EXCESSIVE soil water lessens the exchange of air between soil and atmosphere. Therefore, wet soil conditions are generally accompanied by oxygen deficiency.

This deficiency causes reductions in root  respiration and total root volume, increased resistance to transport of water and nutrients  through the roots, and formation of toxic compounds in soils and plants. Roots of practically all non-aquatic plants are injures if the soil is allowed to remain waterlogged.
Continuously poor aeration results in death of cells, a decrease in cell permeability or death of roots. The amount of injury depends on the plant species, the stage of plant development, soil and air temperature, and on the duration of waterlogging. A considerable amount of oxygen is required in the soil for mineralisation 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 started later,  is slower that 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 transport  carbon dioxide produced by roots, microorganisms, and chemical reactions is facilitated. Lowering soil moisture content also results in a change in heat budget and higher soil temperatures. This change can be expected to occur in well-drained soils.
Therefore, it is necessary especially at this time of the year for farmers (particularly vegetable farmers) to pay strong attention to drainage especially in the clayey soils.