Soil Moisture Retention and Movement

• It is 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 per unit volume of bulk soil.
• 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.
• When salt dissolve in water, it decreases the free energy of water.
• Thus, there are two types of interactions which decrease the free energy of water, namely i) due to the solubility of salts and ii) due to interaction of water and solid surface.
• Thus, the retention of water in the soil and the tendency of water to move in the soil are consequences of energy effects.

Soil Moisture Regimes and their Response to crops

• Moisture regime refers to the moisture or water which is stored in the soil for subsequent used by plants over time.
• The moisture between field capacity and permanent wilting point is referred to as readily available moisture. It is the moisture available for plants to use.
• Available moisture to the plant can be categorized as unavailable, available and gravitational or superfluous.
• Unavailable moisture are held at > 31 bars (Hygroscopic water) , Available water at 1/3 – 1/10 atm and gravitational water moves freely due to action of gravity.