Phosphorus and Potassium Management for Corn Grain

Carrie Laboski, Dept. of Soil Science

With increased interested in corn grain production this year, it is important to consider phosphorus (P) and potassium (K) management as well as nitrogen (N). It is necessary to use a current (within the last 4 years) soil test to guide P and K fertilization decisions. Even if the crop rotation on the soil test report is not the one that you are using for 2007, the soil test levels are still useful. Table 1 provides the soil test P and K levels that are considered optimum and excessively high for corn grain production.

Table 1. Soil test P and K interpretation categories.

*Each soil in Wisconsin has been assigned a soil group. See Table 4.1 in UWEX Publication A2809 for details.

When soil test levels are in the optimum range, nutrient recommendations are equal to the amount of phosphate (P₂O₅) or potash (K₂O) removed in the grain at harvest. At excessively high soil test levels, no fertilizer is recommended. The P and K nutrient application guidelines for corn grain are provided in Tables 2 and 3.

Table 2. Phosphorus application rate guidelines for corn grain.

* Total amount of P₂O₅ to apply including starter fertilizer.

Table 3. Potassium application rate guidelines for corn grain.

* Total amount of K₂O to apply including starter fertilizer.

Crop removal rates of P and K for corn grain are different than for other crops. Table 4 provides a comparison of the amount of P and K removed in the harvest portion of the crop for corn grain, corn silage, soybean, and alfalfa. If converting corn silage or alfalfa acres to corn grain, much less P and K will be removed by corn grain. This could impact nutrient management plans with regard to the amount of P that could be applied to a field based on crop removal rates for the rotation and subsequent changes in soil test P levels.

Table 4. Crop removal of P₂O₅ and K₂O for selected crops and yield levels.

The amount of potash fertilizer sold in Wisconsin over the past four years has been decreasing as reported by the Department of Agriculture, Trade, and Consumer Protection. Over the same time period, potash deficiencies and lower soil test K levels have been observed. While corn grain may have lower K requirements than some other crops, it is still important to ensure adequate K is supplied to the crop to achieve maximum economic yield.

When soil test P or K levels are in the high range or lower, applying some (or all) of the required nutrients is starter fertilizer is a good practice. The most consistent responses to starter fertilizer in no-till or high residue situations have been seen when complete (N-P-K) starter fertilizers are applied in a 2- by 2-inch placement relative to the seed. If using pop-up (with seed) starter fertilizers, the N+K₂O in the fertilizer can not exceed 10 lb/a. Application rates typically used with pop-up starter fertilizer may not maximize yield. On high P testing soils, corn yield is often maximized with a 2- by 2-inch side placed starter at rates of about 10-20-20 (N-P₂O₅-K₂O). Additionally, late planted corn can benefit from a complete starter fertilizer when grown on high testing soils because the early season growth is accelerated resulting in quicker attainment of maturity.

If phosphate and/or potash fertilizer supplies become limited, then it may be necessary to prioritize which fields should have fertilizer applied. Below is a list of things to consider when prioritizing fertilizer applications:

·         Soil test to determine P and K needs

·         Fully credit P and K in manures

·         Apply P and/or K first to lowest testing fields

·         Apply some P and/or K to all fields likely to respond (soil test level optimum or lower)

·         Apply banded P and K to increase efficiency

·         Defer P and/or K applications on soils testing high or above

For additional information on P, K, and starter fertilizer see UWEX Publication A2809 Nutrient application guidelines for field, vegetable, and fruit crops in Wisconsin. This publication and other soil fertility information can be found on the Soil Science Extension website: http://www.soils.wisc.edu/extension/