Dr. Dan Talks Agronomy

One of the things I noticed this summer is how soybeans have lagged in development. The canopy has a yellowish hue and a general malaise seems to have settled over many fields. At the same time, immediately adjacent cornfields are deep green and growing like crazy.

These symptoms were not isolated spots in fields that might indicate compaction, water logging, iron chlorosis or even SCN -- all which can cause yellowing and stunting. The malaise I've noticed has been field wide and growth seems to have stalled out. While I've also been hearing about some carryover problems from fall burndown applications, what I'm seeing is different than erratic emergence and damaged plants.

We know that the spring was cold and emergence was delayed and uneven because soil conditions, moisture and temperature were variable. It didn't help when June turned wet with rainfall totals ranging from 6 inches to 24 inches in many regions. Our farm in northeast Nebraska received 15 inches in June (by my dad's rain gauge tally). Peter Bixel with SciMax Solutions told DTN that 23.5 inches of rainfall fell in north-central Iowa where 8 inches would have been plenty.

We have all heard that soybeans don't like wet feet and they are more sensitive than corn to soil that is continually wet or near saturated. Saturated soil leads to less rooting activity, more risk of soil-borne diseases and reduced nodulation/nitrogen fixation.

While wet conditions can cause soil-borne diseases, those problems would typically appear in spots within the field. The widespread yellowing I'm seeing suggests that the wet environment probably affected nodulation and nitrogen fixation during the month of June.

Fred Below, agronomist at the University of Illinois, agrees. "I have seen these same symptoms repeatedly and have wondered if it could be poor nodulation due to the wet conditions. I know that in our Six Secrets [soybean] plots that have the MESZ [MicroEssentials slow-release fertilizer with sulfur and zinc] treatment and our nitrogen on bean plots, we are not seeing these same symptoms," Below said.

The rains have backed off in July, the weather is warming; the soil is drying and the soybean crop is greening up and growing again. The question is will the crop catch up and has this malaise had an impact on yield?

The good news is soybeans have the inherent ability to recover when growth is stalled and compensate for periodic losses in yield due to weather stress. During the 2012 drought, soybeans stopped growing and just survived in the field during the months of June, July and early August -- losing a whole crop of flowers and pods. When the rains returned in mid-August in some parts of the country, soybeans came back to life, the remaining pods survived and filled out and yields were surprisingly near normal and in some cases better than average. This was totally unexpected and showed us how resilient the soybean plant is and how it can compensate for extended periods of stress.

So the malaise we noticed in June is probably due to wet feet, which reduced growth, nitrogen fixation and probably slowed or prevented early flowering. With weather and soil conditions returning to normal and a full soil profile, soybeans appear to be bouncing back and look more normal. They have the rest of July and early August to flower, set and begin to fill pods.

Still, the setback in nodulation and loss of four weeks of optimal nitrogen fixation in June brings up the question of sidedressing. Indeed, this may be a good time to put on 40 or 50 pounds of supplemental nitrogen. Normally, a top-dress application of nitrogen on soybeans leads to inconsistent yield results. But in a year like this with reduced nitrogen fixation, reduced nitrogen mineralization and increased nitrogen loss from leaching and denitrification, you could predict that soybeans would have a stronger nitrogen response.

Dan Davidson can be reached at AskDrDan@dtn.com

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