Since early August Iowa farmers have been reporting soybean fields or parts of fields with yellow or greenish-yellow leaves in the upper canopy which resemble early senescence and sometimes nitrogen (N) deficiency symptoms. The soybean growth stage is mainly R4 to R5. Several factors can cause these symptoms. Rainfall has been extremely variable this season in Iowa, with severe drought still persisting in some areas of the state. Yet, areas in southern and southeast Iowa have experienced normal to excessive rainfall.
Nitrogen deficient soybean in fields that had excessive rainfall early in the season is not surprising. An ICM News article in early July, Nitrogen Deficient Soybean with Excess Soil Moisture, explained that limited soil aeration combined with poor nodulation or inefficient nodule function can cause N deficiency that may not recover after normal weather resumes.
In droughty areas, however, the most likely cause of yellow soybean is induced early senescence due to insufficient soil moisture since metabolic processes, N fixation, and nutrient uptake are all limited. On the other hand, yellow or greenish-yellow soybean leaves in fields that were droughty early in the season but received rainfall in the last couple of weeks could result from insufficient N fixation by most nodules. The number of nodules and their color when cut open gives only hints of current N fixation. However, digging plants may be useful because a common rule of thumb is that a healthy soybean plant requires 8 to 20 nodules with pink to red color. Typically, N fixation by bacteria in the soybean root nodules peak between the R4 and R5 growth stages and declines rapidly. With dry soil, this natural process can occur earlier, and senesced nodules may not reinitiate N fixation once soil moisture improves. In this case, the plant relies mainly on remobilization of N to the growing seeds and pods or soil N uptake. Since nitrate and ammonium N forms usually are low in the soil at this time of the season, the canopy shows N deficiency symptoms.
So, what can you do about it? Research in several states has shown that N fertilization to the soil (as dribbled UAN or granulated urea) with soybean at the R4 to R6 growth stages seldom is effective. Foliar fertilization may be an option when the soil isn’t dry or right after rainfall if the soybean growth stage is at the early R6 growth stage or earlier. A few Iowa experiments during the mid-1970s showed that foliar soybean an N-P-K-S nutrient mixture between the R4 and R6 growth stages increased yield, but numerous follow-up trials in the US during the 1980s did not confirm these results. Work in Kansas during the mid-1990s indicated significant yield response of high-yielding, irrigated soybean to foliar application of N. However, more recent trials in Kansas and other Midwestern states showed no soybean yield increases from spraying N at R2 to R6 growth stages, and sometimes yield decreases.
An Iowa study during 2005 and 2006 at five sites consisting of spraying 10 lb N/acre with 28% UAN at the R2-R3 growth stage (among other treatments) showed a yield response at one site (4 bu/acre), no effect at two sites, and yield decreases at two sites (4 and 6.5 bu/acre). Spraying was done in early morning or in the evening to lessen the risk of leaf burning. However, leaf area burn was 10% in the responsive site, 10 or 17% in the nonresponsive sites, and 22 and 27% in the sites with yield decrease.
In these most recent foliar N trials the spraying was at the R2-R3 growth stage, there was no drought, and no N deficiencies were obvious at any trial. Therefore, chances of a yield response may be higher by spraying fields or field areas as soon as possible (by the early R6 growth stage) where N deficiency is apparent, rainfall records or soil moisture indicate symptoms may not be just drought effects, and by using lower N rates to avoid excessive leaf injury.
Still, in many cases N fertilization will not be a solution. The apparent N deficiency may be due just to dry soil, in soils with sufficient moisture it may occur only in small patches of fields, and the yield response may not offset product and application costs.
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