There is the theory that N (Nitrogen) fixation and soil residual N may not supply enough N for maximum soybean yields, especially in high-yield environments. This has provided impetus for assessing the effect of N fertilizer on soybean yield.

A recent summary of a large number of studies in the Midwest concluded that fixed N and soil N supply may not be great enough to meet the N demands during reproductive development of soybeans growing in high-yield environments.

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 N, and


• N fertilizer applied during reproductive development when N demand is highest.

In the following summaries of results from this research, low residual soil N is generally considered to be well below 50 to 70 lb/acre.

N fertilizer applied to replace fixed N

Studies in Mississippi and Nebraska researched applying a high rate of N fertilizer (>260 lb N/acre) that was deemed sufficient to replace or supplant N fixation. Yields were increased above those obtained from soybeans receiving no N fertilizer, but the increases were not profitable. These results do, however, support the premise that N fixation is not sufficient to maximize soybean seed yield.

Starter N Fertilizer

Normal-time (April-May) plantings

Research in Mississippi (30 lb N/acre), Missouri (25 lb N/acre), and Nebraska (50 lb N/acre) showed that starter N fertilizer either provided no yield increase or a yield increase that did not cover the cost of the fertilizer (this research was conducted before today’s high soybean commodity price). Thus, net returns were lowered in all cases.

Late plantings (June and July)

• In two Alabama studies, starter N fertilizer (~50 lb N/acre) was applied to soybeans planted at a time that mimics those planted as a doublecrop in the midsouthern US. All sites had low residual soil N. Yield increases (~2.2 bu/acre) in one study did not increase profits, whereas yield increases (~7.5 bu/acre) in a second study increased profits over $30/acre ($6/bu commodity price and $0.40/lb N cost).


• The Kansas Soybean Production Handbook states that soybeans planted into large amounts of wheat straw may respond to small amounts (10 to 20 lb N/acre) of starter N fertilizer because inorganic nitrogen is temporarily immobilized by soil microorganisms decomposing the wheat straw.

N fertilizer applied during reproductive (R3 to R4) development.

• Results from a nonirrigated 3-year Delaware study showed that the application of N fertilizers at a rate of 25 lb N/acre at R3 (beginning podset) did not increase yield.


• Results from Kansas (irrigated), Minnesota, and Missouri (some irrigated) studies show mixed results from this practice. Results from Minnesota (75 lb N/acre) and Missouri (25 lb N/acre) showed no yield increase and thus decreased profits. The Kansas (20 lb N/acre) results showed an average yield increase of 7 bu/acre (~12% yield increase). The Kansas sites had low residual soil N, and irrigated yields were generally greater than 55 bu/acre.


• Results from an Iowa study (organic matter 3-4%) showed no yield increase from 40 lb N/acre applied at R3. Yield levels were >50 bu/acre.

Summary

• Soybeans planted in a normal timeframe (April-May) do not respond profitably to application of preplant or “starter” N fertilizer.


• In late plantings, and especially those following a small grain, applying preplant N fertilizer at <50 lb N/acre may increase soybean yields and profits at sites with low residual soil N. This should be verified each season using current commodity and N prices, plus measured soil N.


• Producers desiring to maximize yields from irrigated plantings on soils with low residual soil N should consider applying 20 to 25 lb N/acre at beginning podset to ensure that N deficiency does not limit yields in these high-yield environments. A key point with this option is to irrigate following surface N fertilizer application to ensure immediate uptake.

Additional points

• Adding starter (early-season) N fertilizer to soybeans may delay or impede nodulation, and thus can delay the onset of N fixation that normally would have occurred in the absence of the starter N.


• Soybean plants growing under moisture deficit conditions may appear N-deficient, but in fact the lack of water has suppressed N fixation and this will not be remedied by application of N fertilizer.


• 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 N, low N mineralization capability, or soil pH so low that it inhibits nodulation and N fixation. As stated above, a measure of residual soil N is necessary for making a truly informed decision about adding N to soybeans.


• The above results are from studies that were conducted before the advent of today’s high commodity prices. Thus, economic results with today’s prices may be different than those cited above.


• One of the environmental strong points of growing any legume crop is not having to add supplemental N fertilizer. Therefore, this should be considered if and when this practice is found to be an important component for reaching a higher soybean yield plateau since loss of N from cropland is a significant concern in USDA/NRCS’s conservation practice standard for nutrient management.

The United Soybean Board is again offering free subscriptions to the Plant Management Network. An individual subscription gains access to 12 resources, including archived “Focus on Soybeans” presentations, Plant Disease Management Reports, and Crop Management Journal. These resources provide excellent information for producers growing soybeans in the Midsouth.

To sign up for a subscription, go to www.plantmanagementnetwork.org/soybean for the account activation page. Fill out the items indicated with red asterisks, including the username and password. If you need assistance, contact Phil Bogdan at pbogdan@scisoc.org.

An updated “Planting Date Decisions” White Paper has been posted on this website. For producers in states other than Mississippi, the Freeze/Frost data referred to in that paper can be found on the NOAA website. Just click on your state and then click on “Freeze/Frost Data”.

Posted by Larry G. Heatherly, Mar. 2012, larryheatherly@bellsouth.net