The following points highlight the four soil factors influencing the water absorption rate by roots.
Factor # 1. Concentration of the External Soil Solution:
Absorption of water, in general, depend upon the difference between the osmotic concentration in the cell sap and the external soil solution. Thus an increase in the concentration of the external water medium, must reduce the difference between the osmotic concentration in the external solution and the root hair cell sap at least for some time, thereby reducing the rate of water absorption.
Such reductions are actually observed in the plants growing in normal water culture solutions, where an addition of 1 % NaCl or some other salt immediately brings about a decrease in the rate of water absorption.
This reduction may only be temporary as the plants soon adapt themselves by increasing the osmotic concentration of the cell sap. This adaptation or increasing the osmotic concentration of the cell sap may be due to absorption and diffusion of dissolved substances into the cell from the surrounding medium or it may be due to the production inside the cells of osmotically active substances.
This adaptation is, however, limited. Most species of plants develop normally only when the osmotic concentration of the soil solution does not exceed a few atmospheres. Exceptions are provided by the halophytes which are native to the saline soils of high osmotic pressure. Halophytes adapt themselves to such soils by increasing proportionately the osmotic concentration of the root cells.
Factor # 2. Soil Temperature:
In general, the rate of water absorption by roots increases with rise in temperature of external medium, but at 35° C. or above in most species of plants the water intake is greatly reduced and soon replaced by withdrawal.
The withdrawal of water from the cell sap may be due to the effect of high temperature on the cell organisation, particularly the plasma membrane of the cell which most probably loses its semipermeable nature.
Factor # 3. Aeration of the Soil:
In general, absorption of water by root systems of most species of plants certainly proceeds much more rapidly in well-aerated soil than those which are poorly aerated.
Poor aeration actually results in a reduced water absorption most probably due to a deficiency of oxygen in the neighbourhood of roots or due to an accumulation of carbon dioxide which results from anaerobic respiration of the root cells under such conditions.
The growth of hydrophytes in water-saturated soils with scarcely any oxygen, however, can take place only because these plants have well developed and continuous air passages from the leaves down to the roots which store large amounts of oxygen in their intercellular spaces which is available for the normal respiration of root cells.
Factor # 4. Some Substances Present in the Soil:
Some substances, occurring in the soil, affect the absorption of water on account of their chemical nature quite apart from their osmotic properties. One of them is humus acid, which retards water absorption in bog soils, owing to the chemical nature of the compound which probably is toxic to the plasma membrane of the root cells.