ISSUE 6   June 14, 2007

EXCESSIVE RAINFALL CHALLENGES CROP GROWTH

Rainfall has been abundant in most of the state this season and in the southeast, south-central, and central regions, it has been excessive. May rainfall has been about twice the long-term average this year. And there is more rain on the way! Not surprisingly flooding, ponding and saturated soils are common. Waterlogging (ponding, saturated soils) affects a number of biological process in plants and soils and is damaging to crop growth. A number of factors, however, influence the extent of yield loss caused by excessive moisture.

Effect of water-logging on the developing plant
Crop injury from water-logging is primarily caused by the lack of oxygen. All plants need oxygen for cell division, growth and the uptake and transport of nutrients. When soils become saturated, the amount of oxygen available to plant tissues below the surface of the soil (or water level if ponding occurs) decreases rapidly as plants and microorganisms use up what is available. The movement of oxygen from the air into water/saturated soil is much slower than in a well aerated soil and much less than needed by the various organisms in the soil. The rate of depletion of oxygen in a saturated soil is dependant on a number of factors, but temperature is the most important and predictable factor; the higher the temperature the faster the rate of oxygen depletion. Generally, the oxygen level in a saturated soil reaches the point that is harmful to plant growth after about 48-96 hours. In an effort to survive, tissues growing under reduced oxygen levels use alternate metabolic pathways that produce by-products, such as lactic acid. These processes help to keep the plant alive in the short-term but elevated levels of these by-products reduce the ability of a cell to function optimally.

In cereals, the growing point is below or near the soilís surface during early vegetative growth (5-6 leaf stage in corn and small grains). While the growing point is below the soilís surface, cereals are quite sensitive to waterlogged conditions. In fields that permitted timely planting, corn is now approaching the 4-leaf stage and small grains are starting to joint and are therefore very sensitive to waterlogging. Young plants can be killed if soils are saturated beyond 48 hours, particularly when soil temperatures are high (i.e. above 65 degrees) Water-logged conditions also reduce root growth and can predispose the plant to root rots later in the season, 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 to display nitrogen and phosphorus deficiencies later in the season due to restricted root development. Leaf yellowing is the most common symptom of waterlogging in plants not killed by excessive soil moisture. Yield losses can occur, however, even if these visible symptoms are not observed.

Effect of water logging on the soil
Water-logging can also indirectly impact cereal growth by affecting the availability of nitrogen in the soil. Excessive water can cause leaching of nitrate nitrogen beyond the rooting zone of the developing plant, particularly in lighter textured soils. Furthermore, when oxygen levels become depleted, soil microbes extract oxygen from the nitrate molecule, causing nitrogen to be converted to a gaseous form that is lost to the air (denitrification). The amount of N loss through denitrification depends on the amount of nitrate in the soil (the ammonium form of nitrogen is not lost through denitrification), soil temperature, and the length of time that the soil is saturated. Research conducted in other states has found losses between 1 and 5% of the nitrate N lost for each day that the soil remains saturated. Adding additional nitrogen to fields that have had significant N losses, once they have dried, can remedy these losses, particularly for corn, which can effectively utilize N applied much later in the season than small grains. In small grains, in order to impact yield, additional N should be applied prior to the 6 leaf stage. Additional N, after this stage, however, does have the potential to increase protein levels. Before adding extra nitrogen to fields that experienced waterlogging, you should first consider the likely yield potential of the crop that has probably already been damaged. Additionally, N losses are not likely to be uniform throughout the field and additional N may only be needed in low spots where losses were the highest. If you do decide to apply some additional N, you should consider varying the rate to target those areas in the field where N is likely to be the most limiting.

Excessive moisture and disease development
The recent rainfall has made conditions ideal for the development of diseases in small grains. Consider using a fungicide with your herbicide if you are growing wheat following wheat for control of leaf diseases. In winter wheat, which is now about to head, you might consider using a fungicide for scab control as conditions are currently very favorable for scab development.

Joel Ransom
Extension Agronomist - Cereal Crops
Joel.Ransom@ndsu.edu


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