There is the theory that nitrogen fixation and soil residual nitrogen may not supply enough nitrogen for soybeans to maximize yield, especially in high-yield environments. This has provided impetus for assessing the effect of nitrogen (N) fertilizer on soybean yield.
Research in several states provides results that address using starter N fertilizer applied to normal and late-planted soybeans, N fertilizer applied in an amount to replace bacteria-fixed nitrogen, and N fertilizer applied during reproductive development when nitrogen demand is highest. In the following summary of results from this research, low residual soil nitrogen is generally considered to be below 50 to 70 pounds per acre. Profitability is measured using a nitrogen price of $0.40 per pound applied by air at a cost of $5 per acre and a soybean price of $6 per bushel.
Starter N fertilizer, normal-time plantings. When soybeans were planted at the normal time (April–May) in Mississippi (30 pounds N per acre), Missouri (25 pounds N per acre), and Nebraska (50 pounds N per acre) , starter N fertilizer either provided no yield increase or a yield increase that did not cover the cost of the fertilizer. Thus, net returns were lowered in both cases. Adding starter N fertilizer to soybeans may delay or impede nodulation, and thus can reduce nitrogen fixation.
Starter N fertilizer, late plantings. Two Alabama studies looked at applying starter N fertilizer (about 50 pounds N per acre) to soybeans planted at a time (mid-June to late July) that mimics those planted as a doublecrop in the midsouthern US. All sites had low residual soil nitrogen. Yield increases in one study did not increase profits, whereas yield increases in a second study increased profits over $30 per acre.
The Kansas Soybean Production Handbook states that soybeans planted into large amounts of wheat straw may respond to small amounts (10 to 20 pounds N per acre) of starter N fertilizer because inorganic nitrogen is temporarily immobilized by soil microorganisms decomposing the wheat straw.
N fertilizer applied to replace fixed Nitrogen. Studies in Mississippi and Nebraska researched applying a high rate of N fertilizer (>200 pounds N per acre) that was deemed sufficient to replace or supplant nitrogen fixation. Yields were increased above those obtained from soybeans receiving no N fertilizer, but the increases were not profitable. These results indicate that nitrogen fixation is not sufficient to maximize seed yield, but replacing fixed nitrogen with N fertilizer is unprofitable.
N fertilizer applied during reproductive (R3 to R4) development. Results from Kansas (irrigated), Minnesota, and Missouri (some irrigated) studies show mixed results from this practice. Results from Minnesota (75 pounds applied N per acre) and Missouri (25 pounds applied N per acre) showed no yield increase and thus decreased profits. The Kansas (20 pounds applied N per acre) results showed an average yield increase of 7 bushels per acre and almost $30 per acre greater returns. The Kansas sites had low residual soil nitrogen, and irrigated yields were generally greater than 55 bushels per acre.
Summary. 1) Soybeans planted in a normal timeframe do not respond profitably to preplant or "starter" N fertilizer. 2) In extremely late plantings and especially those following a small grain, applying preplant N fertilizer may increase soybean yields and profits at sites with low residual soil nitrogen. 3) Producers desiring to maximize yields from irrigated plantings on soils with low residual soil nitrogen should consider applying 20 to 25 pounds N per acre at beginning podset to ensure nitrogen deficiency does not limit yields in these high-yield (> 55 bushels per acre) environments. A key point with this management option is to irrigate following surface N fertilizer application to ensure immediate uptake.
Additional points. First, plants growing under droughty conditions may appear nitrogen-deficient, but in fact the lack of water has suppressed nitrogen fixation and this will not be remedied by application of N fertilizer. Second, it is not possible to predict soybean response to N fertilizer based on soil properties. However, situations with positive responses generally have either very low residual soil nitrogen, low nitrogen mineralization capability, or soil pH so low that it inhibits nodulation and nitrogen fixation.