By Hans Kandel
Extension Agronomist Broadleaf Crops
Soybean plants have the ability to form a symbiotic relationship with nitrogen-fixing bacteria (Bradyrhizobium japonicum), which cause the formation of nodules (small swellings) on the root system. Fields that have no history of soybean production require inoculation with the soybean specific bacteria. Nodules on soybean roots can be found starting at the V2 or V3 (second or third trifoliolate growth stage). The number of nodules formed increases till the R5 growth stage as does the nitrogen fixation. Nodules that are fixing nitrogen are pink or red inside. There are a few factors that can reduce the formation of nodules. Elevated levels of nitrogen in the soil may reduce the nodulation. Very wet conditions early in the season, or extremely dry soil conditions may lead to smaller number and size of nodules. The presence of fungi causing root disease or iron deficiency chlorosis (IDC) may also negatively influence nodulation. Lastly, limited nodulation may occur in fields in which soybeans are being grown for the first time.
The beginning of July is a good time to check the roots for nodules. When digging up the plants be careful to gently remove the dirt, possibly washing the roots in a bucket of water. In fields, with no previous soybean history, with low nodule numbers, and if the plants show a pale green color (make sure it is not IDC) a nitrogen rescue treatment should be considered. When nitrogen deficiency symptoms are beginning to appear nitrogen should be applied. If soil nitrate levels were high at planting time, then nodulation may be low due to high N level in the soil, and the crop should be able to utilize nitrogen from the soil.
At Cathay (Wells County) a trial was conducted by the Carrington Research Extension Center to measure crop response with rescue POST applied nitrogen treatments to soybean visually yellow, with no detectable nodules, grown in a low N soil, and in a field with no prior history of soybean production. Nitrogen treatments included: untreated check, broadcast urea (46-0-0) at 50 and 100 lb N/acre soil applied at the R3 growth stage (early pod formation). About 0.85 inch of rainfall occurred 8 days after urea application. Yield with 100 lb acre N yielded significantly more than the control. With good nodulation there is no reason to apply additional nitrogen.