Optimal rates of side-dressed nitrogen in 22 strip-plot trials

Mounting evidence suggests that the optimal rate of nitrogen (N) application for an individual cornfield varies greatly with percentage of the N lost between application and early June. This evidence prompts questions about optimal rates of application when the fertilizer is applied in June.

Precision farming technologies are being used to learn more about optimal rates of fertilization when a 28 percent N solution is knifed into soil midway between rows in June. Table 1 lists the yields obtained in 22 trials having N applied at rates of 50, 100, and 150 lb/acre.

The studies involved application of N at constant rates in 6- or 8-row strips usually more than 2,000 ft in length. The treatments were replicated at least four times and harvested with yield-monitoring combines. All of the fields were planted to corn following soybean.

The studies were conducted in fields having average to above-average variability in soil type. The strips were positioned to capture as much variability in soil type as possible. The strips often included knolls and poorly drained areas with relatively low yield potential.

A major objective was to assess the potential benefits of variable-rate fertilization, but only whole-strip averages are presented in this article. Whole-strip averages indicate optimal rates when variable-rate applicators are not used.

The results can be interpreted by considering the amounts of grain needed to purchase the fertilizer. It often takes about 4 bu of corn (priced before drying and storage) to purchase 50 lb of fertilizer N. The following interpretations are based on these costs.

If all fields received the same rate of N, then it would have been profitable to increase rates from 50 to 100 lb/acre. Increasing rates from 100 to 150 lb/acre, however, would have resulted in economic losses.

If each field received the best rate, eight fields would have received 50 lb, nine fields would have received 100 lb, and five fields would have received 150 lb of N/acre. The average rate across all fields would have been 93 lb of N/acre.

Most corn producers have learned that fertilizer N usually must be applied at rates greater than 100 lb of N/acre to avoid clear visual symptoms of N deficiency when N is applied in the fall or early spring before planting. Extra N seems to be needed to compensate for fertilizer N that escapes from the field before it can be used by the crop.

Delaying all but starter applications until June offers clear economic and environmental benefits. These benefits could be important as profit margins narrow and environmental concerns increase.

Table 1. Results from 22 corn-after-soybean strip trials.

Yields obtained with various rates of N applied in June
Year-site 50 lb/acre 100 lb/acre 150 lb/acre
bu/acre
1995-2 135 151 154
1995-3 101 124 114
1995-4 141 140 135
1995-5 151 152 151
1995-6 141 150 156
1995-7 104 129 129
1995-8 91 120 138
1996-1 126 128 135
1996-2 153 157 158
1996-3 146 150 149
1996-4 134 150 151
1996-5 133 142 143
1996-6 150 161 163
1996-7 137 137 137
1997-1 121 161 177
1997-2 120 139 150
1997-3 128 153 155
1997-4 114 126 127
1997-5 118 134 129
1997-6 147 151 150
1997-7 160 165 171
Mean 131 143 146

This article originally appeared on page 11 of the IC-482 (PrecisAg) -- May 5, 1999 issue.

Updated 05/04/1999 - 1:00pm