The 1996 growing season has managed to continuously baffle and frustrate farmers, literally flip-flopping from one extreme to the next. The early season started out dry and had many predicting another drought. Then in May, the rains began and it appeared there would be no end to the deluge in many parts of the state. In many cases, field work was halted for more than six weeks. The last couple of weeks has seen a return to warmer and drier conditions. This has many farmers wondering, once again, if drought conditions will resume and what will be the effect of drier conditions on the 1996 corn crop.
Temperature alone is not a good indicator of stress in corn plants. The term "discomfort index" often is used to describe how uncomfortable a person becomes on days when temperatures climb and relative humidity varies. With regard to humidity, the discomfort index of plants is opposite to that of people. People need to lose water to keep cool; plants need to retain water to avoid wilting. The discomfort level for plants is, therefore, highest on clear, bright days with low relative humidity. This is because on bright, sunny days with low humidity, transpiration may exceed the rate of water uptake by roots causing the plant to wilt. Loss of turgidity in plant cells also results from moisture stress. This loss of turgidity causes stomata to close, which lowers CO2 uptake by the leaf and reduces dry matter accumulation. When humidity is high, crops can tolerate high temperatures because there is little moisture lost through evaporation.
Corn has a relatively high water require-ment. In the central Corn Belt, the amount of water used by the crop, plus that lost by evaporation from the soil surface, generally exceeds normal season rainfall by 3 to 5 inches. This precipitation deficit is offset by water stored in the soil from early season rainfall. Most Iowa soils will hold approximately 2 inches of available water per foot of depth in the upper 5 feet of soil. During rapid growth in the later vegetative stages of development, a corn plant will use about 0.2 to 0.25 inches of water per day. This may increase to almost 0.33 inches of water per day during pollination. Stress conditions will become evident in the corn plant when 50 percent of the available soil water has been depleted.
A consequence of particular concern is the extent of corn root development given the high moisture conditions in the early 1996 growing season. In soils with good structure, corn roots tend to follow the moist soil zone as it moves downward during dry weather. When soil moisture is adequate to excessive in the top foot of soil, corn roots do not extend to lower depths, choosing to grow where their needs are met closer to the soil surface. As soils dry, the lack of root extension into lower soil depths results in corn plants that are moisture stressed, even though subsoil moisture levels may be adequate to support corn growth. Another irregularity in an already anomalous year!
This article originally appeared on pages 127-128 of the IC-476(18) -- July 15, 1996 issue.