1. Both soil and water are essential for plant growth. The soil provides a structural base to the plants and allows the root system (the foundation of the plant) to spread and get a strong hold. The pores of the soil within the root zone hold moisture which clings to the soil particles by surface tension in the driest state or may fill up the pores partially or fully saturating with it useful nutrients dissolved in water, essential for the growth of the plants. The roots of most plants also require oxygen for respiration. Hence, full saturation of the soil pores leads to restricted root growth for these plants.

2. Since irrigation practice is essentially, an adequate and timely supply of water to the plant root zone for optimum crop yield, the study of the inter relationship between soil pores, its water-holding capacity and plant water absorption rate is fundamentally important.

Soil Water System

1. Soil is a heterogeneous mass consisting of a three-phase system of solid, liquid and gas. Mineral matter, consisting of sand, silt and clay and organic matter form the largest fraction of soil and serves as a framework (matrix) with numerous pores of various proportions. The void space within the solid particles is called the soil pore space. Decayed organic matter derived from the plant and animal remains are dispersed within the pore space. The soil air is totally expelled from soil when water is present in excess amount than can be stored.

2. On the other extreme, when the total soil is dry as in a hot region without any supply of water either naturally by rain or artificially by irrigation, the water molecules surround the soil particles as a thin film. In such a case, pressure lower than atmospheric thus results due to surface tension capillarity and it is not possible to drain out the water by gravity. The salts present in soil water further add to these forces by way of osmotic pressure. The roots of the plants in such a soil state need to exert at least an equal amount of force for extracting water from the soil mass for their growth.

Soil Properties

Soil is a complex mass of mineral and organic particles. The important properties that classify soil according to its relevance to making crop production (which in turn affects the decision-making process of irrigation engineering) are: 1. Soil texture 2. Soil structure

1. Soil texture: This refers to the relative sizes of soil particles in a given soil. According to their sizes, soil particles are grouped into gravel, sand, silt and day. The relative proportions of sand, silt and clay is a soil mass determines the soil texture. Figure 2 presents the textural classification of 12 main classes as identified by the US department of agriculture, which is also followed by the soil survey organizations of India.

$\text{FIGURE 1: SOIL TEXTURAL CLASSIFICATION BY USDA}$

According to textural gradations a soil may be broadly classified as:

• Open or light textural soils: these are mainly coarse or sandy with low content of silt and clay.

• Medium textured soils: these contain sand, silt and clay in sizeable proportions, like loamy soil.

• Tight or heavy textured soils: these contain high proportion of clay

2. Soil Structure: This refers to the arrangement of soil particles and aggregates with respect to each other. Aggregates are groups of individual soil particles adhering together. Soil structure is recognized as one of the most important properties of soil mass, since it influences aeration, permeability, water holding capacity, etc. The classification of soil structure is done according to three indicators as:

• Type: there are four types of primary structures-platy, prism-like, block like and spheroidal.

• Class: there are five recognized classes in each of the primary types. These are very fine, fine, medium, coarse and very coarse.

• Grade: this represents the degree of aggregation that is the proportion between aggregate and unaggregated material that results when the aggregates are displaced or gently crushed. Grades are termed as structure less, weak, moderate, strong and very strong depending on the stability of the aggregates when disturbed.

Soil Classification

Soils vary widely in their characteristics and properties. In order to establish the interrelation ship between their characteristics, they need to be classified. In India, the soils may be grouped into the following types:

• Alluvial soils: These soils are formed by successive deposition of silt transported by rivers during floods, in the flood plains and along the coastal belts. This group is by far the largest and most important soil group of India contributing the greatest share to its agricultural wealth. Though a great deal of variation exists in the type of alluvial soil available throughout India, the main features of the soils are derived from the deposition laid by the numerous tributaries of the Indus, the Ganges and the Brahmaputra river systems. These streams, draining the Himalayas, bring with them the products of weathering rocks constituting the mountains, in various degrees of fineness and deposit them as they traverse the plains. Alluvial soils textures vary from clayey loam to sandy loam. The water holding capacity of these soils is fairly good and is good for irrigation.

• Black soils: This type of soil has evolved from the weathering of rocks such as basalts, traps, granites and gneisses. Black soils are derived from the Deccan trap and are found in Maharashtra, western parts of Madhya Pradesh, parts of Andhra Pradesh, parts of Gujarat and some parts of Tamilnadu. These soils are heavy textured with the clay content varying from 40 to 60 percent. The soils possess high water holding capacity but are poor in drainage.

• Red soils: These soils are formed by the weathering of igneous and metamorphic rock comprising gneisses and schist’s. They comprise of vast areas of Tamilnadu, Karnataka, Goa, Daman & Diu, south-eastern Maharashtra, Eastern Andhra Pradesh, Orissa and Jharkhand. They also are in the Birbhum district of West Bengal and Mirzapur, Jhansi and Hamirpur districts of Uttar Pradesh. The red soils have low water holding capacity and hence well drained.

• Laterites and Lateritic soils: Laterite is a formation peculiar to India and some other tropical countries, with an intermittently moist climate. Laterite soils are derived from the weathering of the laterite rocks and are well developed on the summits of the hills of the Karnataka, Kerala, Madhya Pradesh, The eastern ghats of Orissa, Maharashtra, West Bengal, Tamilnadu and Assam. These soils have low clay content and hence possess good drainage characteristics.

• Desert soils: A large part of the arid region, belonging to western Rajasthan, Haryana, Punjab, lying between the Indus river and the Aravalli range is affected by the desert conditions of the geologically recent origin. This part is covered by a mantle of blown sand which, combined with the arid climate, results in poor soil development. They are light textured sandy soils and react well to the application of irrigation water.

• Problem soils: The problem soils are those, which owing to land or soil characteristics cannot be used for the cultivation of crops without adopting proper reclamation measures. Highly eroded soils, ravine lands, soils on steeply sloping lands etc. constitute one set of problem soils. Acid, saline and alkaline soils constitute another set of problem soil.

Classification of soil water
As stated earlier, water may occur in the soil pores in varying proportions. Some of the definitions related to the water held in the soil pores are as follows:

• Gravitational water: A soil sample saturated with water and left to drain the excess out by gravity holds on to a certain amount of water. The volume of water that could easily drain off is termed as the gravitational water. This water is not available for plants use as it drains off rapidly from the root zone.

• Capillary water: the water content retained in the soil after the gravitational water has drained off from the soil is known as the capillary water. This water is held in the soil by surface tension. Plant roots gradually absorb the capillary water and thus constitute the principle source of water for plant growth.

• Hygroscopic water: the water that an oven dry sample of soil absorbs when exposed to moist air is termed as hygroscopic water. It is held as a very thin film over the surface of the soil particles and is under tremendous negative (gauge) pressure. This water is not available to plants.

$\text{FIGURE 2: CLASSES OF SOIL WATER}$

Water may be classified as unavailable, available and superfluous. This classification is based on the availability of soil water to the plants.

1. Soil Moisture Tension: The force per unit area that must be exerted in order to extract water from the soil is known as soil moisture tension and is expressed in terms of atmosphere (atm). It is also known as Capillary potential, Capillary tension or force of suction. Soil moisture tension is inversely proportional to moisture content of soil of given texture ad structure. It is measured in the laboratory with the help of various instruments such as centrifuge, tensiometer etc.
2. Soil Moisture Stress: Soil moisture stress is defined as the sum of the soil moisture tension and osmotic pressure of soil solution. Osmotic pressure in the increase in the force (or tension) caused by the salts present in the soil solution. The osmotic pressure, and hence is function of soil moisture stress. The osmotic pressure of the soil solution should be maintained as low as possible by controlled leaching, thus maintaining the soil moisture stress at the root zone in the range that will provide adequate moisture to the roots of the plants.

$\text{FIGURE 3: CLASSES OF SOIL WATER}$

Soil water constants

For a particular soil, certain soil water proportions are defined which dictate whether the water is available or not for plant growth. These are called the soil water constants, which are described below.

1. Saturation capacity: It is the total water content of the soil when all the pores of the soil are filled with water. It is also termed as the maximum water holding capacity of the soil. At saturation capacity, the soil moisture tension is almost equal to zero
2. Field capacity: It is the water retained by an initially saturated soil against the force of gravity. Hence, as the gravitational water gets drained off from the soil, it is said to reach the field capacity. At field capacity, the macro-pores of the soil are drained off, but water is retained in the micropores. Though the soil moisture tension at field capacity varies from soil to soil, it is normally between 1/10 (for clayey soils) to 1/3 (for sandy soils) atmospheres.

3. Permanent wilting point: Plant roots are able to extract water from a soil matrix, which is saturated up to field capacity. However, as the water extraction proceeds, the moisture content diminishes and the negative (gauge) pressure increases. At one point, the plant cannot extract any further water and thus wilts. Two stages of wilting points are recognized and they are:

   • Temporary wilting point: this denotes the soil water content at which the plant wilts at day time, but recovers during right or when water is added to the soil.
   • Ultimate wilting point: at such a soil water content, the plant wilts and fails to regain life even after addition of water to soil.

4. Available Moisture: The difference in water content of the soil between field capacity and permanent wilting point is known as available water or available moisture.

5. Readily Available Moisture: It is that portion of the available moisture that is most easily extracted by plants, an is approximately 75% of the available moisture.
6. Moisture Equivalent: This is an artificial moisture capacity property of the soil and is used as an index of the natural properties. It is the percentage of the moisture retained in a small sample of wet soil 1 cm deep when subjected to a centrifugal force of 1000 times as great as gravity, usually of a minute of 30 minutes. Moisture equivalent is used as a single factor to which the properties of soil can be related within reasonable limits. The moisture equivalent roughly equals to field capacity for a medium textured soil/ The relation between thee are as follows: Moisture equivalent = Field capacity = 1.8 to 2 Permanent wilting point = 2.7 hygroscopic coefficient.
7. Soil Moisture Deficiency: Soil moisture deficiency or field moisture deficiency is the water required to bring the soil moisture content of the soil to the field capacity.