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Soil Test Interpretations For Vegetable Crops

Soil test interpretation is an important part of nutrient management for crop production. The following tables include soil test interpretations for nitrogen (N), phosphorus (P), and potassium (K) the three most commonly deficient plant nutrients used by vegetable crops. The suggested interpretations are valid for results from the OSU Soil, Water, and Forage Analytical Laboratory, but should not be utilized directly for interpreting results tested with different methods by other soil testing laboratories. Nitrogen requirements are based on crops and yield goals. The actual amount of N needed is the N requirement minus soil nitrate-N. Requirements for P and K are based on soil test values. Keep in mind that vegetable crop yield goals are based on the potential yield achievable in the state. Actual yield may vary due to other conditions.

 

Determining Fertilizer Needs by Soil Testing

The starting point for vegetable crop fertilization is determining the residual fertility available in the soil by soil testing.  Soil tests are only as good as the soil sample that represents the field and proper sampling is a key part of obtaining reliable soil test results.  Extension Fact Sheet PSS-2207 explains proper soil sampling techniques.

 

County Extension offices throughout Oklahoma have guidelines and materials for collecting and sending soil samples to the OSU Soil Testing Laboratory.  The standard soil test includes soil reaction (pH), and levels of plant available N, P, and K.  Micronutrients, secondary nutrients, soil organic matter, soil salinity and soil textural classification can also be determined by the soil testing lab for additional charges.

 

Soil pH

Soil pH is a measure of the acidity/alkalinity of the soil. The pH ranges from 0 to 14 with 7.0 being neutral, less than 7.0 is considered acidic and greater than 7.0 is alkaline. The main influence of soil pH is the availability of nutrients in the soil. A majority of plant nutrients are most available in mildly acidic soils ranging from 6.0 to 7.0. Agricultural soils have a tendency to become more acidic over time. Excess acidity is corrected through the application of lime to the soil, with each soil varying in the amount of lime required to increase soil pH. Buffer index (BI) is a measure of the soil’s ability to resist changes in pH and will be listed on the OSU soil test report if pH is less than 6.2. Lime is applied according to the Buffer Index (Table 1) which is further explained in fact sheet PSS-2229. Vegetable crops differ in their ability to take up nutrients at a given pH. Matching crops to soils with the appropriate pH is important because vegetable crops have different tolerance levels to soil pH (Table 2).

 

Table 1. Tons of ECCE* lime required to raise soil pH of a 6-7 inch furrow slice to pH 6.5.

*Effective calcium carbonate equivalent guaranteed by lime supplier.

Tons lime material needed = Tons ECCE required/percent ECCE x 100.

 

Table 2. Soil pH preference of selected vegetable crops*.

  pH 6.0-6.8 pH 5.8-6.8** pH 5.5-6.8
  Asparagus Bean, snap Bean, snap
  Beet Bean, Lima Rhubarb
  Broccoli Brussels sprouts Sweet potato
  Cabbage Carrot Watermelon
  Cantaloupe Collard  
  Cauliflower Cucumber  
  Celery Eggplant  
  Chinese cabbage Garlic  
  Lettuce Kale  
  Okra Kohlrabi  
  Onion Mustard  
  Spinach Parsley  
    Pea  
    Pepper  
    Pumpkin  
    Radish  
    Squash  
    Sweet corn  
    Tomato  
    Turnip  

* Adapted from Knott’s handbook for vegetable growers.

** Although crops can tolerate lower pH, they will be more successfully grown at soil pH’s between 6.0-6.8.

 

Vegetable Fertilizer Recommendations

Nitrogen, phosphorous, and potassium fertilizer recommendations are given on the soil test report. The recommendations are in pounds of N, P2O5, and K2O per acre as listed on a fertilizer analysis. Soil test interpretations for various vegetable crops are given in Tables 3 through 7. Table 8 provides timing suggestions for nitrogen fertilization of various vegetable crops. Crop requirements for nitrogen will vary due to differences in plant populations i.e. where higher population plantings will require more nitrogen than lower plant populations. Phosphorus and potassium are generally applied pre-plant or at planting.

 

Table 3. Primary Nutrient Recommendations for Brassica Head Crops Including: Broccoli, Brussels sprouts, Cabbage, and Cauliflower; Leafy Greens Crops including: Lettuce, Cilantro, Collard, Kale, Mustard, Spinach, and Turnip.

Nitrogen Requirements (lbs N/Acre)    
N1 Brassica Head Crops Leafy Greens
  175 (lbs/Acre) 120 (lbs/Acre)
Phosphorous Requirements (P2O5 lbs/Acre)    
P soil test Index Brassica Head Crops Leafy Greens3
0 150 150
10 125 125
20 100 100
40 55 55
>65 0 0
Potassium Requirements (lbs K2O/Acre)    
K Soil Test Index Brassica Head Crops Leafy Greens
0 150 150
75 125 125
125 100 100
200 50 50
>250 0 0

1 The amount of fertilizer N needed equals the N requirement minus the available N from a soil test. Recommended nitrogen should be split into several applications, such as pre-plant and side-dress applications.

 

Table 4. Primary Nutrient Recommendations for Asparagus, Okra, Sweet Corn, and Bulb crops including: Garlic, and Onion.

Nitrogen Requirements (lbs/Acre)          
N* Asparagus** Okra Sweet Corn Garlic Onion
  70-80 80 200 175 150
Phosphorous Requirements (P2O5 lbs/Acre)          
P Soil Test index Asparagus Okra Sweet Corn Garlic Onion
0 200 150 150 150 150
10 170 125 125 125 125
20 140 100 100 100 100
40 80 55 55 55 55
>65 0 0 0 0 0
Potassium Requirements (lbs K2O/Acre)          
K Soil Test index Asparagus Okra Sweet Corn Garlic Onion
0 200 150 150 150 150
75 155 125 125 125 125
125 120 100 100 100 100
200 50 50 50 50 50
>250 0 0 0 0 0

 * The amount of fertilizer N needed equals the N requirement minus the available N from a soil test. Recommended nitrogen should be split into several applications, such as pre-plant and side-dress applications.

 

** Asparagus rates are based on established plantings, for establishment purposes phosphorous and potassium rates will need to be increased 1.3 to 2 times while nitrogen rates will be 50% of that recommended for established plantings.

 

Table 5. Primary Nutrient Recommendations for Cucurbit Crops including: Cucumber, Melons, Pumpkin, Squash, and Watermelon.

Nitrogen Requirements (lbs/Acre)          
N* Cucumber Melons Pumpkin Squash Watermelon
  150 150 150 150 150
Phosphorous Requirements (P2O5 lbs/Acre)          
P Soil Test Index Cucumber Melons Pumpkin Squash Watermelon
0 1200 150 120 120 150
10 100 125 100 100 125
20 80 100 80 80 100
40 45 55 45 45 55
>65 0 0 0 0 0
Potassium Requirements (lbs K2O/Acre)          
K Soil Test index Cucumber Melons Pumpkin Squash Watermelon
0 120 150 120 120 150
75 100 125 100 100 125
125 80 100 80 80 100
200 40 50 40 40 50
>250 0 0 0 0 0

 *The amount of fertilizer N needed equals the N requirement minus the available N from a soil test. Recommended nitrogen should be split into several applications, such as pre-plant and side-dress applications.

 

Table 6. Primary Nutrient Recommendations for Fruiting Vegetable Crops including: Eggplant, Pepper, Tomato, and Legume Vegetable Crops including: Bean, Cowpea, and Garden Pea.

Nitrogen Requirements (lbs/Acre)            
N* Eggplant Pepper Tomato Bean Cowpea Garden Pea
  100 100 100 75 50 60
Phosphorous Requirements (P2O5 lbs/Acre)            
P Soil Test index Eggplant Pepper Tomato Bean Cowpea Garden Pea
0 150 150 150 120 80 80
10 125 125 125 100 70 70
20 100 100 100 80 55 55
40 55 55 55 45 30 30
>65 0 0 0 0 0 0
Potassium Requirements (lbs K2O/Acre)            
K Soil Test index Eggplant Pepper Tomato Bean Cowpea Garden Pea
0 150 200 225 120 80 80
75 125 155 165 100 70 70
125 100 120 120 80 55 55
200 50 50 50 40 25 25
>250 0 0 0 0 0 0

 

*The amount of fertilizer N needed equals the N requirement minus the available N from a soil test. Recommended nitrogen should be split into several applications, such as pre-plant and side-dress applications.

 

Table 7. Primary Nutrient Recommendations for Root and Tuber Vegetable Crops including: Beet, Carrot, Potato, Radish, Sweet Potato, Turnip.

Nitrogen Requirements (lbs/Acre)            
N* Beet Carrot Potato Radish Sweet Potato Turnip
  120 175 200 80 60 80
Phosphorous Requirements (P2O5 lbs/Acre)            
P Soil Test index Beet Carrot Potato Radish Sweet Potato Turnip
0 120 150 120 120 120 150
10 100 125 100 100 100 125
20 80 100 80 80 80 100
40 45 55 45 45 45 55
>65 0 0 0 0 0 0
Potassium Requirements (lbs K2O/Acre)            
K Soil Test index Beet Carrot Potato Radish Sweet Potato Turnip
0 120 150 300 120 120 150
75 100 125 250 100 100 125
125 80 100 200 80 80 100
200 40 50 100 40 40 50
>250 0 0 0 0 0 0

 

*The amount of fertilizer N needed equals the N requirement minus the available N from a soil test. Recommended nitrogen should be split into several applications, such as pre-plant and side-dress applications.

 

Table 8. Timing of nitrogen fertilization for vegetable crops.

Crop Total N lbs/acre* N preplant lbs/acre 1st N side/topdress lbs/acre 2nd N side/topdress lbs/acre 3rd N side/topdress lbs/acre
Asparagus** 70-80 50 30-70 NA*** NA
Bean 75 25 50 NA NA
Beet 120 55 65 NA NA
Broccoli 175 40 65 70 NA
Brussels sprouts 175 40 65 70 NA
Cabbage 175 40 65 70 NA
Carrot 175 50 65 60 NA
Cauliflower 175 40 65 70 NA
Cilantro 120 70 50 NA NA
Collard 120 70 50 NA NA
Cowpea 50 30 20 NA NA
Cucumber 150 50 50 50 NA
Eggplant 100 25 25 25 25
Garden pea 60 30 30 NA NA
Garlic 175 50 65 60 NA
Kale 120 70 50 NA NA
Lettuce 120 70 50 NA NA
Melons 125 50 50 25 NA
Mustard 120 70 50 NA NA
Okra 80 40 20 20 NA
Onion 150 20 55 45 NA
Pepper 100 25 25 25 25
Potato 200 55 75 70 NA
Pumpkin 150 50 60 40 NA
Radish 80 40 40 NA NA
Spinach 120 70 50 NA NA
Squash 150 50 60 40 NA
Sweet corn 200 40 80 80 NA
Sweet potato 60 30 30 NA NA
Tomato 100 25 25 25 25
Turnip (roots) 80 40 40 NA NA
Turnip (tops) 120 70 50 NA NA
Watermelon 150 50 60 40 NA

 * Attention should be paid to nitrogen applications in light of the fact that nitrogen is easily leached from soil by excessive rainfall.

** Asparagus is a perennial crop and fertility rates will vary according to establishment year vs. years following establishment.  Establishment year apply 50 lbs N preplant and 30 lbs N as a sidedress 2 months after establishment.  Years following establishment apply 70 lbs N near end of harvest season.

***NA = Not Applicable.

 

 

Lynn Brandenberger
Extension/Research Vegetable Crops

 

Hailin Zhang
Director, Soil, Water, and Forage Analytical Laboratory

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