Soil Water: Classification and Sources | Soil Science

In this article we will discuss about:- 1. Classification of Soil Water 2. Sources of Soil Water 3. Movement of Water in Soils 4. Losses.

Classification of Soil Water:

Soil water plays very important role in the growth and development of plants. Plant absorbs nutrients from the soil in the form of soil solution. The soil water containing nutrient ions is known as ‘soil solution’.

1. Physical Classification of Soil Water:

This classification is based on moisture coefficient such as:

(a) Gravitational Water:

It is a free water in excess of capillary water. Gravitational water may be defined as the quantity of water removed from the soil by the forces of the gravity. The water saturates the soil and usually percolates downward to the water table under the influence of gravity.

Gravitational water results in the loss of plant nutrients from the soil. This type of water is not available to plants. Gravitational water remains in macropores and is held at a tension of one-third atmosphere or less. Water in excess of field capacity is termed ‘Gravitational water’.

When the gravitational water percolates down and reaches to the level of parent rock, it is called ‘Ground water’. This water is not desirable for the growth of plant, because there will be no proper aeration due to the presence of this water.

Characteristics of Gravitational Water:

(i) Gravitational water is free water and this water drains out by gravity. It can also be called ‘drainage water’.

(ii) It is loosely held at a tension of one third atmosphere or less.

(iii) It is of no use to plants as it is undesirable water and is removed in drainage. The nutrients are leached away with drainage water in which plant nutrients are dissolved.

Factors Affecting the Amount of Gravitational Water:

The amount of gravitational water held by the soil depends on some factors as follows:

(i) Soil Texture – It plays a great role in controlling the movement of gravitational water. The flow of water is proportional to the size of soil particle. Water percolates more easily and rapidly in sandy soils than the clay soil due to big size of sand particles.

(ii) Soil Structure – It also plays a great role in controlling the movement of water. The movement of gravitational water is slow in soil having platy structure. The granular and crumby structure help to improve the movement of gravitational water. The gravitational water percolates more slowly in soil having single grain structure.

(iii) Organic Matter – The organic matter has the property of holding water in the soil. The movement of gravitational water decreases in the soil having high organic matter content.  

(b) Hygroscopic Water:

Hygroscopic water may be defined as the percentage of water occurs as a thin film (4-5 μ thick – 1 μ or micron = 0.0001 milimeter) surrounding the particles of soil or organic matter. It is absorbed by vacuum dried soil from atmosphere of water vapour by attractive force in the surface of soil particles and immobile in nature. This water is held with a tension of 31 atmosphere or greater and it can be driven out from the soil by heating the soil at a temperature of 100°C to 110°C for 8-10 hours. This water is not available to plants. Movement of this water takes place only in gaseous phase.

Characteristics of Hygroscopic Water:

(i) Hygroscopic water is held by hygroscopic coefficient.

(ii) This water is held mostly by soil colloids and organic matter.

(iii) This water is held with a tension of 31 atmosphere. PF of this water is 5.4.

(iv) It moves mostly in vapour form.

(v) This water is not available to plants.

Factors Affecting Hygroscopic Water:

The amount of hygroscopic water depend on some factor as follows:

(i) Soil Texture – The amount of hygroscopic water varies inversely with the size of particles. Fine texture soil (i.e. clay soil) contains more hygroscopic water than coarse texture soil (i.e. sandy soil).

(ii) Soil Structure – Crumb and granular structure contain more hygroscopic water. The montmorillonite clay mineral has the capacity to absorb more water than kaolinite type. But the illite mineral is of intermediate type.

(iii) Organic Matter – The soil rich with organic matter contains more hygroscopic water as it has the property of absorbing more atmospheric and rain water.  

(c) Capillary Water:

The water held between the field capacity and hygroscopic coefficient is known as ‘Capillary Water’. Capillary water is held by the soil due to capillary forces. It is available form of water and is held between field capacity (0.33 atmosphere) and hygroscopic coefficient (31 atmosphere) in microporespaces.

Capillary water is most useful water for plants. All the water is not be available to plants, only a portion of water which is retained in the soil with less force can be utilized by plant. Capillary water is known as available water. Capillary water held at tension greater than 15 atmosphere is not available to plants. This water evaporates easily at ordinary temperature.

Characteristics of Capillary Water:

(i) Capillary water is held in capillary pores present in soil.

(ii) It is held by between field capacity and hygroscopic coefficient in micropore spaces. Capillary water is capable of movement upwards, downwards or laterally.

(iii) The capillary water is held with a tension from 0.33 to 31 atmosphere.

(iv) The movement of water takes place from thicker film to thinner film. Capillary water is retained by surface tension force as a continuous thin film around the soil particles and also in the capillary spaces.

Factors Affecting the Amount of Capillary Water:

The amount of capillary water held by a soil depends on some factors as follows:

(i) Surface tension – The amount of capillary water increases with an increase in surface tension.

(ii) Soil texture – The fine textured soil (i.e. clay soil) has capillary pore in which capillary water is held. This is mainly due to greater surface area and a greater number of micropore spaces.

(iii) Soil structure – Platy structure contains more water than granular structure. Compaction of heavy soil decreases the porespaces and the capillary retention of water also decreases.

(iv) Organic matter – Organic matter increases the capillary pores and it has high capacity of holding capillary water. The humus that is formed on decomposition of organic matter has great capacity for absorbing and holding of water.

(d) Combined Water:

It is the water of chemical compounds held by chemical forces of molecules (e.g. CuSO₄, 2H₂O). It can be driven out from the compound only at bright red heating.

2. Biological Classification of Soil Water:

(i) Available Water:

It is the quantity of water present in the soil from field capacity to permanent wilting point. This water is available to plant for its growth and development. The amount of available water that can be stored in the root zone of the soil depends on its depth, soil type and soil structure.

(ii) Unavailable Water:

The soil water below the wilting co-efficient is not available to plants and is termed as ‘unavailable water’. It is held by the soil particles so tenaciously that plant cannot extract it from the soil.

(iii) Superfluous Water:

It may be defined as the quantity of water in between maximum retentive capacity and field capacity. Gravitational water is known as ‘superfluous water’. This water is subjected to gravitational forces and is removed easily from the soil. It has no use for the plants.

Sources of Soil Water:

Following are the sources of soil water:

(i) Rainfall:

Rainfall is the primary sources of soil water. But it is not available throughout the year in all arable areas and in appropriate quantities required by each crop variety at its different stages of its growth. There are two distinct monsoon winds in India, namely ‘South- West and North-East Monsoon Winds’.

The South-West monsoon begins to blow over the country early in June. By the middle of July, the whole country is in its grip. The rainy season continues upto the first week of October when the South-West monsoon withdraws rapidly from the country. The South-West monsoon is called ‘grand period of rainfall in India’. In November-December, the South eastern portion of Peninsula is affected by North-East monsoon which is main source of irrigation for this part of this country.

(ii) Irrigation:

Rainfall reserved in other water bodies is used as resources of irrigation water for an indefinite time.

Following are sources of irrigation water:

(a) Surface Water:

Surface water is provided by flowing water of river or from still water of tanks, ponds, lakes, pools or artificial reservoirs such as dams, barrages and diversionary bunds with different storage capacity. Irrigation through canals under different river valley project in our country is done profitably as the water from the river is carried to the field by flow due to gravity and is therefore cheap.

Water from other sources is carried to the field by lifting them with the help of water lifting devices used for irrigation. River lift irrigation project is also functioning in our country. The water of rivers or streams is lifted to irrigate a limited command area adjacent to such water courses.

(b) Ground Water:

Water is reserved underground enriched by percolation of surface water through the porous state of earth’s crust and this type of water is known as ‘ground water’. The subterranean water is tapped by digging or drilling wells. It may be shallow (30-40 ft) or deep (85-100 ft or more). The water from wells is lifted before it can be used for irrigation. The command area for irrigation of shallow tube-wells and deep tube-wells is 10-12 acres and 100 acres respectively.

Movement of Water in Soils:

Soil water movement occurs when there is a potential difference between different points in the soil system. The water moves from higher potential to lower potential.

Different forms of Water Movement:

I. Liquid Water:

(a) Saturated Flow:

When the most of the pores are filled with water, the condition is called ‘saturated’. This occurs in the zone of ground water and sometimes in the soil after heavy rains or during irrigation. Saturated flow takes place when the soil is saturated. The direction of flow is from a zone of higher moisture potential to a lower moisture potential. Generally water percolates down (vertically) into the lower layer. Horizontal flow also occurs with very less rapidity in comparison to vertical flow. Horizontal movement is much more evident in the clay soil.

The movement of percolation water depends on some factors as follows:

(i) Soil texture – The flow of water is proportional to the size of particles. The bigger the particles, the more rapid is the flow or movement. Therefore, water percolates more easily and rapidly in sandy soils than in clay soils.

(ii) Soil structure – In clay soils having single grain structure (structureless), the gravitational water percolates more slowly than in those having an aggregate structure (granular or crumb structure). In platy structure, saturated flow is poor in comparison to granular structure.

(iii) Temperature – High temperature helps the water flow down.

(iv) Organic matter – Organic matter helps to maintain a high proportion of macropores. Larger the porespace, greater the flow.

(v) Pressure – The movement of gravitational water is also influenced by the resistance offered by the entrapped soil air. As a result of pockets of air, the soil air pressure is increased and the percolation is decreased. This is more especially the case in lower layers.

From a practical point of view, saturated flow is very important.

(b) Unsaturated Flow:

Soil pores contain some air as well as water is called ‘Unsaturated soil’. Under field conditions, most soil water movement occurs where the soil pores are not completely saturated with water. The soil macropores are mostly filled with air and the micropores with water and some air. Water movement under these conditions is very slow compared to that occurring when the soil is saturated.

II. Water Vapour:

(a) Diffusion – Water vapour may move by diffusion as a result of vapour pressure (partial pressure) differences due to different temperature or concentration.

(b) Mas flow – Water vapour may flow in a mass with the other gases of the system in response to differences in total pressure.

Losses of Soil Water:

The soil waters are subjected to be lost through some process as follows:

1. Percolation and Run-Off:

The downward movement of water through soil is called ‘percolation’. Percolation losses of water are maximum under humid climate with high rain fall and the percolating waters also leach out important plant nutrients from soil. Percolation losses of water is high in light soil (i.e. Sandy soil) and is low in heavy soil (i.e. clay soil). The water lost by surface flow is known as ‘run-off’. In most soils where the land is slopping or the soil is somewhat impermeable to water, a considerable amount of precipitation is likely to be lost by run off. Run-off water causes erosion of soil.

2. Evaporation:

Evaporation is the process by which the liquid water is gradually changed to vapour from the surface of water or soil. The resulting vapour is lost to the atmosphere. A certain amount of water may be lost by evaporation from lower layer of soil through the cracks in the soil. The rate of evaporation depends upon several factors, such as temperature, wind velocity, humidity, moisture content and nature of the soil.

3. Transpiration:

Water is primarily lost from a plant in the form of vapour through a process known as ‘transpiration’. Transpiration is a physiological process which is very important process for the growth of the plant. The combined loss of moisture by evaporation and transpiration is called ‘evapotranspiration’. Weeds also causes the loss of water from crop field.

Factors Affecting Evapotranspiration:

(i) Radiant Energy – The primary source of radiant energy is the sun. In arid region, more radiant energy is received from the sun and rate of evapotranspiration is increased.

(ii) Atmospheric vapour pressure – The vapour pressure of the atmosphere helps to control evaporation from soils and plants. The evaporation is slow under high vapour pressure as is the case of humid days. In humid region with comparable temperatures, evapotranspiration losses are considerably less.

(iii) Temperature – The evaporation of water is greatly influenced by temperature. The vapour pressure increases at the surface of green leaf or soil with the increase of temperature, but the vapour pressure of the atmosphere is not increased. So the evapotranspiration increases.

(iv) Soil moisture – The evapotranspiration depends on the moisture content of soil and this process will be obviously less when the soil contains less moisture.

(v) Winds – A dry wind will continually sweep away moisture vapour from a wet surface. Wind blowing at a high velocity continuously removes moist air near the surface of the plant or soil and as a result more water evaporates from the said areas.