The spaces present between soils particles in a given volume of soil are called pore spaces. The percentage of soil volume occupied by pore space or by the interstitial space is called porosity of the soil. Porosity of soil depends upon the texture and structure compactness and organic contents of soil. Porosity of the soil increases with the increase in the percentage of organic matter in the soil.
The pore spaces are of two types — (1) Micro-pore spaces (capillary pore spaces) and (2) Macro-pore spaces (non-capillary pore spaces). Capillary pore spaces can hold more water and restrict the free movement of water and air to a considerable extent, whereas macro-pore spaces have little water holding capacity and allow free movement of moisture and air in the soil under normal conditions.
3. Permeability of Soil:
The characteristic of soil that determines the movement of water through pore spaces is known as soil permeability. Soil permeability is directly dependent on the pore size; therefore, it is higher for the loose soil with large macro-pore spaces than it is for compact soil with numerous micro-pore spaces.
4. Soil Temperature:
Soil gets heat energy from different sources such as solar radiation, decomposing organic matter, and heat formed in the interior of earth. The temperature of soil is affected by its colour, texture, water content, and slope, altitude of the land and also by climate and vegetation cover of the soil. Evaporation of water from soil makes it cooler. Black soils absorb more heat than white soils. Sandy soils absorb more heat and radiate it out quickly at night than clay or loam soils.
The soil temperature greatly affects the physico-chemical and biological processes in the soil. For example, the germination of seeds, normal growth of roots and biological activity of soil- inhabiting mioro- and macro-organisms require proper and specific temperature.
5. Soil Water:
In soil, water is not only important as a solvent and transporting agent but in various ways it maintains soil texture, arrangement and compactness of soil particles and makes soil habitat livable for plants and animals. It comes in soil mainly through infiltration of precipitated water (dew, rain, sleet, snow, and hail) and irrigation. Water may be chemically combined or uncombined.
The most important types of chemically combined water arc the water of crystallized on of mineral grains and water of hydration of clay mineral particles. Both kinds of these waters are not at all available to living organisms.
Uncombined water is not chemically combined with soil particles and held in soil by adhesion (attraction of water molecules to solid particles) and cohesion (attraction of water molecules to each other). Following four kinds of uncombined soil water have been differentiated on the basis of their tendency to be retained in soils (Boayoucos 1970; Clapham, Jr. 1973).
In a well-saturated soil, the accessory (extra) amount of water displaces air from the pore spaces between soil particles and percolates downwardly under gravitational influence and finally it is accumulated in the pore spaces.
This accumulated excess water of large soil spaces is called gravitational water. When this gravitational water further percolates down and reaches to the level of patent rock, it is called ground water. Both kinds of this soil water are ecologically important in the leaching of nutrients.
The water which is held by capillary forces (i.e., surface tension and attraction forces of water molecules) in smaller soil channels, when the gravitational water and ground water have been drained, is called capillary water.
Capillary water occurs as a thin-film around soil particles in the capillary spaces and represents the normal available water to the plants. It remains in soil for long periods and carries with it nutrients in solution. Humus has more capillary water than soil minerals.
Hygroscopic water is held very tightly by the small particles of the soil. Plants cannot absorb the hygroscopic water.
Some uncombined soil water occurs as moisture or water vapours in the soil atmosphere.
Further, the total amount of water present in the soil is called holard. The quantity of water that plant-roots can absorb out of bolard is called chresard and that amount of soil water which cannot be absorbed by plant-roots is called echard.
Retention of water in soil and its availability to plants:
The retention of water in a soil depends on the nature of the soil and physical forms of soil water. For example, if a soil is absolutely saturated with water, so that water fills all the pores between soil particles and there is no air space, the soil is said to be at its maximum retentive capacity, or saturation (Fig. 9.5A). If any more water is added, it runs off on the surface.
When water is allowed to drain from a saturated soil under the influence of gravity, the gravitational water is lost, and the larger pores refill with air, leaving only the small pores with capillary water. The amount of water that can be retained in a soil by capillary attraction when it is free to percolate under the influence of gravity is termed the field capacity.
If the soil is dried out so that all capillary water is lost from even the smallest pores, leaving only hygroscopic water, the amount of water remaining in the soil is called hygroscopic coefficient.
If a soil is dried out to a point where the tension at which the remaining water is held precisely equal to the maximal ability of the plants to extract it, the amount of water in the soil is referred to as the permanent wilting point. This is always intermediate between the field capacity and the hygroscopic coefficient. In general, the water which is most useful to plants is capillary water (Clapham Jr., 1973).
6. Soil atmosphere:
Gases found in pore spaces of soil profiles form the soil atmosphere. The soil atmosphere contains three main gases namely O2, CO2, and N2. Soil air differs from atmospheric air in having more of moisture and CO2 and less of O2. The soil atmosphere is affected by temperature, atmospheric pressure, wind, rain-fall, etc. Loam soils with humus contain a normal proportion of air and water (about 34% air and 66% water) and, therefore, are good for majority of crops.