The below mentioned article provides biology notes on Water Absorption from Soil.
Soil is the upper weathered, humus, mineral water and air containing layer of the earth’s crust which supports plant life. Water is an important constituent of soil because all land plants depend upon it for their requirement of water. Deep in the soil and above the impermeable stratum, water occurs freely in the previous rocky matter. It is called ground water. Ground water bearing pervious straturn is known as aquifer.
The upper layer of ground water is called water table. In the region of water table soil is completely saturated with water. Air is excluded. Tube wells and hand pumps bring ground water to the surface. Very few plants can send their roots up to fringe of the water table due to deficiency of air.
The plants which do so, are called phreatophytes, e.g., Populus deltoides, Alhagi pseudalhagi, Tamarix, Prosopis cineraria etc. Phreatophytes have been used successfully to locate under-ground water since the period of Varahmihira.
Soil water, important to most plants, is the one present in 1-2m of soil because their roots are generally restricted to this region.
Water is present in the soil in five forms— capillary water, gravitational water, hygroscopic water, combined water and water vapours. The ultimate source of all soil water is rain or irrigation. A part of rain water does not enter the soil but is drained away from soil-surface along the slope. It is called run-away water or run-off (Fig. 11.18).
It is the water present in soil narrow spaces or microspores of soil having a diameter of 20 pm or below. The amount of capillary water which can be present in a soil depends upon the abundance of microspores.
Capillary water is held in the soil by capillary forces. It, therefore, does not fall down to water table by gravity. Only capillary water is available to plant roots for absorption. However, roots are unable to absorb water from capillaries below the diameter of 0.2 µm.
Excess rain or irrigation is not useful to plants. Rather it can be harmful because excess of water would expel soil air. This happens when flow of gravitational water is prevented and soil is covered with water for longer periods. The optimum or maximum amount of water retained per unit dry weight of soil after the stoppage of gravitational flow is called field capacity. It is 25-35% in common loam soils.
Soil moisture beyond field capacity produces water logging. If soil water is not replenished from time to time, a stage is reached when the plants growing in it become permanently wilted and die.
It is known as permanent wilting percentage (PWP) or permanent wilting coefficient (PWC). At permanent wilting percentage the soil contains about 10% of water which is either present in extremely fine microspores or in the non-available stage.
Available Water (Fig. 11.19):
The total water content present in the soil is called holard. The water available to plants is chresard (= growth water = available water). It consists of roughly 75% of capillary water. The rest of soil water (hygroscopic, combined, water vapour and 25% of capillary water) is called echard or unavailable water.