Significant quantities of rain have fallen over the past two months with the heaviest amounts in the last two weeks falling North of a line from Princeton to Muncie, Indiana. Knox and Dubois counties received amounts in the 4 to 4.5 inch range during this period with heavy rains occurring on June 17. Reporting stations in Tippecanoe, Whitley and Porter counties recorded 5.5 to more than 7 inches during this same period with heavy rainfall on June 24. Much of this rain fell on soils that were already quite wet from and, in some cases, already saturated from previous rainfall.
The net result of all of this rain is saturated soils in much of the state with significant ponding and some flooding. Anytime these types of wetness problems occur, the immediate question relates to the length of time that a given crop can survive if covered by water. For soybeans, the length of time that a plant can remain completely submerged and survive is between 2 and 4 days assuming the soils were not saturated when the flooding occurred.
The length of time is also related to temperature and cloud cover. Hot, sunny conditions may result in death of the plant after 2 days or less while, with cool, cloudy conditions, soybeans may survive after 4 days or more. If a portion of the plant is exposed, soybeans have been known to survive after 7 days of partial submersion. If flooding occurs in fields where the soils have already been saturated for a few days, the soybean plants are less likely to survive.
Soybeans require well-aerated soils to grow vigorously. Saturated soils, with no water on the above ground portion of the plant, can result in poor root and plant growth and some plant death from root rot diseases. After only a few days of saturated soils, soybean plants become a lighter green color due in part to poor nitrogen fixation since little oxygen is present in these saturated soils.
In addition to the saturated soils, nighttime temperatures have been too cool for good soybean plant growth. Soybeans perform best with nighttime temperatures in the 70 to 80 degree range. During the last two-week period, we have had a number of cloudy days that have also contributed to the poor growth and light green or yellow color of the soybeans.
Finally, post-emergence herbicides are being applied to a soybean crop under considerable stress and in many cases adding yet another stress to the plants and contributing to the light green to yellow color.
The bottom line is that growing conditions are far from ideal for the soybean crop. Warm nighttime temperatures, bright sunny days and a few days free of added rainfall will bring this crop around and change its appearance significantly. These changes need to occur shortly since the soybean plant has moved into the reproductive stages of growth and continued stresses can begin to take a toll on yield potential.