Soil Test Interpretations For Vegetable Crops

March 2017


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.

Buffer IndexLime Required pH 6.8
Over 7.1None
7.10.5
70.7
6.91
6.81.2
6.71.4
6.61.9
6.52.5
6.43.1
6.33.7
6.24.2

*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 6.0-6.8pH 5.8-6.8**pH 5.5-6.8
AsparagusBean, snapPotato
BeetBean, LimaRhubarb
BroccoliBrussels sproutsSweet potato
CabbageCarrotWatermelon
CantaloupeCollard
CauliflowerCucumber
CeleryEggplant
Chinese cabbageGarlic
LettuceKale
OkraKohlrabi
OnionMustard
SpinachParsley
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)  
N1Brassica Head Crops Leafy Greens
175 (lbs/Acre)120 (lbs/Acre)
Phosphorous Requirements (P2O5 lbs/Acre)
P soil test IndexBrassica Head Crops Leafy Greens3
0150150
10125125
20100100
405555
>6500
Potassium Requirements (lbs K2O/Acre)
K Soil Test IndexBrassica Head CropsLeafy Greens
0150150
75125125
125100100
2005050
>25000

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**OkraSweet CornGarlicOnion
70-8080200175150
Phosphorous Requirements (P2O5 lbs/Acre)
P Soil Test indexAsparagusOkraSweet CornGarlicOnion
0200150150150150
10170125125125125
20140100100100100
408055555555
>6500000
Potassium Requirements (lbs K2O/Acre)
K Soil Test indexAsparagusOkraSweet CornGarlicOnion
0200150150150150
75155125125125125
125120100100100100
2005050505050
>25000000

* 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*CucumberMelonsPumpkinSquashWatermelon
150125150150150
Phosphorous Requirements (P2O5 lbs/Acre)
P Soil Test IndexCucumberMelonsPumpkinSquashWatermelon
0120150120120150
10100125100100125
20801008080100
404555454555
>6500000
Potassium Requirements (lbs K2O/Acre)
K Soil Test indexCucumberMelonsPumpkinSquashWatermelon
0120150120120150
75100125100100125
125801008080100
2004050404050
>25000000

*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*EggplantPepperTomatoBeanCowpeaGarden Pea
100100100755060
Phosphorous Requirements (P2O5 lbs/Acre)
P Soil Test indexEggplantPepperTomatoBeanCowpeaGarden Pea
01501501501208080
101251251251007070
20100100100805555
40555555453030
>65000000
Potassium Requirements (lbs K2O/Acre)
K Soil Test indexEggplantPepperTomatoBeanCowpeaGarden Pea
01502002251208080
751251551651007070
125100120120805555
200505050402525
>250000000

*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*BeetCarrotPotatoRadishSweet PotatoTurnip
120175200806080
Phosphorous Requirements (P2O5 lbs/Acre)
P Soil Test indexBeetCarrotPotatoRadishSweet PotatoTurnip
0120150120120120150
10100125100100100125
2080100808080100
40455545454555
>65000000
Potassium Requirements (lbs K2O/Acre)
K Soil Test indexBeetCarrotPotatoRadishSweet PotatoTurnip
0120150300120120150
75100125250100100125
125801002008080100
2004050100404050
>250000000

*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.

CropTotal N lbs/acre*N preplant lbs/acre1st N side/topdress lbs/acre2nd N side/topdress lbs/acre3rd N side/topdress lbs/acre
Asparagus**70-805030-70NA***NA
Bean752550NANA
Beet1205565NANA
Broccoli175406570NA
Brussels sprouts175406570NA
Cabbage175406570NA
Carrot175506560NA
Cauliflower175406570NA
Cilantro1207050NANA
Collard1207050NANA
Cowpea503020NANA
Cucumber150505050NA
Eggplant10025252525
Garden pea603030NANA
Garlic175506560NA
Kale1207050NANA
Lettuce1207050NANA
Melons125505025NA
Mustard1207050NANA
Okra80402020NA
Onion150505545NA
Pepper10025252525
Potato200557570NA
Pumpkin150506040NA
Radish804040NANA
Spinach1207050NANA
Squash150506040NA
Sweet corn200408080NA
Sweet potato603030NANA
Tomato10025252525
Turnip (roots)804040NANA
Turnip (tops)1207050NANA
Watermelon150506040NA

* 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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