The 2009 soybean growing season has been filled with questions and concerns regarding the yellow "state" of many soybean fields. This topic has been building from emergence through the current R4/R5 growth stage. I have been in many soybean fields over the past 6 weeks and have seen many of the same culprits. Below is a short list of the main issues I have seen.
- Variable emergence: Variable seeding depth was a major contributor to some of the early season "off color" soybean fields that I investigated. In these fields you would see odd patches of yellow beans. Once you carefully excavated the soybean plants you would see that the yellow beans were planted <1/2 inch deep. Following planting these seeds did not receive enough moisture to stimulate germination. The seed sat in dry soil until a significant rainfall triggered emergence. The delayed emergence and subsequent cool environmental conditions held the soybean plants back developmentally, hence when their neighbors were lush green they appeared light green to yellow. This difference was simply a function of delayed developmental stage (N fixation).
- Cool temperatures and delayed development: Please refer to the following article to address this topic. "Yellow soybeans and nitrogen fixation"
- Potassium (K) deficiency: Many soybean growers either cut-back on their K rate or flat out "took a year off". This has lead to an increased number of fields showing K deficiency. This not only has a direct impact on yield but may increase soybean aphid populations (Please see "K deficiency and the soybean aphid"). To further confound this issue Sale and Campbell (1987) found a differential response to K deficiency among soybean cultivars. This may explain why some varieties or fields are showing greater symptoms than others.
- Drought: Many soybean fields in Wisconsin have experienced droughty conditions. Research has shown that nitrogen fixation in soybean is extremely sensitive to drought and that this sensitivity to soil drying constitutes a serious constraint to N fixation (Serraj et al., 1998; Sinclair et al., 1987).
- Poor inoculant application method: Lastly, I have walked several fields where an inoculant was either not applied or improperly applied to "new" soybean ground leading to variable nodulation (one plant would have nodules and the neighbor lacked nodules) across a field.
Cited article:
Sale, P. W. G. and L. C. Campbell. 1987. Differential responses to K deficiency among soybean cultivars. Plant and Soil. 104:2:183-190.
Serraj, R., T. R. Sinclair, and L. H. Allen. 1998. Soybean nodulation and N2 fixation response to drought under carbon dioxide enrichment. Plant, Cell, and Environment. 21:491-500.
Sinclair, T. R., R. C. Munchow, J.M. Bennett, and L.C. Hammond. 1987. Relative sensitivity of nitrogen and biomass accumulation to drought in field-grown soybean. Agronomy Journal. 79:986-991.
