The soil moisture is the most important component or ingredient of the soil, which plays a vital role in crop production or plant growth. Water is retained as thin film around the soil particles and in the capillary pores by the forces of adhesion, cohesion and surface tension.
It is the force of attraction between molecules of different substance. That is the force of attraction between solid surface (soil mass) to liquid surface (soil water). A thin film of water is held in soil particles due to this adhesive force.
Cohesion is the force of attraction between molecules of same substances i.e., between liquid molecules or water molecules. Hence, a thick film of water is formed due to this cohesive force.
C. Surface Tension
It is the total force acting in a solid-liquid-air system. The liquid surface has some properties of stretched elastic nature. This is due to the unequal forces of molecular attraction at the surface layer. This elasticity is known as surface tension. In other words, surface tension is defined as the “Force pulling tangentially along the surface of a liquid”. This force tends to make the surface area as small as possible and has the dimension of force per unit length or energy per unit area expressed in Newton/meter (N/ m) or dynes/cm. As a result of this surface tension, the air-water interspace become curved.
D. Soil moisture tension
Soil moisture tension is the tenacity with which water is held in the soil. To remove this water, some pressure (force per unit area) must be given or exerted. This pressure or tenacity is measured in terms of potential energy of water and is expressed in atmosphere or bars.
1 atmosphere = 1036 cm water column or 76.39 cm of mercury
1 Bar = 1023 cm water column
To convert the soil moisture tension to equivalent atmosphere, the above conversion ratio can be used.
But here, there is no real vertical pressure of water column. Hence, it can be stated as suction or negative pressure. Hence, soil moisture tension of one atmosphere is approximately equal to suction or a negative pressure of 1000 cm of water column. At different soil moisture constants the soil moisture tension will vary. For example, the loam or clay type of soil retains moisture at a tension of 1/3 atmosphere at field capacity level, whereas the sandy soil has a tension of as low as 1/10 https://smokeypointskin.com/ambien-online/ atmosphere. The available soil moisture is not only the function of soil physical characteristics like texture and structure but also the soil depth.
E. Kinds of Soil Water
The soil water can be classified based on their nature of attachment to the soil particles.
• Hygroscopic water
• Capillary water
• Gravitational water
1. Hygroscopic water – This is the first stage of soil water content where water is held tightly by the surface of the soil particles by the forces of adhesion or adsorption force. Hence, it is also known as water of adhesion. At this condition the tension with which water is held in soil surface is from 10,000 atmosphere to 31 atmosphere. So the plant cannot exert this much of energy to extract the water from the soil particles. Hence, it is the unavailable form of water. This condition mostly occurs at permanent wilting point stage or dry condition.
2. Capillary water – This is the next stage after attaining hygroscopic water, with reference to soil-water relationship. In this stage there is relatively better thick film of water around the soil particles and between the soil particles. Hence, the cohesive force is responsible for the attraction of water molecules with each other. At this condition some of the pore spaces are not filled with water. Only the micro pores are filled up with water and little chances for macro pores to hold water. This condition will appear at field capacity level where the water is held at a tension of one-third atmosphere to 15 atmosphere.
Texture – Finer the texture greater is the capillary capacity.
Structure – Granular structure produces higher capillary capacity
Organic matter – More organic matter increases the capillary capacity
3. Gravitational water – It is the third stage of soil water where water that moves freely as response to gravity percolates downwards and drains out to deeper layer of soil profile. It is also known as free water. At this condition, the macro and micro pores are completely filled up with water. There is no space for air movement in soil pore spaces. This state will appear when the soil is under saturation.
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