October 27, 2008

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.