Retention of Water by Soil: 3 Ways

This article throws light upon the three ways by which water is retained in the soil. The ways are: 1. Cohesion and Adhesion Forces 2. Surface Tension 3. Polarity or Dipole Character.

Way # 1. Cohesion and Adhesion Forces:

These two basic forces are responsible for water retention in the soil. One is the attraction of molecules for each other i.e., cohesion. The other is the attraction of water molecules for the solid surface of soil i.e., adhesion. By adhesion, solids (soil) hold water molecules rigidly at their soil—water interfaces.

These are molecules in turn hold by cohesion. Together, these forces make it possible for the soil solids to retain water.

Way # 2. Surface Tension:

The phenomena is commonly evidenced at water—air interfaces. Water behaves as if its surface is covered with a stretched elastic membrane. At the surface, the attraction of the air for the water molecules is much less than that of water molecules for each other.

Consequently, there is a net downward force on the surface molecules, resulting in sort of a compressed film (membrane) at the surface. This phenomena is called surface tension.

Way # 3. Polarity or Dipole Character:

The retention of water molecules on the surface of clay micelle is based on the dipole character of the molecule of water. The water molecules are held by electrostatic forces that exist on the surface of colloidal particles. By virtue of their dipole character and under the influence of electrostatic forces, the molecules of water get oriented (arranged) on the surface of the clay particles in a particular manner.

Each water molecule carries both negative and positive charges. The hydrogen end of a water molecule is positive while oxygen end is negative. The clay particle is negatively charged. The positive end of water molecule gets attached to the negatively charged surface of clay and leaving its negative end outward.

The water molecules attached to the clay surface in this way present a layer of negative charges to which another layer of oriented water molecules is attached (Fig. 4.1). The number of successive molecular layers goes on increasing as long as the water molecules orientate.

As the molecular layer gets thicker orientation becomes weaker, and at a certain distance from the particle surface the water molecules ceases to orientate and capillary water (liquid water) begins to appear.

Due to the forces of adsorption (attraction) exerted by the surface of soil particles, water gets attached on the soil surface. The force of gravity also acts simultaneously, which tries to pull it downwards. The surface force is far greater than the force of gravity so water may remain attached to the soil particle.

The water may remain attached to the soil particle or move downwards into the lower layers, depending on the magnitude of the resultant force.