Why Soil pH Matters?

High soil pH can negatively impact lawns and gardens primarily by reducing nutrient availability rather than by directly harming plant roots. As pH rises above the optimal range (generally 6.0–7.0 for most turfgrasses and vegetables), several essential nutrients become less soluble and therefore less available for plant uptake. Iron, manganese, zinc and phosphorus are particularly affected under alkaline conditions. This often results in visible deficiency symptoms such as iron chlorosis (yellowing between leaf veins), reduced growth, thinning turf and poor overall plant vigor. Even when nutrients are present in adequate amounts, high pH can limit their accessibility, leading to decreased performance and aesthetic quality in lawns and reduced productivity in garden crops.

In addition, some plant species are naturally adapted to acidic soils and perform poorly when grown in alkaline conditions. Acid-loving ornamentals such as azalea, rhododendron, camellia and blueberry require lower soil pH for proper nutrient uptake and root function. Pin oak and certain maple species commonly develop iron chlorosis when planted in soils with high pH. Even turfgrass species such as centipede grass perform best in slightly acidic conditions and may decline as soil pH approaches neutral or alkaline levels. Matching plant selection to soil pH — or adjusting soil pH where practical — is therefore an important component of successful lawn and garden management.

For more information, see fact sheet HLA-6468: Understanding your lawn and garden soil test.

How Lawn and Garden Soils Become Alkaline

Lawn and garden soils become alkaline (high pH) through both natural soil-forming processes and management influences. In much of the central and western United States, including Oklahoma, many soils developed from calcareous parent materials that contain calcium carbonate (lime). As soils weather, these carbonates remain in the profile and act as a buffering agent, resisting acidification and maintaining a pH above 7.0. Limited rainfall in semi-arid and subhumid regions also slows the natural leaching of basic cations such as calcium, magnesium, potassium and sodium, allowing them to accumulate and maintain alkaline conditions over time.

In managed landscapes, irrigation water is often the primary contributor to increasing soil pH. Many municipal and well water sources contain dissolved bicarbonates and carbonates. When applied repeatedly through lawn irrigation systems, these dissolved minerals are left behind as water evaporates. Over time, bicarbonates neutralize soil acidity and increase soil pH, particularly in the surface layer where irrigation water is most concentrated. In areas with irrigation water of moderate to high alkalinity, gradual increases in soil pH are common, even if the native soil was only slightly alkaline to begin with.

Because alkaline soils are strongly buffered — especially where free lime is present — once pH rises, it can be difficult to reverse without deliberate acidification practices guided by soil testing. If you have concerns about your water quality, it is advised to submit a sample to the Oklahoma State University Soil Testing Lab (SWFAL) for analysis of pH, bicarbonate, carbonate and total dissolved solids (TDS).

For more information, see fact sheet PSS-2401: Classification of irrigation water quality.

Lowering Soil pH: Elemental Sulfur and Aluminum Sulfate (Alum)

Soil pH should not be adjusted without first confirming the need through a laboratory soil test. A soil test determines the current pH and provides recommendations for amendments to reach the target pH for lawns or garden crops. In Oklahoma, soil texture and initial pH strongly influence the amount of material required to change pH, making laboratory guidance essential. Applying acidifying materials without a soil test can result in over-application, plant injury or unnecessary expense.

Two materials are commonly recommended for lowering soil pH in lawn and garden settings: elemental sulfur and aluminum sulfate. Although both are effective, they differ in how they react in soil and how quickly pH changes.

Elemental sulfur is the most widely recommended amendment for long-term pH adjustment. After application, soil microorganisms oxidize sulfur to sulfuric acid, releasing hydrogen ions that reduce soil pH. Because this reaction is biological, it proceeds gradually and is influenced by soil temperature, moisture, aeration and microbial activity.

Under favorable conditions, a measurable pH change may take several months. While this slower response requires advance planning, it is often desirable. Gradual acidification allows the soil system and plant roots to adjust without the stress that can accompany sudden chemical change. For new garden beds or when preparing soil ahead of planting, elemental sulfur provides a controlled and durable adjustment. Incorporating sulfur into the upper 4–6 inches of soil improves contact with microbes and enhances effectiveness.

Aluminum sulfate lowers soil pH more rapidly because it reacts chemically when dissolved in soil water. The acidity is released soon after application, and soil pH may decrease within days to weeks. This faster response can be useful when a more immediate adjustment is needed, such as in established ornamental beds showing nutrient-deficiency symptoms associated with high pH. However, aluminum sulfate must be used carefully. Excessive application can raise soluble aluminum levels to the point of injuring roots, particularly in poorly drained soils. It also generally requires larger quantities of material per unit of pH change compared to elemental sulfur.

In most lawn and garden situations, elemental sulfur is preferred for broad area applications and long-term management, while aluminum sulfate may be considered for smaller areas or situations where quicker change is needed. Regardless of the material chosen, applications should follow soil test recommendations, and pH should be re-evaluated several months after treatment to monitor progress.

Lowering soil pH is not an instantaneous correction but a management practice. When properly planned and based on soil test data, acidification can improve nutrient availability, turf performance and garden productivity while avoiding unnecessary input costs or plant stress.

Depth Assumptions and Maximum Single-application Rates

The amendment rates in the tables assume the material is mixed into the top six inches of soil. If the amendment is incorporated more shallowly or surface-applied to the existing lawn, less total soil is treated, and less amendment is needed to produce the same pH change in that layer. In the case of surface application, assume the amendment will only impact the surface 3" of soil; therefore, reduce the application rate by 50%.

A second consideration is avoiding large one-time applications to actively growing areas. Lowering soil pH too quickly can stress plants, and very high rates of acidifying materials can create localized “hot spots” of acidity or salts. In addition, aluminum sulfate produces a rapid pH shift and can increase soluble aluminum in the root zone if overapplied. For these reasons, large pH corrections should be made gradually. As a general guideline, do not attempt to lower soil pH by more than one pH unit in a single season, and when table rates exceed that amount, split the total into two or more applications separated by time (for example, fall and spring), with follow-up soil testing to confirm progress. For lawns and established landscapes, conservative single-application limits are recommended; when higher totals are needed, multiple smaller applications are safer and typically more effective.

Maintaining Soil pH After Correction

Once a target soil pH has been achieved, which is one of the challenges in management, maintaining it over time requires ongoing monitoring and periodic re-treatment. Irrigation water in many Oklahoma landscapes contains elevated bicarbonate levels, which tend to drive soil pH back toward alkalinity over time. Homeowners and growers should plan to retest soil pH every two to three years in areas where soil pH has been adjusted and monitor every re-treatment.

Take Home Messages

• Soil pH strongly influences nutrient availability. When soil pH rises above about 7.0, nutrients such as iron, manganese, zinc and phosphorus become less available, leading to chlorosis, reduced growth, and poor turf or garden performance.
• Many Oklahoma soils naturally become alkaline, and irrigation can further increase alkalinity. Calcareous parent materials and irrigation water containing bicarbonates can gradually increase soil pH in lawns and gardens over time.
• Always confirm the need for acidification with a soil test. Soil texture and starting pH determine how much amendment is required, and applying materials without testing can waste money or injure plants.
• Elemental sulfur provides gradual long-term pH reduction, while aluminum sulfate works faster. Sulfur relies on microbial conversion and changes soil pH slowly, whereas aluminum sulfate produces a quicker chemical reaction but must be applied carefully.
• Lowering soil pH is a management process, not a one-time fix. Amendments should be applied gradually, and soil should be retested every few years to monitor pH and maintain desired conditions.

References

Colorado State University Extension. (2014). Changing soil pH (CMG Garden Notes No. 222). Colorado State University, Fort Collins, CO. https://extension.colostate.edu/topic-areas/yard-garden/changing-soil-ph-7-222/

Iowa State University Extension and Outreach. (n.d.). How to change your soil’s pH. Yard and Garden Resources. Iowa State University, Ames, IA. https://yardandgarden.extension.iastate.edu/how-to/how-change-your-soils-ph

Ohio State University Extension. (2012). Soil acidification: How to lower soil pH (AGF-507). Ohioline, The Ohio State University, Columbus, OH. https://ohioline.osu.edu/factsheet/agf-507

Oregon State University Extension Service. (2011). Acidifying soil for blueberries and ornamental plants in the yard and garden (EC 1585). Oregon State University, Corvallis, OR. https://extension.oregonstate.edu/sites/extd8/files/documents/ec1585.pdf

University of Wisconsin–Extension. (n.d.). Reducing soil pH. Garden Facts. Division of Extension, University of Wisconsin–Madison, Madison, WI. https://hort.extension.wisc.edu/articles/reducing-soil-ph/

West Virginia University Extension Service. (n.d.). Lowering soil pH. West Virginia University, Morgantown, WV. https://extension.wvu.edu/agriculture/horticulture/lowering-soil-ph