ISSUE 8 July 2, 2009
CEREAL CROP GROWTH IN FLOODED AND SATURATED SOILS
The recent heavy rains in eastern North Dakota falling on soils that were already nearly saturated have resulted in flooding, ponding, and saturated soils. Waterlogging (ponding/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. The primary cause of damage to cereal crops by waterlogging is oxygen deprivation or anoxia. 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 regulated by temperature, with faster depletion occurring when temperatures are higher. The cooler weather we have been experiencing this week, will delay the adverse effects of waterlogging, somewhat.
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, some of which are toxic at elevated levels. 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. Much of the late planted small grain and corn crops in northern ND may still be in this sensitive stage and can be killed if soils are saturated beyond 48 hours when soil temperatures exceed 65 degrees. In southern ND, early planted corn is now in the six leaf stage and wheat and barley is jointing or in the boot. These older plants will probably not be kill by a few days of flooding. They will appear yellow and will grow slowly, however. Water-logged 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 to display nitrogen and phosphorus deficiencies later in the season due to restricted root development. Yield losses can occur, however, even if these obvious visible symptoms are not observed.
Water-logging can also indirectly impact cereal growth by affecting the availability of nitrogen in the soil. Excessive water will leach nitrate nitrogen beyond the rooting zone of the developing plant, particularly in well-drained lighter textured soils. In heavier soils, nitrate nitrogen will be lost through denitrification. The amount of loss 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 found losses from denitrification between 1 and 5% 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. The yield response of small grains to late applied N may not be as significant, but increases in protein levels can be achieved through this practice. 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.
Extension Agronomist for Cereal Crops