Water and Soil Solution

Water is the largest component of the soil, next to mineral matter. In the soil water are dissolved many chemical compounds. These are formed from weathering of rock particles, decomposition of organic matter, activities of micro-organisms or reactions between roots of plants and soil constituents. Soil water, therefore, may rightly be spoken of as soil solution.

Before studying the relation of soil water to plants, it is necessary to consider the relations between soil water and soil itself. The water content of soil under certain conditions is defined as the weight of water in a given weight of soil in an oven at 105°C. It is expressed in terms of percentage of dry weight. Field capacity of the soil is the amount of water held under field conditions when the soil is wet by rain or irrigation and the excess has drained out. When rain falls on dry earth, it percolates down through capillaries formed by continuation of the interstices between soil particles. The soil becomes moistened to a certain depth below which percolation stops. At this stage, the water becomes distributed through the soil and water films become thin and capillary movement of water stops. This is due to force of gravity being equal to force of capillarity, which does not allow any more water to go down by gravity. Field capacity may thus be defined as water content of the moist soil after capillary movement of water has become negligible. The water held between the soil particles in the form of thin films is called capillary water.

If more rain falls on the same soil before any considerable amount of water has been lost from it by evaporation or transportation, the already moistened portion will not show any increase in water content but the layers of soil just below it will become moist up to field capacity. This is because of further capillary movement of water. The rain water is simply pulled through the upper moist zone by gravity. Successive rain-falls will deepen the layer of soil which has been moistened. Ultimately, water reaches a certain maximum depth where the soil is completely saturated with water. This level of complete saturation of water is called water-table and the water collected there is known as gravitational water.

If available capillary water is removed by transpiration or by drying of the soil in air, water is retained in the finest capillaries as well as in the form of thin films a few molecules thick, surrounding the soil particles. This water is referred to as hygroscopic water. Water may also be chemically combined in the structure of soil minerals. Neither hygroscopic nor chemically combined water is availed of by the plants. The capillary water is principally available to the plant and is utilized by it.

Types of Soil Water

The gravitational water stored in the water-table is also unavailable to plant because neither the roots of most plants reach the water-table nor does water rise up to the level of roots by capillarity from the water-table.

A plant growing in a soil can not extract all the water from it but will always wilt even though there is some water left. This water remaining in the soil is known as unavailable water. The water content of the soil to which it is reduced, when a plant growing in it has just reached a condition of permanent wilting, is termed as permanent wilting point or wilting co-efficient or wilting percentage. A permanently wilted plant is that which will not recover its turgidity unless water is supplied to the soil.

The plant may also wilt when water in the soil is still easily available. This is called transient or incipient wilting. This occurs when rate of loss of water is more than rate of absorption.

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