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Yellow Soybeans Need Sun

By Chad Lee, Extension Professor and Agronomist
 
Numerous fields of soybeans appear yellow or bright green right now. The symptoms mimic nitrogen deficiency, but the plants are really starving for air to the roots and sun on the leaves. Everyone is thinking about applying some nitrogen to help make the plants greener. But what these plants really need is for sunshine to help partially dry the soils. 
 
 
 
 
 
 
Saturated soils in the low areas result in yellow plants.
 
Rains have saturated our soils and pushed air out of the root zone. That lack of oxygen can lead to slowed activity of the Bradyrhizobium japonicum which leads to reduced nitrogen fixation and uptake. Extended periods of saturated soils will kill some of the roots and some of the Bradyrhizobium japonicum. Both will rebound quickly once sunshine and air return to the system. 
 
 
 
Nodules are present but probably dying in the saturated soils.
 
Most of the soybeans in Kentucky are flowering, so the rainfall we get now helps to build the size of plant and structures to produce pods and seeds. These yellow plants will likely lose some flowers, but even a healthy soybean plant will lose about half of its flowers. If the sun comes out soon, these plants have plenty of time to build more flowers and pods.
 
 
 
Standing water in a low area. No room for air in this soil.
 
Rainfall and water stress during soybean seed fill is our most critical period for yield determination. The current saturated conditions likely will have very little impact on final yields. (All of these comments are based on the premise that the plants survive. Soybeans completely flooded and choking out is a completely different scenario.) While we need some sunshine now to help the plants recover, we need to continue getting timely rains to help this crop yield well. 
 
 
 
Roots from a green plant. Viable nodules actively fixing nitrogen.
 
Once the fields dry out enough to allow spray equipment across without causing compaction, the soybeans are probably a couple days away from becoming green again. A dose of foliar nitrogen may get one extra day of green plants, but probably will not contribute much to the overall yield of the plants.
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