Stunted, yellow areas in my soybeans: disease, nutrient deficiency or just too much water?
Depending on your circumstances it could be one or all the above. By far the most likely culprit is the excessive amounts of moisture we’ve received lately. Yellowing in a plant is often nutrient-related but the reason for the shortage in the plant is often not a lack of nutrient in the soil. More often it is related to some soil property or root issue that is interfering with nutrient uptake OR a physiological problem that is affecting protein synthesis and nutrient utilization in the plant. Prolonged flooding and/or saturated soils frequently result in all these issues. Roots cannot grow or function properly in anaerobic (no oxygen) soil conditions.
Some of the most common nutrient deficiencies I am seeing in soybeans in excessively wet soils are iron, nitrogen, potassium, manganese or even sulfur. If you have any high pH soils (pH > 7.0) you are more likely to see iron and manganese deficiency. These issues can be helped with a foliar nutrient application, but in many cases plants will eventually recover on their own when better root growth and function resumes.
In soils that are very saturated for a prolonged period of time the nodules on the soybean roots can be damaged or even killed. This means the plant is unable or less able to fix nitrogen until the nodules can start to function again or new nodules form and this might not happen until July. Soybeans will often just sit there in a yellow condition until the soil dries out and air moves back into the soil. Potassium deficiency may be more severe if soil potassium levels are low. However, even in soils with good potassium levels you can see potassium deficiency when root growth and function are being impeded by saturated soils.
Some soil-borne diseases that are favored by wet soils include Phytophthora, Pythium and Rhizoctonia. These diseases can flare up and become an issue when plants are under stress and environmental conditions that favor the pathogen exist. Most soybean seed we plant these days is fungicide treated, but those products are designed to provide protection for 2 – 3 weeks after planting. If your soybeans have been in the ground for 6+ weeks the protection from those products is long gone.
What our soybeans (and all crops) need most of all is a drier soil, warmer and sunnier weather, and better growing conditions. Until this happens not much will help, and in most cases when conditions improve the plants will recover and resume normal growth.