CFAES Give Today
Ohioline

Ohio State University Extension

CFAES

Developing Phosphorus and Potassium Recommendations for Field Crops

AGF-0515
Agriculture and Natural Resources
Date: 
10/18/2022
Greg LaBarge, Professor, Field Specialist, Agronomic Systems, Ohio State University Extension

Land-grant university fertilizer recommendations are determined using field studies to correlate soil test levels to crop yield response. Ohio data was used to develop response curves for phosphorus and potassium in the Tri-State Fertilizer Recommendations for Corn, Soybean Wheat, and Alfalfa (Culman et al. 2020). The publication is a coordinated effort of Michigan State University, The Ohio State University, and Purdue University. This fact sheet shows how to use a soil test result and tables found in the Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa to develop phosphorus (P) and potassium (K) nutrient recommendations. The nutrient recommendations are then developed into a fertilizer plan. This fact sheet does not discuss lime recommendations, but soil pH should be considered first in developing a nutrient plan since the availability of many nutrients is pH dependent. Soil pH and lime recommendation development are covered in the Ohio State University Extension fact sheet, Soil Acidity and Liming for Agronomic Production (Mullen, Lentz, and Watson 2016). For help to understand values found on a soil test report see Interpreting a Soil Test Report (Labarge 2020).

According to Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa, “The tri-state fertilizer recommendations for P and K are based on a build-up and maintenance approach. This framework strives to build soil test levels up to and beyond a critical level, and then maintain these levels over time. The critical level is a key component to this framework. Soil test levels above the critical level are ‘optimal,’ and are unlikely to be responsive to fertilizer application. Soil test levels below the critical level are ‘deficient,’ and are more likely to have a yield response to fertilizer application. The critical level has been determined empirically from the results of hundreds of field trials across the tri-state region (Culman et al. 2020).” For more information on the recommendation framework, refer to the Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa.

Tri-state Recommendation Tables

The tri-state recommendation tables are a quick tool to use in developing P and K recommendations for a variety of row and forage crops. The tables have a consistent layout that follows the tri-state recommendation framework based on soil test values shown on the left-hand side of the chart. The framework defines an “optimal” soil test range. Soil test values that are lower than the “optimal” range are “deficient.” Soil test values higher than the optimal range are “sufficient.”

Figure 1 shows recommendations for corn P:

  • Soil test values in the “optimal” range are shown in a single row. The recommendation shown in this line is a replacement amount of nutrients based on the removal rate for the crop and the yield potential. The optimal soil-test range for corn is 20–40 ppm.Table graphic showing the corn yield, or bushels per acre, potential of a field based on soil test values that are either deficient, optimal, or sufficient, and the corresponding amounts of P2O5 that are recommended to reach the field’s potential yield.
  • When soil test levels are in the “deficient” categories the recommendation shown includes extra fertilizer with the goal to ”buildup” soil test levels to the optimal range in four applications with this increased rate. The buildup portion of the recommendation may not be desirable in all fields. Farmers can default to the “optimal” range recommendation to meet immediate crop needs if buildup is not desired.
  • When soil tests are above the “optimal” range, no fertilizer is needed, as shown in the “sufficient” range.

The use of the table to identify a recommended nutrient rate is straight forward:

  • The crop and the range of yield potentials are shown in the two top rows of the table.
  • The nutrient recommendation is found at the intersection of the soil test value for your field (left-hand side) and the crop yield potential on the top of the table. (i.e., The intersection of a 200-bushel yield potential with a soil test in the “optimal” range of 20–40 is 70 pounds of P2O5 per acre.)

Steps to Develop Phosphorus and Potassium Fertilizer Recommendations

Three essential pieces of information are needed to develop a nutrient recommendation:

  • soil test results
  • nutrient recommendation tables
  • information on the crop to be grown

Table 1 shows example soil test results. Nutrient recommendations from Figure 2 (Table 18 corn phosphorus) and Figure 3 (Table 23 corn potassium) from tables in the Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa publication are provided. The crop to be grown is corn with a 150-bushel yield potential. Yield potential is determined by averaging yields from the last five corn crops and then removing the highest and lowest yield. Table 2 provides the desirable nutrient ranges, blank columns to fill in soil test values from your report, and nutrient recommendations from the tables provided.

Phosphorous (P) is reported as 23 ppm in Table 1. The 23 ppm P value is in the “optimal” range, between 20–40 ppm. Potassium is reported as 100 ppm in Table 1. In addition to the potassium (K) soil test result, cation exchange capacity (CEC) is also used to develop the K recommendation. For this example, Table 1 shows CEC of 10 milliequivalents per 100 grams (meq/g) for CEC. For soil with a CEC of 10 ppm, the “optimal” range is 120–170 ppm. The soil test K of 100 ppm is below the "optimal” range and is “deficient” as defined by the tri-state recommendations.

Table 1. Example Soil Test Results
Table displaying Example Soil Test Results   

Table 1. (PDF Download) Example Soil Test Results

Table 2. Nutrient and Fertilizer Recommendation Worksheet with Desirable pH and Nutrient Ranges. (Completed Example Report)
Table 2. Nutrient and Fertilizer Recommendation Worksheet with Desirable pH and Nutrient Ranges. (Completed Example Report)  

Table 2. (PDF Download) Nutrient and Fertilizer Recommendation Worksheet with Desirable pH and Nutrient Ranges. (Completed Example Report)

To develop the P recommendation, use Figure 2 (Table 18) from Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa (shown below). Locate the yield potential (top of the chart) and the soil test result (left side of the chart). For this example, the corn yield potential is 150 bushels/acre and P is reported as 23 ppm on the soil test, which falls in the 20–40 ppm range. The resulting P recommendation (circled in red) is 55 pounds of P2O5/acre.Table graphic showing the corn yield, or bushels per acre, potential of a field, highlighting the corn yield potential of 150 bushels per acre, the soil test value of 20–40 Mehlich-3 P, and the corresponding amount of 55 pounds of P2O5 per acre that is recommended to reach the field’s potential yield.

The phosphorus recommendation in this example is based on the phosphorus maintenance equation in Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa.

Phosphorus Maintenance Equation:

lb P2O5 /acre to apply = YP x CR

  • Yield Potential (YP) in bushels per acre for corn grain.
  • Crop Removal (CR) in pounds of P2O5 per bushel. Corn grain removal for phosphorus is 0.37 pounds of P2O5 per bushel.

The equations for P buildup and maintenance recommendations are provided in Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa.

To develop the potassium (K) recommendation, use Figure 3 (Table 23) of the Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa. Locate the yield potential (top of the chart) and the soil test value (left side of the chart). For this example, the corn yield potential is 150 bushels/acre, and the K on the soil test is reported as 100 ppm, which is below the "optimal” range. The CEC of this soil is 10 meq/100g (Table 1). The K recommendation is 85 pounds of K2O per acre, which is based on crop removal plus additional K to build soil test levels to the critical level.Table graphic showing the corn yield, or bushels per acre, potential of a field based on soil test values that are either deficient, optimal, or sufficient, and the corresponding amounts of P2O5 that are recommended to reach to the field’s potential yield.

The potassium recommendation in this example is based on the potassium buildup equation in Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa.

Potassium Buildup Equation:

lb K2O/acre to apply = [(CL - STL) x (1 + (0.05 x CEC))] + (YP x CR) + 20

  • Critical Level (CL), which is 100 ppm at a CEC of 10
  • Soil Test Level (STL) in ppm
  • Cation Exchange Capacity (CEC) in meq/100g
  • Yield Potential (YP) in bushels per acre for corn grain
  • Crop Removal (CR) in pounds of K2O per bushel. Crop removal is 0.27 pounds of K2O per bushel of grain corn

The equations for buildup and maintenance recommendations are provided in Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa.

Calculating the amount of fertilizer needed to meet the nutrient recommendation requires two additional steps.

The first step is to read the fertilizer label and locate the percentage of the target nutrient in the fertilizer source. This percentage is used to calculate the amount of fertilizer needed to meet the nutrient recommendation. Commercial fertilizer sources are required by law to give a guaranteed analysis of the nutrients claimed on the label. The first three values given on the fertilizer label are nitrogen, phosphorus, and potassium (N-P-K). A fourth number may be included for nutrients such as sulfur or other micronutrients that are guaranteed on the label.

Nitrogen (N) is shown as pounds of N per 100 pounds of product. Phosphorus (P) is shown in pounds of P2O5 per 100 pounds of product. Potassium (K) is shown in pounds of K2O per 100 pounds of product. For example, in Table 3 below, 11-52-0 is the fertilizer source needed to meet the phosphorus requirement, and 0-0-60 is the fertilizer source needed to meet the potassium requirement.

In the second step, the amount of fertilizer product to be applied needs to be calculated from the guaranteed analysis. Calculating the amount of nutrients applied in addition to the target nutrient is useful to understand their amount and potential impact on nutrient management. For example, the fertilizer 11-52-0 being used to meet phosphorus requirements also contains nitrogen. Pounds of fertilizer to apply per acre and the calculation of additional nutrients applied are shown in Table 3.

Table 3. Pounds of Fertilizer to Apply per Acre to Meet Phosphorus, Potassium, and Additional Nutrient Applications
Table 3. Pounds of Fertilizer to Apply per Acre to Meet Phosphorus, Potassium, and Additional Nutrient Applications   

Table 3. (PDF Download) Nutrient and Fertilizer Recommendation Worksheet with Desirable pH and Nutrient Ranges. (Completed Example Report)

Tables for other crops are in Tri-State Fertilizer Recommendations for Corn, Soybean Wheat, and Alfalfa. Appendix A contains a blank template that can be used for your fields.

Appendix A. Nutrient and Fertilizer Recommendation Worksheet.
Appendix A. Nutrient and Fertilizer Recommendation Worksheet.  

Appendix. (PDF Download) Nutrient and Fertilizer Recommendation Worksheet.

References

Culman, Steve, Anthony Fulford, James Camberato, Kurt Steinke, Laura Lindsey, Greg LaBarge, Harold Watters, et.al. 2020. Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa, Bulletin 974. PDF.
extensionpubs.osu.edu/tri-state-fertilizer-recommendations-for-corn-soybean-wheat-and-alfalfa-pdf.

LaBarge, Greg. 2022. “Interpreting a Soil Test Report” (AGF-0514). Ohioline, The Ohio State University.
ohioline.osu.edu/factsheet/agf-0514.

Mullen, Robert, Edwin Lentz, and Maurice Watson. 2016. “Soil Acidity and Liming for Agronomic Production” (AGF-505). Ohioline. The Ohio State University.
ohioline.osu.edu/factsheet/AGF-505-07.

Originally posted Oct 18, 2022.
Ohioline https://ohioline.osu.edu