Application of Nitrogen Fertilizer in Soybeans

South Dakota is a major soybean producing state in the US. During the 2016 growing season, 5.17 million acres of South Dakota cropland had soybean harvested. Producers generally rotate soybeans with corn or occasionally wheat and nitrogen fertilizer management is usually performed on the corn or wheat prior to or succeeding the soybean crop.

Soybean and Nitrogen
Nitrogen (N) is one of the primary nutrients that crops require for optimal growth and grain production. Soybean seeds are high in protein (~40%) and have relatively high nitrogen requirement to produce the high-protein grain and stover. Nitrogen is added from external source/s on other major crops such as corn, wheat, etc., however, since soybeans are a legume plant, they possess the capability to fix their own N from the atmosphere through a symbiotic relationship with rhizobium bacteria. Soybean nitrogen requirement can reach almost 325 lb/a for 70 bu/a yield with about 50-60% of the N coming from nitrogen fixation. The remaining N requirement (i.e.130-160 lb/a) must come from either soil inorganic N, mineralization of soil organic matter or breakdown of previous years residue. Producers and scientific community are generating questions concerning the ability of soybeans to fix adequate nitrogen in high yield environments, especially above 70 bu/a. Other researchers have shown that increased level of nitrate-N in the soil can inhibit the N fixation process that is physiologically high energy demanding.

Effects on Soybean Yields
SDSU Extension conducted a study during the 2016 growing season at five Eastern SD sites to evaluate the effects of added N fertilizer (as urea) on soybean yields. Urea at 500 lbs/a N rate was applied at two leaf (early season) and pod set (late season) stages and compared with ‘check’ plots that did not receive any N fertilizer. The study was established at the SDSU Northeast Research Farm and four on-farm cooperators’ fields in Clark, Kingsbury, and Minnehaha (2 sites) counties. At each site, the ‘check’ and the two fertilizer rate treatments were arranged in a Randomized design with four replications. Plot size at all sites was 10’ x 20’.

Grain yield analyses did not show any significant statistical differences between the treatments at any site, which suggested that plots without N fertilizer were able to fix adequate N to produce grain yields comparable to plots that received N. Numerically, treatment plots that received late-season N yielded the highest at three of the five sites. Likewise, check plot yields were lowest at four out of five test sites. There were no strong statistical trend/s from the treatments tested during 2016 growing season. However, there is a need for additional investigation in topic particularly different N rates applied at different growth stages to confirm the relationship between added N fertilizer and overall soybean yields.

Source: David Karki, iGrow