Excessive rainfall this spring, following an unusually wet winter, has resulted in extensive flooding in many regions of North Dakota. Even soils that are not visibly flooded quickly become saturated after a rain because there is little evapotranspiration occurring as a result of the low temperatures and lack of an established crop.
Waterlogging (flooded/ponded/saturated soils) affects a number of biological and chemical processes in plants and soils that can affect crop growth in the short and long term.
"The primary cause of waterlogging in crop plants is oxygen deprivation or anoxia because excess water does not react chemically with the plant," says Joel Ransom, North Dakota State University Extension Serviceagronomist. "Plants need oxygen for cell division, growth, and the uptake and transportation of nutrients. Since oxygen diffuses through undisturbed water much more slowly than well-drained soil, oxygen requirements rapidly exceed that which is available when soils are saturated."
The rate of oxygen depletion in saturated soil is affected by the temperature and rate of biological activity in the soil. Faster oxygen depletion occurs when temperatures are higher and when soils are actively metabolizing organic matter.
Cooler weather will delay the adverse effects of waterlogging on emerged crops. 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 alternative metabolic pathways that produce byproducts. Some of the byproducts are toxic at elevated levels.
"Germinating seeds or emerging seedlings are very sensitive to waterlogging because their level of metabolism is high," Ransom says. "Crops, such as small grains and corn, tend to be more sensitive to waterlogging when their growing point is below the surface of the soil (before the five to six leaf stage)."
With the exception of winter wheat, all of the small-grain and corn crops in the state still are in these sensitive stages (if planted at all) and can be killed if soils are saturated beyond 48 hours and the soil temperature exceeds 65° F.
Crops can differ in their tolerance to waterlogging. Data from differing sources suggest a possible ranking of waterlogging tolerance. The most tolerant to most susceptible are rice, soybeans, oats, wheat, corn, barley, canola, peas, dry beans and lentils. Growth stage and variety can affect this ranking.
"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," Ransom says. "It is common to observe plants that have experienced waterlogging to be especially sensitive to hot temperatures and to display nitrogen (N) 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."
Waterlogging can impact cereal plant growth indirectly by affecting the availability of N in the soil. Excessive water can leach nitrate N beyond the rooting zone of the developing plant, particularly in well-drained, lighter-textured soils.
In heavier soils, nitrate N 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 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," Ransom says.