The complex composite mass made up of both inorganic and organic, components containing water and air is called soil. There are different amount of various components of soil. Their amount and depends upon the nature of the soil, its depth and the state of its irrigation. The ordinary garden soil has the following composition by volume:
The inorganic component of the soil consists of mineral particles. They have different sizes. They are formed by the breaking of rocks. This process is Ailed weathering. Weathering is caused by frost, snow, rain and wind. (he rocks from which the mineral matter is derived are either limestone or :ncIstones. Limestone produces chalky soil and sand stones produces saw, • –1.
The characteristics of a soil depend mainly on the texture or the size of the mineral particles:
Texture of soil
Relative proportion of the soil particles size is known as soil texture. There are following types of textures of soil:
- Gravel: It is composed of large minerals Size of its particles is more than 2.00mm.
- Sand: Soils in which the mineral ‘natter consists mainly of sand particles are called sandy soils. Sandy soils have little water absorbing properties and are loose, dry and poor in salutes. However, large interspaces are present between the sand particles. Thus water and air can move freely.
- Loam: A soil in u Inch the sand. silt and clay particles are present in equal proportions is called loam. It is most suitable for plant growth. A loam soil in which sand particles predominate is called sandy loam and the one in which chi\ particles dominates is called sandy loam.
- Silt: Size of its particles is less than sand.
- Clay: The soil consists mainly of clay particles are called clayey soils. The clay particles are most l) of colloidal nature. They possess high water holding properties. 13ut they have small interspaces between them. Therefore. water and air cannot move freely among these particles.
- Profile of soilThe arrangement of soil particles in layers is called soil profile. The composition and appearance of the soil changes from the surface downwards. Soil shows distinct layers.
- A — Horizon: The uppermost layer is called A-horizon. It contains finer rock particles. It is also rich in organic matter.
- B — Horizon: It is present immediately below the B- horizon. It is made up of comparativelytcoarse mineral matter.– It also contains some clay.
- C — Horizon: It consists of the parent rock material or the mother earth.
Soil also contains organic matter. The organic matter formed by the
decomposition of the dead plant is called humus. Humus is dark in 4
colour and light in weight. It is an essential constituent of all soils. Humus and clay are the two colloidal components of the soil. They are closely associated with each other. Humus and clay form colloidal complex of the soil. The colloidal complex gives structure to the soil. It improves aeration of the soil. Thus it improves the water-holding capacity of sandy soils.
Soil is a great reservoir of water. The amount of water contained in this reservoir varies greatly in different soils and under different conditions. Its amount in ordinary garden soil is about 25 per cent by volume. The principal source of soil Water is rain. There are following type of water present in the soil:
- Run away water: Some of the water drains away along the slopes
- after a heavy rainfall or irrigation. This is called run-away water. It is not available to the plant.
- Gravitational water: Some of the water is absorbed by the soil and saturates its upper layers. It percolates downwards through the larger pores between the soils particles due to gravity. Finally, it reaches tilL..vater table. This is called the gravitational water. Gravitational. Water is also not available to the plant.
- Soil holding water: Much of the rainwater is retained by the soil particles. It makes the soil wet. The amount of water retained by soil after the removal of excess water by gravitation is called the field capacity or the water holding capacity of the soil.
- Hygroscopic water: Water adsorbed on the surface of soil colloids is called hygroscopic water. It is tightly held by soil. It forms very thin films. It is non-available to the plant.
- Capillary water: The water which fills the spaces between the non colloidal smaller soil particles is called capillary water. It has the greatest importance for the plant. This water is available to plant. The plant absorbs this water through roots.
- Chemically combined water: A small amount of water is bound to the molecules of some soil minerals by strong chemical bonds. It is also not available to the root. This is called chemically combined water.
- Water table: The water level at some depth from the soil where all pore spaces are filled with water is called water table. It may be present from few feet to several hundred feet. Water rises from water table by capillary action. It is used by the plants.
- Water availability for plants
- Field capacity of the soil: The hygroscopic and capillary water forms the field capacity of soil. It is different for different types of soils. Field capacity varies from 5 to 35%. The clay soil has the highest field capacity.
- Wilting coefficient or permanent wilting point: The amount of moisture left in the soil after a plant has permanently wilt is called wilting coefficient. Soil water is now empty for plant. More water must be added.to the soil. Otherwise, the plant will die. The value of wilting coefficient depends upon the nature of the soil. It is lowest for the sandy soil. But it is highest for the clayey soil.
The soil particles are irregular in size and shape. Therefore, interspaces are present between them. These interspaces are filled by air or partly by water. This soil air is essential for the proper growth of plant.
Soil also contains many organisms. It is rich in algae, bacteria, fungi and soil fauna.
- Bacteria: Bacteria are the most abundant in the soil. They play an important role in the soil formation. Some important soil bacteria are the ammonifying, nitrifying and nitrogen fixing bacteria.
- Fungi: Bacteria and soil fungi decompose the organic compounds of dead plant..1They convert them into hinnus.
- Animals: Some soil animals are protozoa, earthworms, insects, and burrowing animals. The earthworms are especially important in loosening the soil. It improves aeration and movement of water.