Making P and K fertilizer recommendations

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Soil testing is the foundation upon which we make nutrient recommendations. Something equally important that is often overlooked is the crop response correlation work that gives those soil test numbers meaning. Together these two pieces are the essential components of making nutrient recommendations to maximize crop production and limit off site movement of nutrients that result in economic loss and potentially affect water quality. The Tri-State Fertilizer Recommendations used by The Ohio State University follows a build, maintenance and drawdown approach to soil fertility management.

The Tri-State Fertilizer Recommendation for both phosphorus and potassium are built around defining a critical level. The authors of the Tri-State define the Critical Level for both immobile nutrients as “…the soil test level above which the soil can supply adequate quantities of a nutrient to support optimum economic growth.” Below the Critical Level “… the soil is not able to provide P and K requirements of the crop.” Above the Critical Level “… the soil is capable of supplying the nutrient required by the crop and no response to fertilizer would be expected.”

The Maintenance Limit is the upper end of the scale.  The Tri-State authors define the Maintenance Limit as “There is no agronomic reason to apply nutrients when soil test levels are above the maintenance limit level.”

Soil test levels between the Critical Level and Maintenance Limit are define as the Maintenance Plateau Range. In this range, the recommendations are “Designed to replace nutrients lost each year through crop removal. No response to fertilizer in the year of application is expected. And no response to placement technique such as banding or stripping or the use of P and K starter fertilizer…”

click image to zoom Chart 1 show the Critical Level, Maintenance Limit and Maintenance Plateau for a 160 bushel corn crop and recommends fertilizer recommendation based on soil test level for 160 bushel corn.

Critical levels for phosphorus are defined in the Tristate Fertilizer Recommendations depending on crop. The critical P level for corn and soybean rotation is 15 ppm (30 pounds per acre) or if wheat and alfalfa are included in the rotation 25 ppm (50 pounds per acre). The Maintenance Limit for a corn and soybean rotation is 30 ppm (60 pounds per acre) or if wheat and alfalfa are included in the rotation 40 ppm (80 pounds per acre).

click image to zoom Critical levels and Maintenance Limits for potassium is defined in the Tri-state Fertilizer Recommendations based on CEC (Cation Exchange Capacity) of the soil and are shown in Table 1.

Three publications that can help farmers looking to develop fertilizer recommendations are:

The Fertilizer Recommendations for Corn, Soybeans, Wheat and Alfalfa can be found at

Developing Phosphorus and Potassium Recommendations for Field Crops (AGF-515-12) walks through developing a nutrient recommendation and how much fertilizer is needed to meet that need found at

An Excel Spreadsheet to calculate P, K and Lime Recommendations (Beta Version-11/2012) for 7 fields or zones and a three crop rotation can be found at  

Other helpful nutrient related publications can be found at

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