SOIL-WATER SYSTEM

Available water for crop plants.   Soil is a heterogeneous mass and consists of three phases, viz. The solid phase, the liquid phase and the gaseous phase. Mineral matter, consisting of sand, silt and clay and organic matter, forms the solid phase which serves as a framework (matrix) with numerous pores of irregular shapes and different sizes holding air and water in various proportions. Soil is a porous medium, and serves as a water reservoir or bank. Water is deposited in this bank as rain or irrigation, and plants withdraw it during their growth.

Water is retained by a soil particle in the form of a thin film around it, and in the numerous small pores of the soil matrix with forces, such as surface tension capillarity, cohesion and adhesion. The salts present in soil water further add to these forces by way of osmotic pressure. Plants, therefore, need to exert at least an equal amount of force for extracting water from the soil mass for their growth.

Immediately after rain or irrigation, water infiltrates into the soil and continues to move in the soil mass to deeper layers because of the gravitational force. The downward movement of water practically ceases after a certain time (normally after 48 to 72 hours). The water retained in the soil under this situation is termed 'field capacity' which forms the upper limit of the available soil moisture for crop plant. In other words, any further addition of water will not be retained by the soil, but will be lost through deep percolation beyond the roots of a crop, thus making it unavailable for the growth of its plants. After the wetting of the soil, as evaporation and transpiration continue, the soil water goes on diminishing till a point is reached when plants are unable to extract it. The moisture content at this stage is termed 'permanent wilting-point' and this sets the lower limit of the availability of soil water. In other words, any moisture below this point will not support plant growth. The range of soil water between the field capacity and the permanent wilting-point is termed 'available soil water for crop growth'. The values of the available water-holding capacity of different major soil groups are shown in Table 5. The available soil water-holding capacity increases mainly with the fineness of texture and the content of organic matter.

Availability of soil water for crop growth.   Three classical hypotheses have been put forth for the relative availability of soil water in the available range.

(i) Water availability and, consequently, the crop growth is equal and uniform over the entire range from the field capacity to the permanent wilting-point. This holds good generally for perennial species, such as orchard and tree crops whose dense root mass permeates the soil matrix thoroughly.

(ii) Water availability and crop growth proceed uniformly from the field capacity to a certain critical point beyond which crop growth decreases rapidly till the permanent wilting-point is reached. This view holds good for most of the seasonal field crops maturing up to the seed stage.

(iii) The availability of water and the rate of crop growth decrease gradually as the soil water content decreases from the field capacity to the permanent wilting-point. This holds good generally for most of the forage crops and those grown vegetatively.

The effects of the crop and climatic factors on the availability of soil water can also be significant. The responses of crops to soil water are ultimately reflected through the plant-water status, and the atmospheric aridity acts as the primary driving force for the absorption of moisture from the soil. A given crop loses water at different rayes under different conditions of evaporative demands of the climate.

SUITABILITY OF SOIL FOR IRRIGATION

For most crop plants, except rice, the ideal soil for irrigation is that which is deep, without any water-table, has high water-holding capacity, infiltration rate and permeability, and low salt content. The loams and clay loams are generally good soils for irrigation, since the run-off, the number of irrigations necessary and the investment for drainage are low as compared with those in the case of other soils. Any soil can be put under irrigated agriculture permanently but only with due care and caution which, on certain soils, may be beyond economic limits. The heavy soils often need surface as well as subsoil drainage. The light sandy soils involve a high application of costly inputs besides a considerable wastage of water.

In the case of rice, the soils with low percolation rates are ideal for economizing on water. Land submergence has been found to be a beneficial practice for obtaining high rice yields. But during this submergence, huge quantities of water, as high as 60-70 per cent, are lost through deep percolation. In India, nearly half the water resources are diverted to rice alone. The proper selection of land for growing rice is, therefore, vital for utilizing the water resources efficiently and economically.

PLANT AND WATER ABSORPTION

A plant extracts water from the soil with the hairs on the absorbing surface of their elongating roots, through osmosis and diffusion. Inside the plant, water moves along the concentration gradient across the root tissues to the xylem vessels and from there upwards to the stem and the leaves. On the surface of a leaf, there are numerous openings, stomata, through which water is lost in the form of vapour on account of the incident heat energy. The accumulated water vapour around the leaf tissues is further carried away by the wind into the atmosphere. The process of loss of water from the plants through the stomata and plant surface is known as transpiration. Huge amounts of water, practically all that is absorbed, is lost in this way by the plants continuously throughout their life. The quantity of water utilized in metabolic activities by a plant is insignificantly small as compared with the amount lost through transpiration. The primary cause of transpiration is the solar energy available for the vaporization of water from the leaf surface. Water is pulled out as a continuous film from the root hairs to the leaf surface because of the sink strength of the atmosphere.

Stomata are photosensitive cells and are open only during the day in most of the plant species. They close at night and hence there is practically no nocturnal transpiration. The stomata regulate transpiration to some extent by closing their apertures when the soil is dry and there is shortage of moisture in the atmosphere. The exchange of gases for photosynthesis also takes place through the stomata and, hence, the closure of stomata means no photosynthesis, i.e. no growth and accumulation of dry matter.

Plant nutrients also move primarily through water in the plants and are translocated to different plant parts along with the transpiration stream. The closure of the stomata, therefore, also reduces the nutrient uptake.