- The moisture content of a sample of soil is usually defined as the amount of water
lost when dried at 1050C, expressed either as the weight of water per unit weight of dry
soil or as the volume of water pr unit volume of bulk soil. Although useful, such
information is not a clear indication of the availability of water for plant growth. The
difference exists because the water retention characteristics may be different for different
soils. - The forces that keep soil and water together are based on the attraction between
the individual molecules, both between water and soil molecules (adhesion) and among
water molecules themselves (cohesion). In the wet range surface tension is the most
important force, while in the dry range adsorption is the main factor. - Thus, the higher the moisture content, the smaller is the attraction of the soil for water. The energy of water tension in a soil depends on the specific surface as well as the structure of the soil and on
its solute content. When water is present in fine capillaries, the energy with which it is attached is a function of the surface tension and capillary size but when it is present in bigger pores, it is bound loosely to the soil and can be acted upon by gravity. - When salts dissolve in water, they decrease the free energy of water. Soil water, by virtue of the salts dissolved in it has a lower free energy than pure water. Further, soil water that is bound to solid particles as hygroscopic water is tightly held by the surface of contact and has a low free energy by virtue of binding forces.
- Thus, there are two types of interactions which decrease the free energy of water, namely, (i) due to the solubility of salts, (ii) due to the interaction of water and solid surface. Both these add together in decreasing the energy of soil water. Thus, the retention of water in the soil and the tendency of water to
move in the soil are consequences of energy effects.
