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Issue No. 19, Article 3/August 13, 2010

Good Root Systems May Prove Important As Soybean Seeds Fill in The Heat

Temperatures have remained high over the last few weeks, and in a few areas, dry soil conditions are a concern. Illinois temperatures have been 2 to 4°F above normal daily averages over the last 30 days, and 3 to 7°F above over the past 7 days. The southern region has been the farthest above normal and has been the driest. Over the last 30 days Illinois has ranged drastically in rainfall; areas in the north and west are 3 to 7 inches above normal, and areas in the central, east, and south are normal to 2 inches below normal (Figure 1). Unfortunately, the areas rated above normal received much of their precipitation from heavy rainfall events that also caused late-season flooding damage. The last week has been starting to put us on the drier-than-normal side, with little rain in the near-future forecasts.

Figure 1. Departures in 2010 from normal precipitation for July 11 (left) to August 9 and August 1 to August 9. Maps generated by the Illinois State Climate Office, Illinois State Water Survey, University of Illinois at Urbana-Champaign.

For the most part, however, soybeans look pretty good, with few visual signs of drought stress so far across much of the state. Some fields have plants that are curling leaves in the heat of the day to conserve water, and many fields could use rain due to the critical yield-setting growth stages currently developing. August rainfall is always a primary factor in yield-determining models, and this year will be no exception. The August 8 USDA report has soybeans 94% blooming and 71% setting pods, numbers that are ahead of the 5-year averages by 4% and 5%, respectively. In our trials, fields planted in mid-June are approaching R4 or full pod, when pods are 3/4 inch long at one of the upper four nodes. The earlier-planted fields (April to Mid-May) are approaching R6, or full seed, when a pod on one of the upper four nodes has its cavities filled with seed. A pod from one of the upper four nodes from a May 5 planting in Champaign has seed cavities 50% to 70% full.

Soybean pod with seed partially filling the seed cavity; from an upper node of a May 5 planting near Champaign.

The great majority of acres in the state are somewhere between full pod and full seed, so seed fill is occurring rapidly. The entire R5 growth period of beginning seed to full seed is a very critical time for avoiding plant stresses. Under limited heat and drought stress, the total crop growth rate, and thus the total plant dry weight accumulation, remains high. The demand for water for evapotranspiration to move nutrients (N, P, and K) is thus also very high during these seed development stages. Throughout R5, the nutritional need of the plant to fill seeds is acquired roughly half from soil uptake and half from plant redistribution. This ratio favors soil uptake earlier in R5 and shifts slowly to plant redistribution later in R5. Because nutrient uptake from the soil is so critical and water demand remains high, root health may be crucial in determining yield in drier areas.

As you scout for the variety of diseases and insects that we have received reports about, pay attention to root health by digging roots in areas that show more or fewer drought stresses. A strong root system should have good depth from a relatively straight taproot, good secondary branching with plenty of active nodules (pink inside when split), and minimal or no signs of soybean nematode cysts.

Healthy root system with a good straight taproot, secondary roots and nodulation, and a few soybean nematode cysts; dug from a May 5 planting near Champaign.

--Vince M. Davis

Vince Davis

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