Adhesion Cohesion and Soil Moisture tension
What are the forces responsible for water retention in Soil?
Hydrogen bonding accounts for two basic forces responsible for water retention and movement in soils
These are Adhesion and Cohesion
The forces of adhesion and cohesion make it possible for the soil solids to retain water and control its movement and use
What is Adhesion?
Adhesion besically is the force of attraction between the two dissimilar molecules
Eg – attraction between soil and water molecule
In case of Soil, it is force of attraction between water molecules and solid surfaces
By adhesion some water molecules are held rigidly at the surfaces of soil solids
The adhesive force diminishes rapidly with distance from the solid surface
What is Cohesion?
Cohesion is basically the force of attraction between similar molecules
Eg – force of attraction between water and water molecule
In case of soil, it is the force of attraction between the water molecules for each other
These tightly bound water molecules hold by cohesion other water molecules farther away from the solid surfaces
The cohesion of one water molecule to another results in water molecules forming temporary clusters that are constantly changing in size and shape as individual water molecules break free to join up with others
This cohesive force between water molecules also allows the solid to indirectly restrict the freedom of water for some distance beyond the solid-liquid interface
What is Soil moisture tension?
Soil moisture tension is a measure of the tenacity with which water is retained in the soil and reflects the force per unit area that must be exerted by plants to remove water from the soil
Any thing in nature is attracted by gravitational force
Accordingly, even the water i.e. moisture in the soil is also attracted by gravitational force acting downwards
The plants have to exert some force to pull this water against the gravity into their own systems
What is the unit of measuring Soil moisture tension?
A common means of expressing tension is in terms of a bar, which equals the pressure exerted by a vertical water column
A pressure of one bar is approximately equal to the hydrostatic pressure exerted by a vertical column of water having a height of 1023 cm or a hydraulic head of 1023 cm
Similarly 1.0 bar is equivalent to 0.9869 atmospheres
This value approximates the standard atmosphere, which is the average air pressure at sea level i.e., equal to 14.7 lbs/in2 or a mercury head of 76 cm or 760mm.
The suction of water having a height of 10cm is equal to 0.01 bars or 10 millibars, that of a column of 100 cm high about 0.1 bar or 100millibars
Similarly 1.0 bar is equal to 100 centibars
The higher the height of water column or bars or atmospheres the greater the tension or suction measure
Further readings and references on Adhesion Cohesion and Soil Moisture tension
Handbook of Agriculture