



Gypsum applications are needed when the exchangeable sodium percentage, ESP, approaches 15 percent. Calcium ions (Ca2+) in gypsum replace sodium ions (Na+) on the colloids which results in improved soil physical conditions. The amount of gypsum required will vary widely depending upon the percentage of exchangeable sodium and the soil texture, as determined by the soil test. This relationship is shown in Table 3.4.


When the required amount of gypsum exceeds 5 tons per acre, the rate should be split into two or more applications of no more than 5 tons at one time. Successive applications should not be made until time has allowed for some leaching to occur, and the need has been verified by a second soil test. The gypsum should be incorporated only to a depth of about 1 to 2 inches, which is enough to mix it well with the surface soil and keep it from blowing away. However, if the soil was contaminated by brine spill, the gypsum needs to be mixed to 6 to 7 inches deep to create a favorable rooting zone.


Leach Soil

Leaching (or washing out) the soil is essential to reduce the amount of salts or sodium in the soil. In order for this leaching process to occur, water must enter the soil in excess of what is used by growing crops and lost by evaporation. How fast and to what extent the reclamation is successful will depend on how much good quality water passes through the soil in a given period of time. The shorter the time interval over which excess water is applied, the more effective that amount of water is in reclamation. For this reason, rainfall is most effective when it falls on soil that is already wet.


Avoid Deep Tillage and Establish Cover

Once the leaching process has been started, deep tillage such as moldboard plowing should be avoided for several years to promote uninterrupted downward movement of the salts. Such tillage will bring salt back up to the soil surface, and leaching will be required again. As soon as the salt level in the soil is low enough, a salt-tolerant crop such as barley or Bermudagrass should be established on the problem area to provide a cover for as much of each growing season as possible. It is especially important to have the cover crop during midsummer when evaporation is high. Adequately fertilized Bermudagrass does a good job of drying the soil. To minimize soil compaction it should be cut for hay instead of pastured. Make sure to keep heavy equipment off the area when it is wet.


Some problem areas may be too salty to establish a cover crop until some salts have been leached. A cover crop can be established when there is no longer a white salty film on the soil surface, following a week or two of dry weather, or when weeds begin to grow.



The final step in reclamation is simply to wait for the previous practices to work. Except for brine spills, these problem soils developed over a period of several years. Reclamation may not take as long, but depending on how well reclamation practices can be carried out, may take one or more years.



Learn to Live With It

The key to successful reclamation is good internal soil drainage. If salts or sodium cannot be leached out, the soil cannot be reclaimed by conventional methods. However, most soils have some internal soil drainage, and although drainage may not be good, over several years time it may be sufficient to lower the salt concentration to near normal. During this time it will be important to practice some of the same steps outlined above. Especially important are the following:


  1. Avoid excessive fertilization.
  2. Avoid traffic on field when wet.
  3. Apply gypsum to slick spots.
  4. Establish a cover crop.
  5. Maintain a high level of crop residue.
  6. Be patient!

Depending on the severity of the problem it may be necessary to select a different crop than has been grown in the past. A list of crops and their relative tolerance to salt is provided in Table 3.5.


Table 3.5: The relative salt tolerance of crops.*



Moderately Tolerant
In increasing order of tolerance






7,800-10,400 ppm


Sugar beet

Barley (grain)s


3,900-7,800 ppm




Grain  sorghum

Oats (grain)

Wheat (grain)

Rye (grain)

2600 ppm

Field beans


7,800-11,700 ppm


Birdsfoot trefoil

Barley (hay)

Rescue grass



SaltgrassAlkali sacaton    

2,600-7,800 ppm

Smooth bromegrass


Blue grama

Oats (hay)

Wheat (hay)
Rye (hay)

Perennial ryegrass

Yellow sweet

cloverWhite sweetclover

1,300-2,000 ppm

Ladino clover

Red clover

White Dutch clover



6,500-7,800 ppm




Garden beets

2,600-6,500 ppm






Bell pepper

Sweet potato & yam


Sweet corn






1,950-2,600 ppm

Green beans












 * Salt tolerance values at which 50 percent yield reduction may be expected compared to nonsaline conditions. Salt concentrations are from a soil saturated paste extract.


 Chapter 4. Determining Fertilizer Needs

 Determining fertilizer and lime needs for selected fields and crops are critical management decisions that often mean the difference between profit and loss for farmers. Applying too little fertilizer or lime when deficiencies exist hurts yield and profit potential. Too much fertilizer reduces nutrient use efficiency, cutting into profits and in some cases, negatively impacting the environment. In today’s economic and political atmosphere, farmers must be concerned about both effects.


At one time, determining fertilizer and lime requirements of Oklahoma crops was simple. If a fertilizer contained phosphate, it was good because almost all Oklahoma soils were low in phosphorus. Because of this, in the early days of fertilizer use, 10-20-10 or 19-19-19 was an effective fertilizer that gained popular use. This thinking no longer applies. Many soils have been fertilized with this practice for many years, increasing soil fertility much above native levels. In other soils, continuous cropping has decreased soil pH values to yield-robbing levels or depleted once abundant supplies of nutrients. Farmers can no longer afford to guess about their fertilizer and lime needs. The fertility levels of each field must be known in order to best manage the entire farm.


There are three approaches to determining fertilizer needs: (1) soil testing, (2) scouting for nutrient deficiency symptoms, and (3) plant analysis. Soil testing is by far the most successful method. To obtain maximum benefit, it must be done on a regular basis and should therefore be viewed as a routine component of an overall soil fertility program. A soil fertility program can be enhanced by scouting for nutrient deficiency symptoms and by using plant analysis when applicable, but soil testing remains as the foundation.


Use of Soil Testing

Soil testing evolved from an understanding by soil scientists that plants require chemical elements as nutrients. Thirteen of the essential nutrient elements for plants come from the soil. The soil’s nutrient-supplying capacity is a chemical characteristic of the soil, and therefore, is most reliably measured or estimated by chemical tests (i.e., soil testing). The concept of soil testing is not new. Even in ancient times, farmers had a limited understanding of basic soil fertility concepts as can be gathered from the ancient agricultural practices documented in Table 4.1. Modernization of soil fertility principles and the refinement of soil testing began in the mid 1800s with advances continuing to this day (Table 4.2).


Table 4.1: Ancient agricultural practices related to soil testing.

Time Location Agricultural Practice
2500 B.C. Mesopotamia  First recorded writings mentioning soil fertility. Barley yields observed to range from 86 to 300 times that planted depending on the area in which the crop was grown.
900 B.C. Greece Manuring was an agricultural practice known to improve soil productivity. 
300 B.C. Greece Various sources of manure were classified according to their value as a soil amendment. Green manure crops, especially legumes, were also known to enrich the soil.
100 B.C Rome The value of using marl and other liming materials as soil amendments was  recognized.
50 B.C. Rome  Considered to be when the first soil fertility test was developed. Columella recommended using a taste test to measure the degree of acidity and salinity of soils. 


Table 4.2. Modernization of soil testing.

Time Location Event
1842 Germany Justus von Liebig stated his “law of the minimum.”
1843 England J.B. Lawes and J.H. Gilbert established the Rothamsted Experimental Station.
1892 U.S.A. Magruder Plots established by Alexander C. Magruder in Stillwater, Oklahoma.
Late 1800s U.S.A. E.W. Hilgard promoted the use of hydrochloric acid as an extractant for determining fertility status of soils.
1909 Germany E.A. Mitscherlich developed his equation relating growth to the supply of plant  nutrients.
Early 1900s U.S.A. C.G. Hopkins promoted the importance of monitoring changes in soil fertility status to prevent decreases in productivity as a result of nutrient depletion.
1940s and 50s U.S.A. Introduction of new crop varieties and hybrids and increases in the availability and use of fertilizers spurred interest in soil testing as a management tool.
1960’s to present   U.S.A. Evolution of soil testing continues on all fronts as technological advances allow improvements in the areas of analysis, correlation, calibration and interpretation.


Soil testing in Oklahoma first became popular in the 1950s. Soil testing for farmers primarily was performed by county extension agents (now called educators) who operated small laboratories out of their county offices. Samples periodically were analyzed by researchers at the OSU campus to verify their accuracy. In the 1960s, Dr. Billy Tucker, an extension soil fertility specialist, and Dr. Lester Reed, a soil chemist, helped analyze approximately 200 to 300 samples per year for the county agents.


After several years, Dr. Tucker realized advances in research and technology were causing the county soil testing laboratories to become outdated. In order to maintain a quality soil testing/soil fertility program at OSU, a centralized state soil testing laboratory was needed that used standardized methods and interpretations based on statewide research.


The task was easier said than done. Much resistance was met from the county agents, who took pride in their soil testing skills and also saw their laboratories as a means of making contacts with farmers and generating extra income for other Extension programs. After much public and private debate, Dr. Tucker finally convinced the director of Extension and most county agents to support the establishment of a centralized soil testing laboratory on the OSU campus. Since that time (1969), sample activity at the OSU laboratory has grown to approximately 28,000 soil samples per year.


Value of Soil Testing

Soil tests are designed to estimate plant-available fractions of selected nutrients, that is, the portion of a nutrient present in the soil that a plant can take up. Soil fertility tests do not measure total amounts of nutrients in the soil because not all chemical forms of the nutrient can be used by the plant. As a soil test level increases for a particular nutrient, the ability of the soil to supply that nutrient also increases and less fertilizer needs to be added to adequately supply food for the plant.


Much field and laboratory research must be conducted to accurately interpret soil tests so proper amounts of fertilizer are recommended for application. This process is called calibration. During the calibration process, a relationship is established between the soil test value and the amount of fertilizer needed by the plant. Soil tests are calibrated by establishing fertilizer rate experiments on soils with different soil test levels to determine the best fertilizer rate for each level. Once a number of fertilizer experiments have been conducted, the data can be summarized and fertilizer recommendation guides can be developed. Agricultural Experiment Stations provide this information.


Soil Sampling

Producers and fertilizer dealers must remember a good soil sample is obtained by sampling a uniform field area. Avoid sampling “odd-ball” areas. Sample each field separately, as well as dissimilar soil types within the same field. A core or slice from the surface to a depth of 6 inch or plow layer should be taken from 15 to 20 locations in the field and composited into one representative sample to be tested.


Noncultivated fields should be sampled to a depth of six inches, again because this is the effective depth of most treatments and the depth of most root activity. Nutrients from fertilizer, animal manure and lime can be accumulated on the surface if they are surface applied without incorporation. A set of samples from the top two inches will help identify stratification of nutrients and is especially important for pH determination for no-till fields. If nutrient loss in runoff is the main concern, the two-inch sample is better than a six-inch sample because only the surface inch or two is in direct contact with surface runoff.


Special attentions should to be paid when sampling fields where fertilizers are banded. See Fact Sheet PSS-2207 How to Get a Good Soil Sample for details.


Subsoil samples for nitrates are valuable for estimating fertilizer nitrogen carryover. The nitrogen fertilizer rate easily is adjusted to take advantage of “leftover” nitrate. The subsoil test should be taken from 6 to 18 inches. Sample depth should be indicated when submitting subsoil samples for the nitrate test. Subsoil sample analysis can help provide a more reliable estimate of other nutrients that are mobile in the soil, such as boron, sulfur, and chloride.


Soil samples may be submitted to your county OSU Extension office. They will send the samples to the Soil, Water and Forage Analytical Laboratory for testing and then send the results back to you with fertilizer recommendations. Soil samples are analyzed routinely for pH, nitrate nitrogen, plant available phosphorus and potassium, while calcium, magnesium, sulfur (secondary nutrients), zinc, iron and boron (micronutrients) are tested on request. The subsoil is analyzed only for nitrate unless otherwise requested. A number of other tests also are available through the lab.


Preparing for No-till Production Systems

While the decision to switch from conventional tillage systems to no-till production can be challenging in many aspects, several soil components need to be addressed prior to this switch. One of the biggest issues to be addressed is deep profile soil pH. Soil pH issues at depth can drastically limit overall root development into deeper soils, limiting access to potential nutrients and moisture lower in the soil profile. While lime can be applied to a no-till system, the ability for neutralization is limited without incorporation. Therefore, these deeper soil pH issues should be remedied prior to moving into no-till production.


Soil Sampling in No-till Production Systems

Soil sampling between no-till production systems and conventionally tilled production systems can vary drastically or be quite similar, all depending on management of the system and issues to be addressed. One of the most noted soil fertility characteristic of no-till production is nutrient stratification (or the formation of layers of that are non-uniform with depth in the soil). While this historically has been seen as a negative characteristic of no-till production, research has suggested very little impact for most nutrients managed within typical production systems.


The major issue with nutrient stratification in no-till production is soil pH. If soil pH was rectified prior to implementation of no-till production, little short-term issues should arise at depth with pH. The major issue comes at the surface. This especially is true in systems that have had continual surface applications of urea- and/or ammonium-based fertilizers. This fertilizer will undergo a transformation process in the soil that can decrease the overall soil pH.


Since these are issues at the surface of the soil, these should be able to be corrected rather easily. However, the identification of these issues from a traditional 0-to-6-inch soil sample can result in no application when one would be justified. Therefore, a sample of 0 to 6 inches and a 0 to 2 inches sample would be encouraged. These varied depths allow for identification of issues within the traditional soil zone (0 to 6 inches) as well as potential issues due to nutrient stratification (0 to 2 inches). One issue with collection of samples from only a 2-inch section of soil is collecting enough soil to get an adequate sample. Twenty cores typically are suggested for a traditional sample to get a representative sample across the field. However, it always is better to collect too much than not enough. The second thing to be considered is the current recommendations for lime application for correction of soil pH is based on a 0- to 6-inch sample. Since neutralization of soil acidity will not occur at these lower depths, recommendations can overestimate the amount of lime needed. Therefore, it often is recommended to lower the lime application rate by half to a third in no-till for a 0- to 2-inch sample.


The other major difference between soil sampling in no-till systems are production systems that have had banded fertilizer. The primary issue from no-till banded fertilizers is not the no-till nor the banding, but the combination of the two. When producers band fertilizers in a conventionally tilled system, the band does not behave any differently in season. However, without tillage to mix the banded and non-banded fertilizer together, a soil sample collected within these bands can grossly overestimate the concentrations of nutrients in the soil system. This can lead to an under-application of nutrient, which could result in a critical yield loss. Additionally, this issue typically is associated with phosphorus within the soil system but other nutrients, especially non-mobile nutrients, can be a concern as well. For sampling in fields that have had banded fertilizer in the past, the collector needs to ask a couple of questions to achieve a proper soil sample.


First, what crop with what row spacing has been previously planted? If the row spacing is narrower than 12 inches, a normal sampling pattern can be used to collect a proper sample. If the row-spacing is wider, is it known where the previous bands have been placed? Will the successive crops be planted over the previous rows? When planting over the previous row, collection of samples should be focused around these rows. This sampling will provide an indication of the residual fertilizer left from the banding and what nutrients were not taken up by the previous crop. If the previous rows are known (along with the bands) but it is not known what successive crop will not be planted over the previous rows, sampling must be done to estimate residual fertilizer in band and outside of the previous bands. This will involve the collection of soil from both these locations. However, as high potential residual levels of fertilizers still within the band to drastically skew the results, a proper ratio of soil from inside and outside the bands must be collected. For example, if the previous crop was on 30-inch row-spacing, for every one sample collected within the banded zone, 20 samples need to be collected outside. The final scenario is if the previous rows are not known. This can be the most challenging as the collection of soil is essentially blind to where the previous banded rows. The best method to collection is to collect a sample and conducted a paired collect half the distance of the previous row-spacing. For example, if 30-inch row spacing was previously used collect a sample from a location and collect a paired sample 15 inches from the previous sample in the direction thought to be across rows. These paired samples would still be considered a single sample. Therefore, a 20-core sample would consist of 40 individual cores or 20 paired cores. 


Laboratory Soil Tests

A brief description of laboratory tests currently used at the OSU lab follows.



This test measures the active soil acidity or alkalinity. Soils with a pH of 7.0 are neutral soil; pH less than 7 is acid and soil pH values higher than 7.0 are alkaline. Under normal conditions, most plants grow well when soil pH is in the range of 6.0 to 7.5. An application of lime should be considered for most non-legume crops when soil pH is 5.5 or less. Legumes usually grow best when the pH is 6.0 or higher.


Buffer Index

When soil pH is less than 6.3, a buffer index reading is obtained. This value estimates the amount of lime required to correct soil acidity. The buffer index value is not a standard pH reading and means nothing without a calibration table that relates it to the amount of lime to apply. The lower the buffer index, the higher the lime requirement (See Chapter 3 for more details about pH and liming).



The nitrate soil test measures the actual amount of nitrate-nitrogen in the soil available to plants. The nitrogen fertilizer requirement can be determined by subtracting the pounds of nitrate-nitrogen in the soil from the total nitrogen requirement for a selected yield goal.



The phosphorus soil test estimates the amount of available soil phosphorus. The actual amount cannot be measured because of chemical reactions occurring in the soil. The estimated availability is reported as soil test index and a percent sufficiency in the soil. A soil test with 40 percent sufficiency means 40 percent of plant phosphorus needs will be supplied by the soil. The remainder must be provided by adding fertilizer to reach the 100 percent potential yields. If no phosphorus is added, the yield will only be 40 percent of its potential. Much field calibration work must be done to correctly interpret this type of test. The Mehlich-3 procedure is used for extraction of soil phosphorus and potassium in Oklahoma. Other labs may use different procedures. Oklahoma calibration may not be appropriate if soils are tested with a different method.



Like phosphorus soil tests, potassium tests estimate availability and indicate a certain percent sufficiency.


Calcium and Magnesium

These two elements and potassium are referred to as exchangeable cations and are found on the cation exchange sites of the soil. The soil tests measure the exchangeable portion of the cations. Oklahoma research has found that calcium and magnesium additions can increase yields when individual tests are low. Percent of base saturation or ratios of calcium/magnesium, potassium/magnesium, calcium/potassium or calcium/magnesium/potassium have not been useful in depicting deficiencies on most Oklahoma soils.



The sulfur soil test measures the amount of available sulfate-sulfur. The amount found in the soil test can be subtracted from crop requirements based upon a yield goal similar to the approach used for nitrogen. Unlike nitrogen, most soils contain adequate available sulfur for most crops. Additionally, annual sulfur contributions from rainfall are high enough to meet the needs of a 60-bushel wheat crop.


Zinc, Iron and Boron

Availability of these trace or micronutrient elements can be estimated from soil tests. Trace element deficiencies occur only on certain soils and with certain crops. Knowledge of crop needs and soil deficiencies will help determine when trace element tests need to be run.


Soil Test Interpretations

After soil samples have been tested, the results need to be examined to see if they identify nutrient deficiencies in any of the fields. This step is called interpreting the test results. Interpretation can only be done reliably if the soil test has been calibrated by field research. Usually calibration research is on-going at land-grant universities, such as OSU, and has its best application for soils in that state. The calibration should identify the deficiency and estimate its severity.


OSU interpretations are based on research calibration tables published in Extension Fact Sheet PSS-2225. The same calibration tables are included here as a reference (Tables 4.3 to 4.10). The tables in PS


Primary Nutrient Interpretations

Soil test interpretations for nitrogen, phosphorus and potassium are presented in Tables 4.3-4.6. Fertilizer requirements for common Oklahoma crops and forages can be determined from these tables. Nitrogen requirements are based on yield goal, while phosphorus and potassium requirements are based on soil test values and their corresponding sufficiency levels.


Interpretations of soil test reports obtained from OSU are automatically generated by computer using data from these calibration tables. An example report is shown in Figure 4.1. The report lists the name and address of the sender at the top and presents the sample identification numbers and soil test results in designated boxes below. The soil test interpretation is printed in an area underneath the test results. If no cropping information is provided with a soil sample, then no computer interpretation is generated and fertilizer requirements must be determined by use of the calibration tables in Fact Sheet PSS-2225 or an interactive program on the lab’s website ( A yield goal also is needed to make nitrogen recommendation except for lawn and gardens. 


 Example soil test report from the OSU Soil, Water and Forage Analytical Laboratory.

Figure 4.1: Example soil test report from the OSU Soil, Water and Forage Analytical Laboratory


In the example report, wheat was selected as the crop and 50 bushels per acre was selected as the yield goal. Both selections are listed at the beginning of the interpretation. The pH of the sample was 6.5 which is satisfactory for wheat, therefore no lime was required.


The nitrate test for this sample showed 20 pounds nitrogen per acre in the soil. According to the calibration tables (Table 4.3), 50 bu/acre of wheat requires 100 pounds per acre of nitrogen, Subtracting 20 from 100 results in a deficiency of 80 pounds nitrogen per acre which must be supplied using nitrogen fertilizer.


The phosphorus test index for this sample was 10. The calibration table for wheat (Table 4.3) shows that a phosphorus index of 10 corresponds to a sufficiency level of 45 percent. The corresponding P2 O5 fertilizer requirement to offset this insufficiency is shown on the report or can be read directly from the calibration table as 60 pounds per acre. This rate of P2 O5 must be applied annually to prevent phosphorus deficiency until another soil test is performed.


The potassium test index for this sample was 100. This value is not listed in the potassium calibration table for wheat, so the fertilizer requirement must be estimated using the requirements recommended for the index values, 75 and 125 (Table 4.3). Since 100 is halfway between 75 and 125, the potassium index of 100 corresponds to a sufficiency level of approximately 75 percent (halfway between 70 and 80) and a K2 O requirement of approximately 45 pounds per acre (halfway between 50 and 40). The computer calculated this value and listed the potassium fertilizer requirement as a “75 percent sufficiency, 45 pounds per acre K2 O.” This rate of K2 O, like P2 O5 , must be applied annually to prevent potassium deficiency until another soil test is performed.


Secondary and Micro-nutrient Interpretations


Calcium deficiency has not been observed in any crop in Oklahoma. Gypsum is sometimes applied over the pegging zone of peanuts during early bloom stage to improve quality. Appropriate rates are listed in Table 4.7.


Table 4.3. Primary nutrient soil test calibration tables for small grains and row crops.


Yield Goal(bu/A)  N (lbs/A) Yield Goal (lbs/A) N (lbs/A) Yield Goal(bu/A) N (lbs/A) Yield Goal(bales/a) N (lbs/A) Yield Goal N (lbs/A)
 Wheat Barley Oats                  
15 20 25 30 2000 30 40 40 1 50 1000 50
20 25 35 40 2500 40 50 50 1.5 75 1500 75
30 35 55 60 3000 50 60 60 2 100 2000 100
40 50 70 80 4000 70 85 85 2.5 125 2500 125
50 60 90 100 4500 85 100 110 3 150 3000 150
60 75 105 125 5000 100 120 130 3.5 175 3500 175
70 90 125 155 7000 160 160 190 >3.5 175    
80 100 140 185 8000 195 180 215        
100 125 175 240 9000 230 200 240        





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