Soil Water Content |Its Types, Retention and Measurement

The water present inside the pores of soils is called the soil water. This water comes from the surface of the soil by the process of infiltration (the movement of water into the soil), percolation (the downward movement of water within the soil), and seepage from the nearby water supply canals.

soil water content
Credit: U.S. Fish and Wildlife Service

The movement of water within the soil depends upon its permeability. Permeability is the ability of the soil to transmit the water or air. Soil permeability is commonly measured in terms of the rate of the flow of water through the soil in a given time period.

Structure of Water

Water is a simple compound containing atoms of oxygen and hydrogen. One molecule of water contains one oxygen atom and two hydrogen atoms. These elements are bonded together by a covalent bond, where hydrogen shares its electron with an oxygen atom to form the bond, which is arranged in a ‘V’ shape arrangement.

soil water

The atoms form a 105-degree angle with an oxygen atom. Due to this V-shaped arrangement, water is polar in nature which means there is an uneven distribution of electrons.

The water molecules have a negative charge near the oxygen atom and a positive charge near the hydrogen atom.

Soil moisture retention

Water is retained in the soil due to the force of adhesion, cohesion force, capillary force, and surface tension. These forces hold the water against the gravitational force.

Cohesion force – It is the force of attraction between two similar molecules. Through the cohesive force, the atoms of oxygen get attracted to the other atoms of other water molecules present in the soil to form a bond. This is a cohesion bond. The bond between two molecules through a single hydrogen atom is called a hydrogen bond. The water film in soil maybe 15-20 layers thick, and approximately 60% of this water is available to the plant, which they utilize for their growth.

Adhesive force – Attraction between two different molecules is the adhesive force. Due to adhesive force, the water is attached to the particles of soils. Adhesive water is not available for the plant because the force is strong, so the root of the plant cannot absorb the water.

Water-repellent soils are the soil which is coated with some oily or fatty substance present in the soil or may come from the surface water due to leaching, this coat prevents the attraction of soil with water molecules, and such soils are water repellent soils.

Capillary force – The force that holds water in between the capillaries of the soil particles is the capillary force. The capillary water is retained on the soil particles by adhesive force between the soil particles and water molecules.

The heat of wetting – when the clay particles are hydrated, a certain amount of energy is released, which is known as the heat of wetting. The heat of wetting decreases as the layers of water molecules increases on the soil particles which means the first layers of water molecules will release more amount of energy as compared to its substituent layers.

Types of soil water

There are two classifications of soil water

  • Physical classification
  • Biological classification

Physical classification

It is further divided into three types

  • Gravitational Water
  • Capillary water
  • Hygroscopic water

Gravitational water – Gravitational water is the water that is held at a potential greater than -1/3 bar. This water moves under the force of gravity. It is mainly found in the macrospores of soil, and this water is littles use to plant.

Capillary water – Capillary water is held at a potential of -1/3 bars to -31 bars. Capillary water is found in the micropores of the soil; these water are retained by the adhesive force between the soil particles and water molecules. Capillary water is available for the use of the plant.

Hygroscopic water – Hygroscopic water is the water that is held at a potential of more than -31 bars. They are mainly found in the vapor form in soil due to which it is not available for the plant.

Biological water

This classification is based on the water which is available to the plant for their utilization.

Available water – Available water is the water that is held in the soil at a pressure of -1/3 bars (field capacity) and -15 bars (permanent wilting coefficient). These water are easily available for plant use.

Unavailable water –Unavailable water is the water that is held in soil at a pressure of -15 bars. These waters are not available for plant use.

Factors affecting the soil water

Soil water is affected by the texture, structure, organic matters, soil pores sizes.

1) Soil pore size – soil pore means the voids present in between soil particles, which are filled by water or air. The size of soil pores affects the soil water as the small and medium-size pore holds the water with more strength than large ones.

2) Soil structure – soil structure differs from soil to soil; soil structure determine the quantity of water a soil a hold

3) Organic matters – Organic matters is very important for the soil as it increases the soil fertility and also soil water holding capacity. It increases the pores in the soil.

4) Soil texture – Soil texture affects the soil water content as the fine texture soil helps water to percolate in soil from the upper surface.

Soil moisture measurement

Soil moisture can be measured by

  • Tensiometer
  • Gravimetric method
  • Electrical conductivity method

Soil moisture by Tensiometer

It is used to determine the soil moisture tension in the soil. It consists of a glass tube with a porous ceramic cup and is filled with water, and the tube is attached with a vacuum gauge.

It is used in irrigation scheduling. It can also be used to determine the amount of water need to irrigate.

Gravimetric method

It is a mathematical method used to determine the soil moisture content. In this method, the soil is taken from the field in a container,  weight is noted, which will be the initial value, and after that, it is oven dried for 24 hrs, and again weight is taken. After that moisture is calculated by

%moisture= (Initial weight – oven dry weight)/oven dry weight x100

Electrical conductivity method

This works on the electrical conductivity method. In this method, gypsum blocks are buried in the soil at the desired depth; these gypsum blocks are connected with two electrodes, then conductivity is measured with the help of the Wheatstone bridge. This method is used to calculate soil moisture.

Importance of soil water

  • Soil water is an important agent for physical, chemical and biological weathering
  • It is an important medium for the transportation of minerals in the plant body
  • It maintains cell turgidity and regulates body temperature.
  • It helps in the gaseous exchange during transpiration.

It is an excellent solvent for most of the soil nutrients.

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