Making P and K fertilization decisions

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

Having current information about the soil pH and available soil nutrient levels is the passport to effective and economical use of nutrient inputs. With the cost of phosphorus and especially potassium remaining high relative to the value of the corn, soybeans and other fields, it is important to understand the response of crops to nutrient additions in relation to what is available in the soil. Through years of good nutrient management nearly two-thirds of Michigan soils contain enough available phosphorus to grow top corn and soybean yields, but only about one-third of Michigan soils contain adequate levels of potassium for producing top yields.

In soils that contain less than the critical level of available P or K (soil test value which results in 95 percent of maximum yield) nutrient additions will improve crop yield. In this responsive soil test zone, significant improvement in crop yield results from the addition of fertilizer P or K. With each increased increment of P or K addition there is less increase in crop yield until the point is reached where the cost of the fertilizer addition is equal to the value of the increased yield. This is the point of most economical return. The amount of P and K to add to reach this point varies with the soil test level, the crop and the soil texture. The critical soil test value for P is 15 ppm for corn and soybeans and is 25 ppm for alfalfa and wheat. The critical soil value for K ranges from 85 to 125 ppm depending on soil texture, being higher for soils with more clay.

With resources being tight for buying seed, fertilizer and other inputs, farmers are wanting to know how best to allocate fertilizer resources. Decisions need to be made. Let soil test information be your guide. We know that maintaining soil test values in the adequate range provides the best opportunity for excellent crop yields from year to year. When resources are limited, it is best to apply needed amounts of fertilizer first on fields where the soil P or K levels are less than adequate and then allocate the remaining fertilizer to fields with adequate levels to offset crop removal.

When allocating fertilizer to fields at less than recommended rates, it is better to reduce the application rate a given proportionate amount on all fields with similar soil test values than to apply the full amount on some fields and none on others. For example, on fields with similar soil test values applying fertilizer at 70 percent of the recommended rate on all the acreage will provide a better economic return than applying the full rate of fertilizer on 70 percent of the field acres and none on 30 percent of the field acres. That is because the biggest yield increase comes from the first increments of fertilizer addition.

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