Furthermore, crops in the low-lying areas of these fields look yellow and are stunted while the crops in the rest of the field are pale green and slow to develop. Weeds seem to be getting ahead of the crops in fields that have been too wet to spray. The rain has also increased the relative humidity, increasing the risk of disease development including the risk of scab on small grain crops that are starting to flower. Crops that were recently planted are struggling to emerge.
Waterlogging (flooded/ponded/ saturated soils) affects a number of biological and chemical processes in plants and soils that can impact crop growth in both the short and long term. Waterlogging deprives plants of oxygen. Plants need oxygen for cell division, growth and the uptake and transport of nutrients. Since oxygen diffuses through undisturbed water much more slowly than a well-drained soil, oxygen requirements rapidly exceed that which is available when soils are saturated. The rate of oxygen depletion in a saturated soil is impacted by temperature and the rate of biological activity in the soil.
The cooler temperatures that have been associated with the recent wet weather have helped delay the adverse effects of waterlogging on any emerged crops. Generally, the oxygen level in a saturated soil reaches the point that is harmful to plant growth after about 48-96 hours (longer when temperatures are cooler). In an effort to survive, tissues growing under reduced oxygen levels use alternate metabolic pathways that produce by-products, some of which are toxic at elevated levels. Crops like small grains and corn tend to be more sensitive to waterlogging when their growing point is still below the surface of the soil (before the 5-6 leaf stage).
Crops can differ in their tolerance to waterlogging. Data from differing sources suggest a possible ranking of waterlogging tolerance as follows (most tolerant to most susceptible): rice, soybean, oats, wheat, corn, barley, canola, peas, dry beans and lentils. Waterlogged conditions also reduce root growth and can predispose the plant to root rots, so the ultimate effect of excess moisture may not be known until late in the season. It is common to observe plants that have experienced waterlogging to be especially sensitive to hot temperatures and display nitrogen and phosphorus deficiencies later in the season due to restricted root development. Yield losses can occur even if these obvious visible symptoms are not observed.
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Waterlogging can also indirectly impact cereal growth by affecting the availability of nitrogen in the soil. Excessive water can leach nitrate nitrogen beyond the rooting zone of the developing plant, particularly in well-drained lighter textured soils. In heavier soils, nitrate nitrogen can be lost through denitrification. The amount of loss depends on the amount of nitrate in the soil, soil temperature, and the length of time that the soil is saturated. Losses of up to 75% of the soil applied N have been reported when soils are saturated for 5 days and soil temperatures are 75 to 80 degrees.
Managing yellow corn and small grains
The yellowing of the crop that can be commonly observed can largely be attributed to the lack of nitrogen in the plant, either because it has been lost from the soil through leaching and/or denitrification or because the roots system has been so restricted by the lack of oxygen in the soil, that the plant is not able to access nitrogen in sufficient quantities.
Late planted small grains that are still tillering may benefit from an addition of nitrogen as far as yield and protein are concerned, if it can be applied. There is little that can be done to enhance the yield of wheat that is in the boot stage or later. An improvement in protein content may be possible with an application of urea or UAN at the boot stage, if there is sufficient rainfall to get the nitrogen into the soil so that plant can take it up by the plants. Foliar applied UAN after flowering can also be used to boost protein.
Corn has not yet reached the period of its most rapid N use, so it is possible to enhance corn yield with an application of N. The sooner that these applications can be made the better, as more green tissue means more potential photosynthesis and more yield.
