Corn Fertilizer Decisions for 2009: Phosphorus, Potassium, Sulfur

Corn Fertilizer Decisions for 2009: Phosphorus, Potassium, Sulfur

March 20, 2009

Also see
UNL Research: Phosphorus Increase Indicated for Continuous Corn

Thirty-four irrigated high yield corn trials were conducted across Nebraska in 2002-2004 to reevaluate UNL fertilizer use recommendations. Following is a review of these results for you as you determine fertilizer needs for your 2009 crops. In these trials the mean yield with fertilizer was 234 bushels per acre.

Phosphorus. The trials found that profitable yield increases can be expected from applying 40 lb/ac phosphate when irrigated high yield corn is:

  1. rotated with soybeans and
  2. soil test P levels are below 10 ppm by the Bray -1 and Mehlich-3 P soil tests, or 7 ppm by the Olsen P soil test (also known as the sodium bicarbonate P test) (Figure 1).


Graphic showing yield response to phosphorus
Figure 1. Corn yield response to applied phosphorus (P) occurred at Bray 1 < 20 ppm when the previous crop was corn, but at Bray 1 < 10 ppm when the previous crop was soybean.
Graph of phosphorus recommendation
Figure 2. Phosphorus recommendation based on soil test phosphorus and previous crop.

The research results indicate that a profitable response of irrigated continuous corn to 40 lb/ac of applied phosphate is likely when soil tests are equal to or below 20 ppm Bray-1 P or Mehlich-3 P. For soil tests greater than 25 ppm Bray-1 P or Mehlich-3, yield increases are unlikely. Profitable response to 80 lb/ac compared with 40 lb/ac phosphate is unlikely unless soil test phosphorus is very low, for example less than 5 ppm by the Bray-1 soil test.

The recommended phosphate rate is related to Bray P-1 for acid and neutral soils and Mehlich-3 for all soils in Figure 2. To use the graph for Olsen P, multiply Olsen P values by 1.5. The associated equations are:

P-rate for continuous corn (lb P2O5/acre) = (25 - Bray-P) x 4
P-rate for corn following soybean (lb P2O5/acre) = (17 - Bray-P) x 6

Apply half the rate when banding preplant or beside the row at planting.

Potassium. The research results verify that profitable corn response to applied potassium is highly unlikely if soil test potassium is above 125 ppm. The results of these 34 trials, as well as results of two series of trials conducted in the 1970s and 1980s, show that potassium application when soil test potassium is above 125 ppm is more likely to cause corn yield loss than gain.

Sulfur. The UNL recommendation does not call for sulfur application for medium and fine texture soils in Nebraska. This recommendation was verified by the results from these trials. While sulfur application often results in greener leaves, increased yield is unlikely on medium and fine texture soils.

Charles Wortmann
Extension Soils Specialist