The following factors affect the rate of absorption of water by the roots:

• Available Soil Water.
• Soil Temperature.
• Aeration of Soil.
• Concentration of the Soil Solution.

Available Soil Water

The part of soil moisture in excess of that present at permanent wilting point is called available water. Between field capacity and permanent wilting point, the absorption goes on fairly well. Below the permanent wilting percentage water uptake is very slow. High soil water content, in the range above field capacity, retards root absorption because of decrease in soil aeration.

Soil Temperature

It has been frequently observed that irrigating plants with cold water results in wilting of plants. At temperature below 10°C absorption is much decreased. At low temperature, water becomes more viscous and hence can not move freely. Permeability of protoplast to water also decreases in low temperature. Another possibility is, that lower temperature decreases rate of respiration, which consequently retards rate of active absorption. At soil temperature above 30°C or 35°C, certain plants, like lemon, vine etc., show retarded absorption.

Aeration of Soil

Absorption of water by roots of most plants is more rapid in well aerated soil than in less aerated one. Oxygen deficiency is the primary factor in the retardation of the rate of absorption. When roots are exposed to common effect of oxygen deficiency and CO2 excess, the effect on water absorption is approximately equal to that under oxygen deficiency alone. Exposure of roots to pure CO2 or to its high concentrations (50% or more) has a remarkable effect on absorption of water. This may be caused by the effect of CO2 in decreasing the permeability of root cells to water.

Concentration of the Soil Solution

In humid regions the concentration of soil solution is small, therefore water potential is higher in the soil solution than the root cells. Hence, absorption of water by the roots increases. In alkali or saline soils, however, the concentration of dissolved salts is very high in the soil solution and hence absorption of water is retarded. Halophytes, the plants which can live in saline or alkali soils, are an exception. Water absorption in such plants is not decreased due to increased concentration of solutes in soil solution.

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