Availability of Soil water:
The source of soil water is rain. Soil is the main reservoir of water for plants. The amount of water in soil is variable, which depends on the nature of soil, amount of rainfall and vegetation. Different types of water present in the soil differ in their availability to the plants.
Types of water:
A part of rainwater does not enter the soil but is drained away from soil surface along the slope and gets collected in water reservoir such as lakes, river, ponds etc. It is called runaway water and is not available to the plants.
After heavy rainfall some amount of water percolates down due do the force of gravity. This water is called gravitational water. It accumulates in the deep situated zone of the earth, called water table or groundwater. It lies beyond the reach of water-absorbing roots and therefore, is not available to the plants.
This water is held by thin capillaries formed by the soil particles, after the movement of gravitational water has stopped. It comprises the bulk of water remaining in soil after gravitational water has drained away.
Water is present in micropores of soil or narrow spaces of soil. It occurs in the form of thin films and retained by forces of surface tension. It does not respond to gravitational pull. Capillary water is readily available to plant and is the major source of water to the plants.
Hygroscopic water :
Hygroscopic water is a part of soil water which is retained very tightly by colloidal soil particles. Water is held tightly around the soil particles as a result of cohesive and adhesive forces and cannot easily be removed by the plants. Hence, it is not available to the plants.
Chemically combined water:
Some amount of water is chemically combined to minerals present in the soil. It occurs in the form of hydrated oxides of aluminum, iron, silicon etc.
Such water is not easily available to the plants as the root hairs are incapable of extracting them from the chemical compounds. This form of water is also not available to the plants.
Wilted plants mostly during summer when receive a shower of rain become fresh because of the immediate absorption of water through the aerial parts.
The amount of available water in a soil depends upon the texture and structure of the soil .For example, the sandy soil which has coarse particles is unable to hold much water due to bigger pore spaces. Clay retains much water due to smaller pore spaces, but easily becomes waterlogged. Loam soil, is a mixture of sand, silt and clay, and is the best for plant growth.
The amount of water actually retained by soil is the field capacity or water holding capacity. Most of the water runs away as a run-away or gravitational water. Soil moisture beyond the field capacity produces water logging.
Field capacity is equal to the sum total of water retained by hygroscopic and retained water.
Wilting is the shrinkage in volume of water in plant cell sufficient to cause them to lose all or most of their turgor . Drooping, rolling and folding of leaves in young plants are examples of wilting.
Incipient wilting is the condition when the wilting symptoms are not visible externally and the water content of plant cell decreases. Temporary or transient wilting is the condition where most of the herbaceous plants usually wilt during hot summer days and regain their freshness or turgor during night. If the absorption of water totally ceases or the plant cells do not get water from any sources, the plant undergoes permanent wilting. The plant does not regain its turgidity unless water is supplied.
Permanent Wilting Percentage
Permanent wiling percentage (PWP) or permanent wilting coefficient (PWC) is the percentage of water in the soil when a plant or plants growing in it has just reached a condition of permanent wilting. It varies in different types of soil, for example, clay has higher Permanent wiling percentage (PWP) than sand. At permanent wilting percentage, the soil contains about 10% of water which is either present in extremely fine micropores or in the non available stage.
The total water content present in the soil is holard. The water available to plant is chresard. It consists roughly of 75% of capillary water. The rest i.e., hygroscopic , combined water, water vapour and 25% of capillary water is echard water .
Importance of water for plants:
Water plays a vital role in the life of a plant. Some of the important functions of water are as follows:
- Water is an excellent solvent.
- Water constitutes more than 80% of the cell protoplasm. Such high proportion of water in the protoplasm is essential for its normal functioning.
- It helps in the maintenance of turgidity in plant cell, which is necessary for growth and maintenance of shape.
- It acts as a reagent in various chemical reactions in plant cell.
- Water is a raw material in the synthesis of organic compounds. Besides, it is an essential medium for the activity of enzymes.
- Water maintains a uniform temperature inside the plant body .
- It helps in translocation of chemical substances.
- Water has high latent heat, and thus they have cooling effect on evaporation.
- Molecules of water have great cohesive force among themselves, which help in ascent of sap.
- Water helps in respiration, photosynthesis, growth, absorption of dissolved substances and many other plant processes.