What is Soils | Types of soils | Properties of soil - Physical - Chemical | Soil Texture | Soil Structure | By - civil Hub

WHAT IS SOILS

Soil mainly consists of finely divided organic matter and minerals (formed due to disintegration of rocks). It holds the plants upright, stores water for plant use, supplies nutrients to the plants and helps in aeration. Soils can be classified in many ways, such as on the basis of size (gravel, sand, silt, clay, etc.), geological process of formation, and so on. Based on their process of formation (or origin), they can be classified into the following categories:

Soils

(i) Residual soils: Disintegration of natural rocks due to the action of air, moisture, frost, and vegetation results in residual soils.
(ii) Alluvial soils: Sediment material deposited in bodies of water, deltas, and along the banks of the overflowing streams forms alluvial soils.
(iii) Aeolian soils: These soils are deposited by wind action.
(iv) Glacial soils: These soils are the products of glacial erosion.
(v) Colluvial soils: These are formed by deposition at foothills due to rain wash.
(vi) Volcanic soil: These are formed due to volcanic eruptions and are commonly called as volcanic wash.

In this Articles :-
Q. What is Soil  ?
Q. Types of Soils ?
Q. Properties of Soil ?

Types of soils :-

(i) Alluvial Soils : Alluvial soils include the deltaic alluvium, calcareous alluvial soils, coastal alluvium, and coastal sands. This is the largest and most important soil group of India. The main features of the alluvial soils of India are derived from the deposition caused by rivers of the Indus, the Ganges, and the Brahmaputra systems. These rivers bring with them the products of weathering of rocks constituting the mountains in various degrees of fineness and deposit them as they traverse the plains. These soils vary from drift sand to loams and from fine silts to stiff clays. Such soils are very fertile and, hence, large irrigation schemes in areas of such soils are feasible. However, the irrigation structures themselves would require strong foundation.

(ii) Black Soils : The black soils vary in depth from a thin layer to a thick stratum. The typical soil derived from the Deccan trap is black cotton soil. It is common in Maharashtra, western parts of Madhya Pradesh, parts of Andhra Pradesh, parts of Gujarat, and some parts of Tamil Nadu. These soils may vary from clay to loam and are also called heavy soils. Many black soil areas have a high degree of fertility but some, especially in the uplands, are rather poor. These are suitable for the cultivation of rice and sugarcane. Drainage is poor in such soils.

(iii) Red Soils : These are crystalline soils formed due to meteoric weathering of the ancient crystalline rocks. Such soils are found in Tamil Nadu, Karnataka, Goa, south-eastern Maharashtra, eastern Andhra Pradesh, Madhya Pradesh, Orissa, Bihar, and some districts of West Bengal and Uttar Pradesh. Many of the so-called red soils of south India are not red. Red soils have also been found under forest vegetation.

(iv) Lateritic Soils : Laterite is a formation peculiar to India and some other tropical countries. Laterite rock is composed of a mixture of the hydrated oxides of aluminium and iron with small amounts of manganese oxides. Under the monsoon conditions, the siliceous matter of the rocks is leached away almost completely during weathering. Laterites are found on the hills of Karnataka, Kerala, Madhya Pradesh, the estern Ghats of Orissa. Maharashtra, West Bengal, Tamil Nadu, and Assam.

(v) Desert Soils : A large part of the arid region belonging to western Rajasthan, Haryana, and Punjab lying between the Indus river and the Aravalli range is affected by desert and conditions of geologically recent origin. This part is covered with a mantle of the blown sand which, combined with the arid climate, results in poor soil development. The Rajasthan desert is a vast sandy plain including isolated hills or rock outcrops at places. The soil in Rajasthan improves in fertility from west and north-west to east and north-east

(vi) Forest Soils : These soils contain high percentage of organic and vegetable matter and are also called humus. These are found in forests and foothills.

Soils suitable for agriculture are called arable soils and other soils are non-arable. Depending upon their degree of arability, these soils are further subdivided as follows:-

(i) Class I: The soils in class I have only a few limitations which restrict their use for cultivation. These soils are nearly level, deep, well-drained, and possess good water-holding capacity. They are fertile and suitable for intensive cropping.

(ii) Class II: These soils have some limitations which reduce the choice of crops and require moderate soil conservation practices to prevent deterioration, when cultivated.

(iii) Class III: These soils have severe limitations which reduce the choice of crops and require special soil conservation measures, when cultivated.

(iv) Class IV: These soils have very severe limitations which restrict the choice of crops to only a few and require very careful management. The cultivation may be restricted to once in three or four years.

Soils of type class I to class IV are called arable soils. Soils inferior to class IV are grouped as non-arable soils. Irrigation practices are greatly influenced by the soil characteristics. From agricultural considerations, the following soil characteristics are of particular significance.

(i) Physical properties of soil
(ii) Chemical properties of soil
(iii) Soil-water relationships

(i) PHYSICAL PROPERTIES OF SOIL

The permeability of soils with respect to air, water, and roots are as important to the growth of crop as an adequate supply of nutrients and water. The permeability of a soil depends on the porosity and the distribution of pore spaces which, in turn, are decided by the texture and structure of the soil.

1. Soil Texture

Soil texture is determined by the size of soil particles. Most soils contain a mixture of sand (particle size ranging from 0.05 to 1.00 mm in diameter), silt (0.002 to 0.05 mm) and clay (smaller than 0.002 mm). If the sand particles dominate in a soil, it is called sand and is a coarse-textured soil. When clay particles dominate, the soil is called clay and is a fine-textured soil. Loam soils (or simply loams) contain about equal amount of sand, silt, and clay and are medium-textured soils.

The texture of a soil affects the flow of water, aeration of soil, and the rate of chemical transformation all of which are important for plant life. The texture also determines the water holding capacity of the soil.

2. Soil Structure

Volume of space (i.e., the pores space) between the soil particles depends on the shape and size distribution of the particles. The pore space in irrigated soils may vary from 35 to 55 percent. The term porosity is used to measure the pore space and is defined as the ratio of the volume of voids (i.e., air and water-filled space) to the total volume of soil (including water and air). The pore space directly affects the soil fertility (i.e., the productive value of soil) due to its influence upon the water-holding capacity and also on the movement of air, water, and roots through the soil.

Soils of uniform particle size have large spaces between the particles, whereas soils of varying particle sizes are closely packed and the space between the particles is less. The particles of a coarse-grained soil function separately but those of fine-grained soils function as granules. Each granule consists of many soil particles. Fine-textured soils offer a favourable soil structure permitting retention of water, proper movement of air and penetration of roots which is essential for the growth of a crop.

The granules are broken due to excessive irrigation, ploughing or working under too wet (puddling) or too dry conditions. Such working affects the soil structure adversely. The structure of the irrigated soil can be maintained and improved by proper irrigation practices some of which are as follows:-

(i) Ploughing up to below the compacted layers,
(ii) After ploughing, allowing sufficient time for soil and air to interact before preparing the seed bed or giving pre-planting irrigation,
(iii) The organic matter spent by the soil for previous crops should be returned in the form of fertilisers, manures, etc.,
(iv) Keeping cultivation and tillage operations to a minimum, and
(v) Adopting a good crop rotation.

3. Depth of Soil

The importance of having an adequate depth of soil for storing sufficient amount of irrigation water and providing space for root penetration cannot be overemphasised. Shallow soils require more frequent irrigations and cause excessive deep percolation losses when shallow soils overlie coarse-textured and highly permeable sands and gravels. On the other hand, deep soils would generally require less frequent irrigations, permit the plant roots to penetrate deeper, and provide for large storage of irrigation water. As a result, actual water requirement for a given crop (or plant) is more in case of shallow soils than in deep soils even though the amount of water actually absorbed by the crop (or plant) may be the same in both types of soils. This is due to the unavoidable water losses at each irrigation.

(ii) CHEMICAL PROPERTIES OF SOIL

For satisfactory crop yield, soils must have sufficient plant nutrients, such as nitrogen, carbon, hydrogen, iron, oxygen, potassium, phosphorus, sulphur, magnesium, and so on. Nitrogen is the most important of all the nutrients. Nitrogenous matter is supplied to the soil from barnyard manure or from the growing of legume crops as green manures, or from commercial fertilisers. Plants absorb nitrogen in the form of soluble nitrates.

Soils having excess (greater than 0.15 to 0.20 per cent) soluble salts are called saline soils and those having excess of exchangeable sodium (more than 15 per cent or pH greater than 8.5) are called alkaline (or sodic) soils. Excessive amounts of useful plant nutrients such as sodium nitrate and potassium nitrate may become toxic to plants. Saline soils delay or prevent crop germination and also reduce the amount and rate of plant growth because of the high osmotic pressures which develop between the soil-water solution and the plants. These pressures adversely affect the ability of the plant to absorb water.

Alkaline (or sodic) soils tend to have inferior soil structure due to swelling of the soil particles. This changes the permeability of the soil. Bacterial environment is also an important feature of the soil-water-plant relationship. The formation of nitrates from nitrogenous compounds is accelerated due to favourable bacterial activity. Bacterial action also converts organic matter and other chemical compounds into forms usable by the plants. Bacterial activity is directly affected by the soil moisture, soil structure, and soil aeration. Compared to humid climate soils, arid soils provide better bacterial environment up to much greater depths because of their open structure. Besides, due to low rainfall in arid regions, leaching (i.e., draining away of useful salts) is relatively less and the arid soils are rich in mineral plant food nutrients, such as calcium and potassium.

(iii) SOIL–WATER RELATIONSHIPS

Any given volume V of soil consists of : (i) volume of solids Vs , (ii) volume of liquids (water) Vw, and (iii) volume of gas (air) Va. Obviously, the volume of voids (or pore spaces) Vv = Vw + Va. For a fully saturated soil sample, Va = 0 and Vv = Vw . Likewise, for a completely dry specimen, Vw = 0 and Vv = Va. The weight of air is considered zero compared to the weights of water and soil grains. The void ratio e, the porosity n, the volumetric moisture content w, and the saturation S are defined as

e =Vv/Vs , n = Vv/V , w = Vw/V , S = Vw/Vv

It should be noted that the value of porosity n is always less than 1.0. But, the value of void ratio e may be less, equal to, or greater than 1.0.

Further, if the weight of water in a wet soil sample is Ww and the dry weight of the sample is Ws , then the dry weight moisture fraction, W is expressed as

W = Ww/Ws

The bulk density (or the bulk specific weight or the bulk unit weight) γb of a soil mass is the total weight of the soil (including water) per unit bulk volume, i.e.,

γb = WT/V

WT = Ws + Ww

The specific weight (or the unit weight) of the solid particles is the ratio of dry weight of the soil particles Ws to the volume of the soil particles Vs, i.e., Ws/Vs. Thus,

Gb γw = Ws/V

i.e., V =Ws/Gb x γw

and  Gs γw = Ws/Vs    

Therefore, the volume of water in the root-zone soil,

Vw = W Ad (1 – n) Gs

This volume of water can also be expressed in terms of depth of water which would be obtained when this volume of water is spread over the soil surface area A.

∴ Depth of water,  

  dw = Vw/A

dw = Gs (1 – n) Wd

or dw = w d