Nitrogen Malaise in Soybeans

Late planting, excessive rainfall and wet soils have slowed down soybean growth. And when soybeans are slow out the gate, particularly when soils are wet, they can exhibit yellowing.

One of the things I am noticing this summer is how soybeans have lagged in development and their vegetative canopy has a yellowish hue. I refer to this condition as a malaise that gives them a sickly, yellowish appearance, limits growth and sets the crop behind. Soybeans showed similar symptoms last summer.

These symptoms aren’t isolated spots in fields that might indicate causes like compaction, water logging, iron chlorosis or even SCN—all which can cause yellowing and stunting. This malaise is field-wide and seems to impact each and every acre to almost the same degree. Because symptoms are field-wide I recognized they can’t be attributed to some of the abiotic or biotic factors that normally stress a crop in isolated spots or regions of a field.

We know that the spring was cold and planting and emergence was delayed and uneven because soil conditions, moisture and temperature were variable. This set the crop back, and then it turned wet in mid May with excessive rainfall totals far exceeding normal.

Considering that soybeans with a yellowish hue seemed to be suffering a general malaise during June at the same time it was raining excessively made me realize they were experiencing “wet feet.” We all know that soybeans don’t like wet feet, meaning they are most sensitive to a soil that is continually wet or near saturated. And a saturated soil means three possible things: less rooting activity, more risk of soil-borne diseases, and reduced nodulation and nitrogen fixation.

I discount soil-borne diseases because the symptoms would be different and more isolated to spots in the field based on the level of pathogen present. The field-wide symptoms of a general malaise with yellowing suggest that the wet environment is impacting nodulation and nitrogen fixation. June is a key month for forming nodules and feeding nitrogen to the soybean plant, and the crop is missing that important nutrient resource.

Soybeans have an inherent ability to recover when growth is stalled and they can compensate for periodic losses in yield due to weather-related stress. Those plants that haven’t drowned out are more than likely experiencing a malaise due to wet feet with reduced growth and nitrogen fixation. Naturally, when weather and soil conditions return to normal, soybeans should bounce back and begin to look more normal. They will have the whole month of July and August to flower, set and begin to fill pods. The question is, how much yield has been lost due to the setback in growth in June?

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com.