soil water is water available for consumption by plants .
Soil water has been classified from a physical and biological point of view as Physical classification of soil water, and biological classification of soil water.
Physical classification of soil water
Gravitational water occupies the larger soil pores (macro pores) and moves down readily under the force of gravity. Water in excess of the field capacity is termed gravitational water.
Gravitational water is of no use to plants because it occupies the larger pores. It reduces aeration in the soil. Thus, its removal from soil is a requisite for optimum plant growth. Soil moisture tension at gravitational state is zero or less than 1/3 atmosphere.
Factors affecting gravitational water
- Texture: Plays a great role in controlling the rate of movement of gravitational water. The flow of water is proportional to the size of particles. The bigger the particle, the more rapid is the flow or movement. Because of the larger size of pore, water percolates more easily and rapidly in sandy soils than in clay soils.
- Structure: It also affects gravitational water. In platy structure movement of gravitational water is slow and water stagnates in the soil. Granular and crumby structure helps to improve gravitational water movement. In clay soils having single grain structure, the gravitational water, percolates more slowly. If clay soils form aggregates (granular structure), the movement of gravitational water improves.
Capillary water is held in the capillary pores (micro pores). Capillary water is retained on the soil particles by surface forces. It is held so strongly that gravity cannot remove it from the soil particles.
The molecules of capillary water are free and mobile and are present in a liquid state. Due to this reason, it evaporates easily at ordinary temperature though it is held firmly by the soil particle; plant roots are able to absorb it. Capillary water is, therefore, known as available water. The capillary water is held between 1/3 and 31 atmosphere pressure.
Factors affecting capillary water:
The amount of capillary water that a soil is able to hold varies considerably. The following factors are responsible for variation in the amount of capillary water.
- Surface tension: An increase in surface tension increases the amount of capillary water.
- Soil texture: The finer the texture of a soil, greater is the amount of capillary water holds. This is mainly due to the greater surface area and a greater number of micro pores.
- Soil structure: Platy structure contains more water than granular structure.
- Organic matter: The presence of organic matter helps to increase the capillary capacity of a soil. Organic matter itself has a great capillary capacity. Undecomposed organic matter is generally porous having a large surface area, which helps to hold more capillary water. The humus that is formed on decomposition has a great capacity for absorbing and holding water. Hence the presence of organic matter in soil increases the amount of capillary water in soil.
The water that is held tightly on the surface of soil colloidal particle is known as hygroscopic water. It is essentially non-liquid and moves primarily in the vapour form. Hygroscopic water held so tenaciously (31 to 10000 atmospheres) by soil particles that plants can not absorb it.
Some microorganisms may utilize hygroscopic water. As hygroscopic water is held tenaciously by surface forces its removal from the soil requires a certain amount of energy. Unlike capillary water which evaporates easily at atmospheric temperature, hygroscopic water cannot be separated from the soil unless it is heated.
Factors affecting hygroscopic water:
Hygroscopic water is held on the surface of colloidal particles by the dipole orientation of water molecules. The amount of hygroscopic water varies inversely with the size of soil particles. The smaller the particle, the greater is the amount of hygroscopic water it adsorbs.
Fine-textured soils like clay contain more hygroscopic water than coarse-textured soils. The amount of clay and also its nature influences the amount of hygroscopic water. Clay minerals of the montmoril1onite type with their large surface area adsorb more water than those of the kaolinite type, while illite minerals are intermediate.
BIOLOGICAL CLASSIFICATION OF SOIL WATER
Biological classification is based on the availability of water to plant. in this classification soil water is classified as available water, unavailable water and super available or superfluous water
available soil water
available soil water is that water that is between wilting coefficient and field capacity. it is obtained by subtracting wilting coefficient from the moisture equivalent. the soil moisture between field capacity (1/3 atmosphere) and wilting point (15 atmospheres) is readily available moisture.
what is field capacity?
field capacity is the maximum quantity of water which a soil can retain against the force of gravity. value of field capacity os -1/3 bar.
what is wilting coefficient?
the wilting coefficient is the level of soil moisture at which water becomes unavailable to plant and permanent wilting ensues.
unavailable soil water
the unavailable water includes the whole of the hygroscopic water plus a part of capillary water below the wilting point.
what is wilting point?
wilting point also known as the permanent wilting point is defined as a minimal point of soil moisture the plant requires not to wilt. if moisture decrease to this or any lower point a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours (wikipedia)
the wilting point depends on plant variety but is usually around 1500 Kpa (15 bars). at this stage, the soil still contains some water but it is difficult for the roots to extract from the soil. nearly 15 bars of tension is required to extract water by plants. at this limit, if no additional water is supplied to the soil, most of the plants die
moreover the moisture content at the wilting point varies with soil texture. fine-textured soil retains a higher amount of water (26%-32% v/v) than coarse-textured soil (10%-15% v/v) at the wilting point.
types of water that are not available to the plants are:
- hygroscopic water
- fraction of inner capillary
- water vapour
super available or superfluous soil water
this is soil water beyond the field capacity stage. it includes gravitational water plus part of the capillary water removed from large interstice. this water is unavailable for the use of plants because it is lost due to deep percolation. the preference for superfluous water in the soil for a longer period is harmful to plant growth.