Classification of Soils
Soils are naturally occurring soft matters forming the chief part of the superficial crust of the earth. They act as substratum or medium for growth of vegetation and constitute one of the most important natural resources for sustaining life on earth.
The classification of soils help in describing, interpreting and understanding what they are, their formation and various other characteristics.
Soil classification is an important tool in narrowing down to specific properties that the individual soil has, which will then lead to proper utilization of that specific type of soil granting that its properties fit with the purpose for which it is being put into use.
Soil Survey of India, established in 1956, made comprehensive studies of soils in selected areas like in the Damodar Valley
Indian Council of Agricultural Research (ICAR) did a lot of studies on Indian soils the ICAR has classified the Indian soils on the basis of their nature and character as per the United States Department of Agriculture (USDA) Soil Taxonomy.
On the basis of genesis, colour, composition and location, the soils of India have been classified into:
- Alluvial soils
- Black soils
- Red and Yellow soils
- Laterite soils
- Arid soils
- Saline soils
- Peaty soils
- Forest soils
Alluvial Soils
Contrary to what the name suggests, alluvial soils are not actually soils. They are deposits of sediments and nutrient rich particles formed in floodplains, deltas, floodplains and alluvial fans.
All alluvial soils form by flooding. Because floods periodically deposit new sediment at the surface, alluvial soils can have a unique layered look.
Dark and light colors alternate, along with assorted sizes of gravel particles. This unique layering process is called stratification.
In many places around the world there are distinctly different soil colors, chemical properties, and human artifacts buried in floodplains. These allow scientists to reconstruct human occupation and land use history.
Black Soil
Black soil is extremely fertile and produces some of the best yields in the country. Despite being rich in nutrients, it has remained underutilized by farmers who prefer to stick to paddy cultivation. It is time that this soil gets the consideration that it deserves.
Black soil covers most of the Deccan Plateau which includes parts of Maharashtra, Madhya Pradesh, Gujarat, Andhra Pradesh and some parts of Tamil Nadu.
They swell and become sticky when wet and shrink when dried. So, during the dry season, these soil develop wide cracks.
Red and Yellow Soil
Red soil is a very important agricultural resource in India. Soils in central and south India are predominantly loam, while soils in the north tend to be sandy. There are three main soil classes in India based on origin, chemical composition and texture.
Along the piedmont zone of the Western Ghat, long stretch of area is occupied by red loamy soil. Yellow and red soils are also found in parts of Odisha and Chattisgarh and in the southern parts of the middle Ganga plain.
The soil develops a reddish colour due to a wide diffusion of iron in crystalline and metamorphic rocks. It looks yellow when it occurs in a hydrated form.
The fine-grained red and yellow soils are normally fertile, whereas coarse-grained soils found in dry upland areas are poor in fertility. They are generally poor in nitrogen, phosphorous and humus.
Laterite Soil
These soils are normally red with varying degree of reddish brown colours due to iron oxides. Laterite soils as such, never displays any type of soil profile as evidenced as in the case of most other soils.
The laterite soils develop in areas with high temperature and high rainfall. These are the result of intense leaching due to tropical rains.
With rain, lime and silica are leached away, and soils rich in iron oxide and aluminium compound are left behind. Humus content of the soil is removed fast by bacteria that thrives well in hightemperature.
Poor in organic matter, nitrogen, phosphate and calcium, while iron oxide and potash are in excess. Hence, laterites are not suitable for cultivation; however, application of manures and fertilisers are required for making the soils fertile for cultivation.
Saline Soils
Saline soils are very common in many parts of the world. The saline soils are also known as alkaline soils. These soils are the particular types of the soil which have the high percentage of sodium chloride along with the other types of salts. Saline soils can be found all over the world hence it is quite surprising to learn about them.
They have nutrient deficiencies and toxicities because of boron, carbonate, and aluminate ions present. These soils are observed to have very low organic matter and biological activity.
Increase in soil salinity results in progressively smaller and more stressed microbial community, which is also seen to be least efficient toward metabolism.
Physical properties of saline soils are observed to be improved by growth of salt-tolerant plants. These plant varieties are shown to improve water uptake in soil, saturated hydraulic conductivity of soil, structural stability, bulk density of soil, and soil porosity.
Peaty Soils
Peaty soils of this sort are often clayey (that is, they feel heavy and sticky when moist and squeak when you grab a handful and pull it hard), but some examples of peat soils that may be sandy or loamy (that is, they feel loose and airy) also fall into the ‘peaty soils’ category.
Cereals produce low yields. On light undrained peats, or where the water table remains high, the main crop is grass.
On some of these areas willows are grown. Peaty loams are suitable for growing root crops and cereals. Where the depth of peat is very shallow (due to wastage) cropping is similar to that for the underlying mineral soil type.
Forest Soils
Forest soils are extremely important because they provide a range of services, from wood production to wildlife habitat. In temperate forests, the soils have been formed under natural forest conditions for many centuries or even millennia.
There are also soils now covered with forest vegetation, often plantations, on lands that were not naturally forested.
These soils are probably undergoing processes that give them ‘forest soil-like’ characteristics, e.g., litter layers from trees, woody organic residues from deep roots, and associated soil microbe and fauna populations.
Like other soils, forest soils have developed, and are developing, from geological parent materials in various topographic positions interacting with climates and organisms.
Forest soils may be young, from ‘raw’ talus, recent glacial till or alluvium, or ‘mature,’ in relatively stable landscape positions. Just as forest vegetation of the world varies greatly, so do forest soils, e.g., they are shallow, deep, sandy, clayey, wet, arid, frigid, or warm.