Phosphorus and Potassium

Build-up and Maintenance FrameworkBuild-up and Maintenance Framework

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, then maintain these levels over time. The original tri-state recommendations had three distinct recommendations based on soil test values: 1) build-up, 2) maintenance, and 3) drawdown. The new tri-state fertilizer recommendations use a similar but simplified framework. The major changes to the new framework include 1) making the build-up phase optional but still recommended and 2) eliminating the drawdown phase. These changes are intended to simplify recommendations and provide farmers with greater flexibility in managing nutrients.

The New Tri-State Fertilizer Recommendation Framework

Overview of Build-up and Maintenance Phases and Associated Fertilizer Recommendations

Under the new framework, the default recommendation remains as build-up (crop removal + additional fertilizer to build soil test levels) if soils test below the critical level or are “deficient.” The recommendations are designed to supply additional nutrients and to raise the soil test to the critical level over a four-year period. For deficient soils, recommended rates of fertilizer should be applied annually.

When soils are in the maintenance range, above the critical level but less than the maintenance limit, soils are “optimal,” capable of supplying nutrients required by the crop. No response to fertilizer is expected. Fertilizer should be applied at some point within the rotation to replace the nutrients removed in the harvested crop each year. Fertilizer applications can be made annually or every other year. In some cases, fertilizer may be applied every three or four years. Soil testing should be used to assess soil test levels and fertilizer requirements no less than every four years. Soils above the maintenance limit are “sufficient.” There is no agronomic reason to apply fertilizer when soil tests are in this range.

P and K Critical Levels

One of the most important components of P and K management is knowing when a crop will need P or K fertilizer. When soil test levels are above the critical level and therefore in the maintenance range or above, the chance of a yield response to P or K fertilizer is highly unlikely. Critical levels for soil test P and K are provided below and are based on more than 200 on-farm P and K trials in corn, soybean, and wheat in the tri-state region over the past 10 years. New critical levels are now reported using Mehlich-3 as the default extractant for soil test P and K.

Recommended Mehlich-3 Soil Test Phosphorus and Potassium Levels for Field Crops in the Tri-State Region

The critical levels for soil test phosphorus have been modified to use Mehlich-3P but are largely consistent with the original tri-state recommendations.

Potassium critical levels have been modified more substantially, primarily simplified. The original tri-state potassium recommendations were based on cation exchange capacity (CEC) levels of the soil. As CEC increased, so did the recommended critical level and the K fertilizer rate in the build-up range. The new recommendations classify all loam and clay soils together (CEC >5 meq/100 g) into a consistent and simplified recommendation.

P and K Crop Nutrient Removal Rates

Over the past several years, thousands of corn, soybean, and wheat grain samples have been analyzed for nutrient content. Corn, soybean, and wheat now yield more grain with less nutrient removed per bushel of grain relative to 20 or 30 years ago. For phosphorus, there was a 5%, 1%, and 22 % reduction in pounds of P2O5 per bushel of corn, soybean, and wheat grain, respectively. For potassium, there was a 26%, 19%, and 35% reduction in pounds of K2O per bushel of corn, soybean, and wheat grain, respectively. Reduced grain nutrient concentrations have also been recently reported in other states.

Nutrients Removed in Harvested Grain   Nutrients Removed in Harvested Forage Biomass

P and K Fertilizer Rate Calculations

The revised tri-state fertilizer recommendations are calculated based on the equations in the table below. The equations require the user to provide the soil test level and the expected yield potential of the crop. The equations for soils in the build-up range differ from those in the maintenance range. These equations remain unchanged from the original recommendations, with the exception that Michigan does not recommend the 20 lbs added to the total nutrient removal for K recommendations. Note that build-up equations have not been recently validated and should be considered a priority for future research.

Equations Used for Calculating New Fertilizer Recommendations

Phosphorus Fertilizer Recommendations

Potassium Fertilizer Recommendations

Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa

Executive Summary  |  Soil Sampling, Handling, and Testing  |  Soil pH and Lime Recommendations  |  Nitrogen  |  Phosphorus and Potassium  |  Calcium, Magnesium, and Sulfur  |  Micronutrients  |  Additional Resources  |  Authors and Acknowledgements

This website provides a summarized version of the Tri-State Fertilizer Recommendations, reporting only the main points of the document, but lacking comprehensive detail. For complete information, please see the full version which we anticipate being available soon from The Ohio State University Extension Publications Store (