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*.

* 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.

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.

 * 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.

 *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.

 

*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.

 

*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.

 * 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