Excessive soil water lessens the exchange of air between soil and the atmosphere. Therefore, wet soil conditions are generally accompanied by oxygen deficiency. This deficiency causes reductions in root respiration and total root volume, increases resistance to the transport of water and nutrients through the roots, and facilitates the formation of toxic compounds in soils and plants. The roots of practically all non-aquatic plants are injured if the soil is allowed to remain waterlogged.
Continuous poor aeration results in the death of cells, a decrease in cell permeability, or even the death of roots. The amount of injury depends on the plant species, the stage of the plant’s development, the soil and air temperature, and the duration of the waterlogging. A considerable amount of oxygen is required in the soil for the mineralization of nutrient elements from organic matter by microbiological activity.
Deficient aeration reduces this microbiological activity, decreasing the rate at which nitrates are supplied. Consequently, a tendency towards nitrogen deficiency exists in waterlogged soils. All biological processes are strongly influenced by temperature. Wet soils are cold and crop growth starts later and is slower than in dryer soils.
The direct aim of drainage systems is to lower the moisture content of the upper soil layers so air can penetrate more easily to the roots, and facilitate the transport of carbon dioxide produced by the roots. In addition, microorganisms and chemical reactions are facilitated. Lowering the soil moisture content also results in higher soil temperatures. This change can be expected to occur in well-drained soils.
Therefore, it is necessary, especially at this time of year, for farmers (particularly vegetable farmers) to pay careful attention to drainage, especially in clayey soils.