No. 21 Article 6/August 15, 2008

Can You Help Your Soybean Crop with a Foliar Nutrient Application?

The soybean crop this year was planted late for the most part. Those acres that were planted early were in many instances worse off than late-planted acres due to the prevailing wet conditions and cool temperatures, for which soybean is not well adapted. In a typical year, soybean starts the reproductive (R) stages around the time the sixth or seventh trifoliate is fully open (V6-V7). Since flowering of soybean is induced by day-length (or rather length of night), the late planting and/or slow early growth forced plants to go into reproductive stages as early as V4 in some of the fields I have seen this year. Thus, many soybean acres started seed filling with an undersized root system and lower capacity to maximize light interception due to small leaf surface area. The question that some producers have is whether foliar fertilization is going to help the soybean crop cope with the late start, a small root system, and limited vegetative growth this year.

Foliar application of nutrients for soybean has been researched intensively under various soil fertility levels and growing conditions for diverse nutrient mixtures applied at different development stages. From the mid-1970s to mid-1980s, research on foliar fertilization of soybean was stimulated by the hypothesis that seed yield could be improved with foliar nutrient application during late reproductive stages. Such application would delay leaf senescence and reduce nutrient stress caused by high nutrient demands of the seed during the late-season (when root activity naturally declines due to plant age, and dry soil conditions can limit nutrient availability). Those studies showed inconsistent results, and typically little or no yield increase was observed. In more recent years, interest in soybean foliar nutrient application has been revisited, and results have been by and large similar to those of previous studies.

The widespread adoption of no-till and other conservation tillage operations in the Midwest led to concerns that such systems, compared to conventional tillage, might limit soybean nutrient uptake because of cooler and wetter soil conditions early in the growing season. Thus, research on foliar application of fertilizers at early vegetative to early reproductive stages of development was warranted. These studies, just like the studies on foliar applications during late reproductive stages, showed inconsistent results. Typically, the more consistent yield increases due to foliar fertilization occurred when early-season conditions limited root growth and nutrient uptake (normally under wet or cool soil conditions), or when soils had inadequate nutrient supply.

Also, studies of foliar applications have been done with addition of micronutrients, split application of different nutrient mixtures with and without micronutrients, and nutrient additions in combination with glyphosate or fungicide applications. These various studies have shown inconsistent responses, with little or no increase in yield.

In summary, a large body of evidence shows that the likelihood of obtaining a yield increase from foliar fertilization of soybean is very limited. Further, those fields where a response is most likely often show such small yield increases that it makes the application not cost effective.--Fabián G. Fernández

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