Giving Proper Nitrogen Credit for Legumes in Corn and Milo Rotations

Giving Proper Nitrogen Credit for Legumes in Corn and Milo Rotations

October 27, 2008
Added profit:
$30.15-$80.40/acrea
Based on an average savings of 45 lb of N after soybeans and 120 lb of N after alfalfa at $0.67/lb of N

Gary Zoubek, Extension Educator
Aaron Nygren, Extension Educator

Research at the University of Nebraska–Lincoln, as well as other land grant universities, has clearly demonstrated that producers can safely reduce their nitrogen input for corn following a legume crop such as soybeans. This legume nitrogen credit is not always due to increased nitrate nitrogen supplied or fixed by the legume plant. In fact, soybean production results in a net removal of nitrogen from the system.

The nitrogen credit from soybeans is largely due to the reduced immobilization of soil nitrogen associated with decomposition of soybean residue as compared to corn or sorghum crop residues. Because of the lower carbon to nitrogen ratio, soybean crop residues will decompose faster with less nitrogen tie-up than cereal crops such as corn or wheat. This faster decomposition will release more nitrogen to the soil to be available to the new crop.

University of Nebraska recommendations for corn following soybean indicate that there can be a credit of 45 lb of nitrogen per acre for non-sandy soils and 25 lb of nitrogen per acre for sandy soils unless the previous soybean yield was less than 30 bushels per acre. For yields less than 30 bushels, one pound of nitrogen per bushel of soybeans harvested can be credited.

Fertilizer nitrogen recommendations for corn following alfalfa can be reduced by 90 to 150 lb on fine textured soils and by 40 to 100 lb on sandy soils depending upon the percent alfalfa stand. Alfalfa with more than four plants per square foot can use the higher nitrogen credits while the lower credits should be used with stands of less than 1.5 plants per square foot. Some universities give a credit into the second year after alfalfa.

UNL On-Farm Nitrogen Research

In 2003 and 2004 UNL Extension Educators in York, Hamilton, and Seward counties compared UNL nitrogen recommendations using both 45 lb and 75 lb nitrogen credits and using actual residual soil nitrate nitrogen as well as an assumed soil nitrate nitrogen level of 3.0 ppm. The research was conducted on nine farms. Results from these replicated research plots over the two years verified that the UNL recommendations of crediting 3.0 ppm soil nitrate nitrogen carryover and 45 lb/acre soybean legume credit is conservative in estimating crop nitrogen needs. All three treatments in these replicated plots exceeded the yield goals of the producers. Following the previous soybean crop, residual soil nitrate nitrogen levels in all nine plots averaged near 6.0 ppm, well above the 3.0 ppm level assumed in the UNL nitrogen formula. The actual data is shown in Tables 1, 2, 3 and 4.

The result of 20 years of research in Nebraska by USDA-ARS finds the mean soybean nitrogen credit to be about 55 lb/acre for dryland and 65 lb/acre for irrigated production. In 34 irrigated high-yield site-years, the economically optimal nitrogen rate was 65 lb/acre less when corn followed soybean compared to corn. However, the credit varies by site-year and a soybean nitrogen credit of 45 lb/acre is recommended to minimize risk due to inadequate nitrogen supply.

Online Corn Nitrogen Calculator for Nebraska

The UNL Corn Nitrogen Calculator for Nebraska accounts for legume credits. In Example 1 a corn price of $4.50 and a fertilizer price of $.67/lb were used. These prices result in a corn price to nitrogen price ratio of 6.7 to 1 resulting in a slight reduction in the amount of fertilizer applied compared to no change if the price ratio was 8 to 1. In this example, an irrigated field with a 200 bushel yield goal would have a recommended nitrogen rate of 171 lb N/acre when corn was the previous crop compared to 130 lbs N/acre when soybean was the previous crop. This calculator is available on the web in the Soil Management section of CropWatch.

Table 1. Commercial nitrogen application comparing three methods of nitrogen recommendations, 2003.
Producer Yield Goal OM Soil N ppm Actual soil N +
75 lb N Credit
Actual soil N +
45 lb N Credit
3.0 ppm + UNL
(45 lb N Credit)
Hamilton No. 1 200 bu 2.4 6.3 75 lbs N
Hamilton No. 2 200 bu 2.5 6.7 84 lbs N 114 lbs N 144 lbs N
York No. 1a
210 bu
2.5
14.6
75 lbs/N
110 lbs N

150 lbs N

York No. 2
220 bu
2.0
6.0
87 lbs N
117 lbs N
141 lbs N
Average
207.5 bu
2.4
6.3b
80 lbs N
111 lbs N
141 lbs N

a Had 75 lbs N applied preplant prior to sampling.
b Average of three fields (York 1 not included).

Table 2. Combine-harvested, replicated yields produced under three methods of nitrogen recommendation.

Producer
Yield Goal

Actual soil N +
75 lb N Credit

Actual soil N +
45 lb N Credit
3.0 ppm +
UNL (45 lb N Credit)
Hamilton No. 1
200 bu/ac
206 bu/ac
217 bu/ac
216 bu/ac
Hamilton No. 2
200 bu/ac
224 bu/ac
227 bu/ac

219 bu/ac

York No. 1
210 bu/ac
226 bu/ac
233 bu/ac

235 bu/ac

York No. 2
220 bu/ac
217 bu/ac
220 bu/ac

222 bu/ac

Average
207.5 bu/ac
217b bu/ac
224a bu/ac

222a bu/ac

Av. N applied
 
80 lbs N
111 lbs N

141 lbs N

* Following dryland soybeans.
Superscripts with different letters are significant at 5%.

Table 3. Commercial nitrogen application comparing three methods of nitrogen recommendations, 2004.

Producer
Soybean Yield
Yield Goal
Organic Matter
Soil N ppm
Actual soil N +
75 lb N Credit
3.0 ppm +
1 lb/bu N Credit

3.0 ppm +
UNL (45 lb N Credit)

York No. 1
55 bu
210 bu
2.4
6.0
110 lbs N
140 lbs N
150 lbs N
York No. 2*
11 bu
210 bu
2.6
10.8
100 lbs N
145 lbs N
190 lbs N
York No. 3
72 bu
200 bu
2.5
5.9
71 lbs N
93 lbs N
116 lbs N
Hamilton No. 1
68 bu
200 bu
2.2
5.3
96 lbs N
121 lbs N

144 lbs N

Seward No. 1
65 bu
200 bu
3.0
9.0
44 lbs N
102 lbs N

122 lbs N

Average
54 bu
204 bu
2.5
7.4
 
84 lbs N
120 lbs N

144 lbs N

* Following dryland soybeans.

Table 4. Combine harvested, replicated yields produced under three methods of nitrogen recommendation.

Producer
Yield Goal
Actual soil N +75 lb N Credit
3.0 ppm + 1 lb/bu N Credit
3.0 ppm + UNL (45 lb N Credit)
York No. 1
210 bu
200 bu/ac
205 bu/ac
201 bu/ac
York No. 2*
210 bu
229 bu/ac
227 bu/ac
233 bu/ac
York No. 3
200 bu
208 bu/ac
210 bu/ac
213 bu/ac
Hamilton No. 1
200 bu
202 bu/ac
205 bu/ac
208 bu/ac
Seward No. 1
200 bu
218 bu/ac
228 bu/ac
230 bu/ac
Average
204 bu
212b bu/ac
215a bu/ac
217a bu/ac
Av. N applied
 
84 lbs N
120 lbs N
144 lbs N
* Following dryland soybeans.
Superscripts with different letters are significant at 5%.