Soil Water: Importance, Concepts and Classification

After reading this article you will learn about:- 1. Importance of Soil Water 2. Structure of Soil Water 3. Different Forces of Retention 4. Energy Concepts 5. Methods of Expression 6. Classification.

Importance of Soil Water:

Water, an excellent solvent for most of the plant nutrients, is a primary requisite for plant growth. Water serves four functions in plants: it is the major constituent of plant protoplasm (85-95%); it is essential for photosynthesis and conversion of starches to sugars; it is the solvent in which nutrients move into and through plant parts; and it provides plant turgidity, which maintains the proper form and position of plant parts to capture sunlight. In fact, the soil water is a great regulator of physical, chemical and biological activities in the soil.

Plants absorb some water through leaf stomata (openings), but most of the water used by plants is absorbed by the roots from the soil. For optimum water used, it is vital to know how water moves into and through the soil, how the soil stores water, how the plant absorbs it, how nutrients are lost from the soil by percolation, and how to measure soil water content and losses.

Soils also serve as a regulated reservoir for water because it receives precipitation and irrigation water. A representative cultivated loam soil contains approximately 50% solid particles (sand, silt, clay and organic matter), 25% air and the rest 25% water (Fig. 7.1). Only half of this water is available to plants because of the mechanics of water storage in the soil.

Structure of Soil Water:

Water can participate in a series of reactions occurring in soils and plants, only because of its structural behaviour. Water is simple compound, its individual molecules containing one oxygen atom and two much smaller hydrogen atoms. The elements are bonded together covalently, each hydrogen or proton sharing its single electron with the oxygen. Instead of the atoms being arranged linearly (H—O—H) the hydrogen atoms are attached to the oxygen as a ‘V’ shaped, arrangement and are separated from each other by angle of only 105° (Fig. 7.2.).

Polarity:

Due to ‘V’ shaped structure of water, the side on which the hydrogen atoms are located tends to be electropositive and the opposite side is electronegative. Because of non-linear positions of H⁺ ions, water is polar. Polar means there is no centre of zero charge from which there is an equal charge at some distance from that centre in all directions.

The H of water in soils may bond to oxygen ions of soil mineral surfaces, thereby holding the water tightly to soil. This accounts for the polarity of water and therefore, water is most important for carrying out many reactions in soils and plants.

Different Forces of Retention of Soil Water:

Soil serves as a water reservoir but a leaky one. When too much water is added, the excess runs-off over the surface or into deeper layers. Why does the soil hold some of the water, yet allow part of it do drain deeper? Water is held in soils because of the attraction between unlike charges—a positive ion attracted to a negatively charged ion.

The positively charged hydrogen’s of water are attracted to nearly negatively charged ions, such as oxygen, even to the oxygen of another adjacent water molecule. Most soil minerals are composed of 70-85% by volume of oxygen. Hydrogen of water bond strongly to these surface oxygen atoms by adhesive bonding (the attraction of unlike molecules).

The hydrogen’s of water are also attracted (bonded) to oxygen of other water molecules, including those already adsorbed on to the soil particle surfaces. The attraction of similar or like molecules for each other is cohesive bonding. Such bonding between two molecules through a single hydrogen atom is called hydrogen bonding.

When fatty or oily substances, which are low in oxygen, coat the soil particles, water is not attracted to and held to the coated surface. Such soils are called water-repellent soils. This type of soil are formed in nature under many plant covers and after forest fires, which tend to vaporize oils and resins and drive them into the soil where they coat the soil particles and cause them to resist wetting.

Strong combined adhesion and cohesion forces cause water films of considerable thickness to be held on the surface of soil particles. Because the forces holding water in soil is attractive forces, the more surface (more clay and organic matter) a soil has, the greater is the amount of adsorbed water.

So soil holds water in two ways in the interstices or pores or capillaries between the solid particles, and by adsorption on the solid surfaces of the clay and organic matter.

The mechanism of adsorption of water on the soil surfaces are related to the adhesion and cohesion forces through hydrogen bonding and also related to the hydration of exchangeable ions which may result in some of them dissociating from the surface into the water (Fig, 7.3).

The effect of the cation on the water molecules is greater. The greater its charge and the smaller its size, so the greater its surface charge density, and these effects are influenced by the relative moisture content of the clay, by the heat evolved during wetting of clays and by the greater apparent density of the clays in water.

Heat of Solution:

When ions are hydrated, a large amount of energy is released and this is known as heat of solution.

Heat of Wetting:

When clay particles are hydrated a certain amount of energy must be released and this phenomenon is known as heat of wetting. So there is a close relationship between moisture retention in soil and the energy. The force, with which water is held, is also termed as suction.

Although soil water is held by adsorption and capillary forces, but at the outset it is important to realise that water held in even a fairly dry soil cannot be sharply separated into capillary and adsorbed water.

The soil capillaries are not straight uniform tubes, and so for that reason it is better to eliminate the word “capillary” and use the words interstices or pores to describe the spaces between soil particles. So surface tension is an important property, especially as a factor in the phenomenon of capillarity.

The phenomenon of surface tension is generally evidenced at water—air interfaces and it may be defined as the forces in dynes acting at right angles to any line of 1 cm length in the surface. At the surface, the attraction of air for the water molecules is much less than that of water molecules for each other.

Consequently there is a net down-ward (in ward) force on the surface molecules, resulting in sort of a compressed film at the surface. This phenomenon is called surface tension.

Consider water in a capillary tube having a boundary with air. The boundary layer between the water and the air is called meniscus. The meniscus is usually curved; it may make a definite angle—the angle of contact with the walls of the tube; and it puts the water column under a tension T, given by

T = 2σ/r cos α

If the water is in a circular tube of radius r, where σ is the surface tension of the water and a is the angle of contact or angle of wetting, which is usually zero for the system soil mineral particles-water-air, but may be appreciable if the soil contains much organic matter.

Energy Concepts of Soil Water:

The retention and movement of water in soils, its uptake and translocation in plants and potential evapotranspiration etc. are also related to energy. Different kinds of energy art involved including potential, kinetic and electrical. By using the term ‘free energy’ (ability to do work) energy status of water can be characterized to indicate the strength with which water is held. Several concepts have been used.

The concept of pressure—the pressure required to force the water off soil and was measured in atmospheres of pressure needed to remove water. The opposite of pressure- moisture suction or tension. Recently soil water potential is used and it may be defined as the work the water can do when it moves from its present state to a pool of water in the reference state.

The movement of water in soil takes place from a higher free energy to a lower free energy level. It is expected that there is great variability in the free energy levels of water in soils. So the tendency for soil water to move from one soil zone to another due to variation in free energy levels.

Concept of Water Potential:

Water in soil has potential energy as well as kinetic energy. Kinetic energy is very small. Potential energy may be defined as the capacity to do work. Pure water has the maximum capacity to do work. Water in soil is held by adsorptive, osmotic and pressure gradient forces and also has relatively lower capacity to do work.

However, work is necessary for the movement of water from one position to another against the force fields to which it is subjected. Extraction of water by plant roots is an example of work done on soil water. Since the term potential refers to the work done per unit quantity, it can be used quantitatively to the work done by water or work done on water as a function of its energy status.

Work is positive when water loses energy and is negative when it gains energy due to movement. For example, a stone sliding down a hill loses potential energy and does positive work, while the stone is moved back up the hill against gravity, it gains potential energy and does negative work.

Adsorbed water is less free to move as compared to water in a pool. Adsorbed water always less free energy (less ability to do work) than water in the pool (zero potential). Therefore, adsorbed water always has a negative potential; work must be done to remove the water to a free pool of water. The more tightly water is adsorbed; the more negative is the number.

Components of Total Soil Water Potential:

Based on the concepts of water potential, the total soil water potential can be defined as the work done per unit quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water from a pool of pure water at the reference point to the point under consideration against the force fields.

At equilibrium, the algebraic sum of all forces would be zero. The total soil water potential at any point of equilibrium would be equal to the algebraic sum of all the component potentials as mentioned. Each of the component potentials may be defined in principle, the work done against the respective force field.

The soil water potential is a combination of the effects of the surface area of soil particles and small soil pores that adsorb water, matric potential Ψ_m) the effects of attraction of ions and other solutes for water, solute or osmotic potential (Ψ_s) and the atmospheric or gas pressure effects, pressure potential (Ψ_p). In salt free well drained soil, matric potential is almost equal to the soil water potential (Ψ_w).

An additional effect of the position of the water (such as being elevated) compared to the reference state (the reference free energy state = 0 and is at a specified elevation) is called the gravitational potential (Ψ_g). Gravitational potential is not related to soil properties, only to the elevation of water in comparison to a reference position.

Various potentials can be written as follows:

Most of the productive soils have no depth of water standing on it and can be written as follows:

Ψ_t⋍Ψ_w⋍Ψ_m

Therefore, among all potentials matric potential (Ψ_m) is the most important and dominant for most soils. A relationship between water potential and water content in soil is presented in Fig. 7.4.

Methods of Expression of Soil Water Potential or Suctions:

Soil water potential can be measured in two units at varying energy levels in soil.

(i) pF Scale:

The free energy is measured in terms of the height of a column of water required to produce necessary suction or water potential at a particular soil moisture level. So the pF may be defined as the logarithm of centimetre height of a water column to give the necessary suction.

Here ‘p’ indicates the logarithmic value and ‘F’ indicates free energy e.g. pF₄ is equal to 10,000 centimeters of a water column height (logarithm of 10,000 = 4).

(ii) Bars or Atmospheres:

Atmosphere or Bar is the average air pressure at sea level. The term millibar (m bar) is equal to [1/10,000] atmosphere. A popular unit bar is equated to a number of other units as follows:

1 bar = 0.9869 atmospheres (approx. 1 atm.)

= weight of a 1020 cm water column

= 14.5 lbs. per sq inch

= 10⁶ dynes per sq cm

= 75.01 cm high mercury column

If the suction is very low as occurs in case of a wet soil containing large amount of water that it can hold, the pressure difference is of the order of about 0.01 atmosphere or 1.01 pF equivalent to 10 cm height of water column. Similarly, if the pressure difference is 0.1 atmosphere the pF will be 2.0 (Table 7.1).

A saturated soil has pF value 0, while an oven dry soil has a pF 7.0.

Classification of Soil Water:

There are generally two types of soil water classification based on drying of wet soils and growing plants therein. (A) physical and (B) biological.

A. Physical Classification:

Under physical classification soil water is grouped into three on the basis of retention: gravitational, capillary and hygroscopic water.

(i) Gravitational Water:

Gravitational water may be defined as the water that is held at a potential greater than -1/3 bar and that portion of the soil water that will drain freely from the soil by the force of gravity. In-spite of having low energy of retention, gravitational water is of little use to plants water occupies the larger pores resulting poor aeration. Therefore, the removal of excess water is a must for the growth of most plants.

(ii) Capillary Water:

Capillary water is held in the micro-pores of soils (capillary pores). Capillary water is retained on the soil particle by force of attraction between soil particles and water molecules (Fig. 7.5).

Capillary water is held so rigidly that the force of gravity is not able to separate it from the soil particles. Capillary water is free and moves through the soil pores because of a water potential gradient.

Capillary water may be defined as the water that is retained in the soil between the water potential of – 1/3 bar to – 31 bars. The force of retention of water molecules by the soil particle is high and part of water is available and part of it is unavailable and so all capillary water is not available to plants.

(iii) Hygroscopic Water:

Hygroscopic water is defined as the water that is held by the soil particles at a suction of more than -31 bars. It is essentially non-liquid and moves primarily in the vapour form. This water is held so tenaciously that plants are not able to absorb it and thereby unavailable to plants. Some micro-organisms can utilize such form of water.

B. Biological Classification:

There is a definite relationship between moisture retention and its utilization by plants. Biological classification is based on the availability of soil moisture to the plant. Soil water under this system of classification can be divided into three categories.

(i) Available Water:

Available water is defined as that portion of water which is retained in the soil between field capacity (-1/3 bar) and the permanent wilting coefficients (-15 bars). This water is easily usable by plants and therefore, it is called plant available water. Plant available water is equal to the difference of water percentage at field capacity and a permanent wilting point.

(ii) Un-Available Water:

Unavailable water is defined as the water which is held at soil water potential greater than -15 bars. It is unavailable to plants. It includes the whole of the hygroscopic water plus a part of the capillary water below the wilting point.

(iii) Superfluous Water:

Superfluous water is defined as the water which is retained in the soil beyond the field capacity soil moisture tension. This water includes gravitational water plus a portion of capillary water removed from large interstices. Such type of water is unavailable to plants and rather presence of such water in the soil for a long period causes harmful effect for plant growth because of lack of air.

Soil Moisture Constants:

Soil moisture constants and their approximate equivalents in bars of water potential as they affect the relative availability of water of plants are shown in Fig. 7.6.

Oven Dry Weight:

Oven dry weight is the basis for all soil moisture calculations. The equilibrium tension of the moisture at oven dryness is 10,000 atmospheres or bars (-10,000 bars of soil moisture potential). It is determined by placing the soil in an oven at 105°C until it loses no more water.

Air Dry Weight:

Air dry weight is a somewhat variable term, mainly because the moisture in the air fluctuates. Moisture at air dryness is held with a force of 1,000 atmospheres or bars (-1,000 bars of soil-moisture potential). This water is not available to plants.

Hygroscopic Co-Efficient:

Hygroscopic coefficient is determined by placing an air-dry soil in a nearly saturated atmosphere at 25°C until soil absorbs no more water. The soil- moisture tension at this point is equal to 31 bars (soil moisture potential -31 bars) and this water is not available to plants, but available to certain micro-organisms.

Wilting Co-Efficient:

Sometimes it is also used as permanent wilting point. The wilting point is defined as that amount of water which is held with water potential less than -15 bars and it is held so strongly that plants are not able to absorb it for their needs.

At this point of soil-moisture potential, the plants begin to wilt and at the very beginning of the wilting condition are sometimes recovered with the addition of water and it is then called temporary wilting point, while such wilting condition of the plant is not recovered in-spite of addition of water and then it is called permanent wilting point. Both the wilting points indicate low moisture availability to plants.

Field Capacity:

Field capacity is defined as the capacity of a soil to retain moisture against the downward pull of the force of gravity and moisture is held with soil water potential less than -1/3 bar. It is used to determine the amount of irrigation water needed and the amount of reserve soil water available to plants.

Moisture equivalent is approximately equal to the amount of moisture held at field capacity soil. The term moisture equivalent is defined as the percentage of water held by a one centimetre thick moist layer of soil after subjected to a centrifugal force of 1,000 times gravity for half an hour.

Another term “maximum water-holding or maximum water retention capacity” is also used. Maximum water holding capacity is defined as the capacity of a soil to retain water is exceeded. At this point all soil pore spaces (macro and micro pore spaces) are filled up with water and the drainage is restricted.

The water at this point is at a low soil moisture tension. Under natural field conditions only poorly drained soils are at their maximum water holding capacity for long periods of time. Soil saturation, field capacity and wilting points are shown diagrammatically as follows (Fig. 7.7).

Factors Affecting Gravitational, Capillary and Hygroscopic Water:

Gravitational:

Soil texture and structure are two main factors that affect the amount of gravitational water.

Texture:

It plays an important role in regulating the flows of gravitational water. The flow of water is directly proportional to the size of the particles. The larger the size of the particle, the more rapid is the movement of water.

Structure:

The different types of soil structure affect the gravitational water by influencing its movement as well as drainage condition of soils. For an example, the rate of movement of gravitational water is slow in platy soil structure which results stagnation of water on the soil surface.

Whereas, spheroidal soil structure helps to improve the movement of gravitational water by increasing its rate of infiltration and percolation. Besides there are other factors like, hard pans in the sub-soil horizon, compactness of soil, organic matter contact in soil etc. also affect the amount and rate of movement of gravitational water.

Capillary Water:

There are various factors to be considered that affect the amount of capillary water in soil namely, soil texture, soil structure, surface tension, organic matter content, size of capillary pores in soil, tortuosity (zigzag path) of capillary soil pores etc.

Soil Texture:

The finer the texture of a soil the larger the quantity of capillary water it holds. This is mainly attributed to the greater surface area and a large number of micro-pore spaces present in such soil.

Soil Structure:

Various types of soil structure present in diversified soils hold water of varying quantities. As for example, soils having platy structure hold excess water as that of granular soil structure.

Surface Tension:

An increase in surface tension increases the amount of capillary water. Surface tension is, therefore, an important property and factor that influence the movement and amount of water in the phenomenon of capillarity.

Organic Matter:

Organic matter plays an important role for the changes in the capillary water in soil. The presence of organic matter in the soil increases the percentage of pore spaces and consequently increases the capillary capacity of a soil.

Organic matter also influences the soil aggregation as well as formation of soil structure which also affect the amount of capillary water. Humus, a decomposed product of organic matter, has a greater capacity for holding water especially capillary water.

Size of Soil Pores:

Different sizes of soil pores hold water with different tenacity. Small and medium sized soil pores tend to hold water with much more tenacity than that of larger size soil pores.

So the movement of capillary water is largely dependent upon the size of capillary pores since different energy levels are associated With Water present in different sizes of pores. Therefore, it affects the availability of such capillary water to the plants.

Tortuosity (zig-zag path) of soil pores and entrapped air in the soil, Soil pores are not continuous, straight and uniform like that capillary glass tubes. Due to such nature of soil capillary pores, the movement of water is somewhat restricted and different. Furthermore, soil pores are field with air which may by entrapped, slowing down or preventing the movement of capillary water.

Hygroscopic Water:

The amount of hygroscopic water varies inversely with the size of soil separates. The smaller the size of soil particles the greater the amount of hygroscopic water it adsorbs. Fine texture soils like clay, clay loam soils contain more hygroscopic water as compared to coarse textured sandy soils.

The amount and nature of clay colloids also influence the amount of hygroscopic water. Soils high in colloidal materials (organic and inorganic soil colloids) will hold more hygroscopic water than soils containing low amount of clay and humus. Clay minerals of montmorillonite type having large surface area adsorb more water than that of kaolinite type of clay minerals.

Factors Affecting Available Water:

The amount of available water is influenced by a number of factors like plant, climatic and soil factors.

The plant and climatic factors are related to the losses of water vapour under the system known as ‘SPAC’ (soil-plant-atmosphere continuum). Among the soil factors matric and osmotic suction, soil depth and soil stratification or layering are most important and are discussed below:

Matric Suction:

The matric suction means suction due to soil matrix and so the matric suction is influenced by soil texture, structure, organic matter etc. Hence, the texture, structure and organic matter content etc. influence the quantity of available water in soil. The general relationship between soil moisture characteristics and soil texture is shown in figure 7.8.

As the fineness of texture increases, there is a general increase in the amount of available water. The comparative available water holding capacities in relation to water content (inches/foot of soil) are also being shown by the figure 7.8.

Organic matter also influences the amount of available soil moisture storage favourably and this favourable effect is attributable to porosity of soil resulting from well aggregation and formation of good soil structure.

Osmotic Suction:

The application of different fertilizers and naturally occurring compounds very often contribute salts to the soil. So the suction develops due to presence of soluble salts in soil and is termed as osmotic suction.

Osmotic suction effects in the soil solution will tend to reduce the range of available water in such soils by increasing the wilting coefficients. The total moisture stress in such soils at this point is matric suction plus the osmotic suction of the soil solution.

Soil Depth:

Keeping all other factors equal, deep soils will have greater available water holding capacities as compared to shallow depth soils. Soil stratification or layering will influence significantly the available water and its movement in the soil. Hardpans or impervious layers drastically reduce the rate of movement of water and also influence the penetration of roots adversely.

Hardpans also reduce the soil depth. Sometimes sandy layers also act as barriers to soil moisture movement from the finer textured layers above. Movement through a sandy layer is very sluggish at intermediate and high tensions.