Formulating Suggestions for Phosphorus and Potassium Utilization in Cultivated Crops

The Tri-State Fertilizer Recommendations, developed by Purdue University, provide a build-up and maintenance approach for corn, soybean, wheat, and alfalfa. These recommendations are designed to help farmers optimise their soil nutrient levels.

The recommendations for phosphorus (P) and potassium (K) are based on the critical level, soil test level, Cation Exchange Capacity, yield potential, and crop removal. The critical levels for both P and K are determined empirically from field trials. For example, the optimal soil-test range for corn P is 20-40 ppm, while the critical level for potassium is determined based on the yield potential, soil test value, Cation Exchange Capacity, and crop removal.

The fertilizer labels show the guaranteed analysis of the product, with the first three values given as nitrogen, phosphorus, and potassium (N-P-K). The potassium is shown in pounds of KO per 100 pounds of product, and the phosphorus in pounds of PO per 100 pounds of product.

To calculate the potassium recommendation, the soil test level is subtracted from the critical level, multiplied by (1 + (0.05 x Cation Exchange Capacity)), and then added to the product of yield potential and crop removal, and finally, 20 is added.

When soil tests for P and K are above the "optimal" range in the Tri-State Fertilizer Recommendations, no fertilizer is needed. However, if the soil test levels are in the "deficient" categories, extra fertilizer is recommended to "build up" soil test levels to the optimal range.

It's important to note that the Tri-State Fertilizer Recommendations do not discuss lime recommendations, but soil pH should be considered first in developing a nutrient plan. For help with understanding values found on a soil test report, see Interpreting a Soil Test Report. For information on soil pH and lime recommendation development, refer to the Ohio State University Extension fact sheet, Soil Acidity and Liming for Agronomic Production.

The use of the Tri-State Fertilizer Recommendations tables is straightforward to identify a recommended nutrient rate for a specific crop. Tables for phosphorus and potassium are provided for various crops, including corn.

Lastly, a fourth number may be included for nutrients such as sulfur or other micronutrients that are guaranteed on the label. The guaranteed analysis of a fertilizer source is required by law to be given on the label.

Formulating Suggestions for Phosphorus and Potassium Utilization in Cultivated Crops