Check Soybean Nodulation (07/01/21)
A good time to evaluate soybean plant nodulation is in the soybean vegetative growth stage, after the second or third trifoliolate leaf has emerged (Figure 1). Use a spade to dig up a number of plants being sure not to damage the root systems. Use a small bucket of water to wash the roots of soil so you can observe the roots properly. When digging up the soybean plants, the nodules should be readily visible. Inspect the nodules by cutting them open, and they should have a pink to red color on the inside if they are healthy. Important factors that affect nodulation are the viability of the inoculum (the number of rhizobium bacteria that were placed on or near the seed), and/or available soil-borne bacteria specific to soybean from previously grown soybean. The viability of the soybean inoculum depends on storage time (shelf life of the inoculum), storage conditions (hot conditions will kill bacteria), and handling of seed before and during seeding. Inoculum is easily killed by direct sunlight. Therefore it is important to check nodulation of the soybean plants to ensure adequate inoculation.
Nodulation may be limited if dry conditions or saturated soils follow soybean planting. Nodules can also be limited by high levels of available soil N, which leads to lower nodule production. Other stress factors that may lead to low nodulation Include: drought, hail damaged plants, root diseases, iron deficiency chlorosis (IDC) symptoms early in the growing season, high salt levels in the soil, and other plant stressing conditions.
If the soybean plants do not have nodules and there are N deficiency symptoms, a “rescue” treatment of N fertilizer may provide the nitrogen needed by the plant. However, in most cases with normal nodulation (Figure 2), there is no need to apply N fertilizer as the plant can biologically fix the required N. For more information about nodulation and rescue treatment see the soybean soil fertility publication SF1164.
Extension Agronomist Broadleaf Crops