416 Mineral Nutrition of Plants 



is attained, however, moisture depletion is largely brought about by 

 plant root extraction. In this case the soluble materials in the soil 

 solution are either absorbed by the plant during moisture uptake or, 

 as is the case in saline soils, the soluble salts are largely left in the soil 

 because of the selective absorption of the plant root. Moisture depletion 

 is thus accompanied by a considerable change in the concentration of 

 the soil solution. This change in the concentration of the soil solution 

 brings about concurrent changes in the equilibrium between the 

 adsorbed and the soluble ion phases. 



It was indicated earlier that the freezing point depression gives a 

 measure of the total soil moisture stress. The aqueous vapor pressure 

 of the soil water is another measure of the free energy of the soil water 

 or the total soil moisture stress. In the plant growth moisture range, 

 the pressure of the water vapor in the soil atmosphere is always very 

 high and near the saturation pressure at the particular temperature. 

 The relative humidity range from 98 to 100 per cent more than covers 

 the available moisture range in all soils. In other words, the relative 

 humidity in soil from which plant roots are extracting water for growth 

 never goes below about 98 per cent. 



MOVEMENT OF MOISTURE IN SOIL 



The movement of water into and through soil can be conveniently 

 expressed in terms of the force which tends to produce the motion of 

 the water. Gravity and the gradient of the moisture tension in the soil 

 water are the two components that must be considered. Gravity always 

 acts in the downward direction. The component of force arising from 

 the tension gradient in the soil water, however, may act in any direction. 

 When water is at rest under gravity and hence is at static equilibrium, 

 the pressure gradient force is equal to and opposite to gravity. The pres- 

 sure gradient force in soil moisture can be conveniently estimated from 

 tensiometer readings and these instruments cover the tension range in 

 which ready movement of soil moisture takes place. The vector sum of 

 the gravity force and the pressure gradient force can be most conveni- 

 ently expressed in terms of the hydraulic gradient. When the pressure 

 force vanishes, i.e., when the soil moisture tension of the soil water is 

 everywhere the same, then the net water moving force is simply and 



