Rock debris is formed of the layers of the Earth by the process of weathering and erosion. This rock waste is found on the surface of the Earth in a thin or thick layer. In ordinary language, it is called Soil. It is comprised of minerals, organic matter (called humus) derived from decomposed plants, living organisms, air, and water.

Soils differ according to climate, parent material, rainfall, relief of the bedrock, and the proportion of organic material. The study of soils is Pedology.

Types Of Soil

Soil is the layer of broken rock particles and decaying organic matter on the surface of earth, which is essential for the growth of plants. It comprises of all the necessary nutrients that aid the growth and development of plants and trees. There are different types of soils, each one of them varying in its nature and texture. While some soils are deep rooted, the others are present on the margin. They also differentiate in their color, as some may be red while others are black. There are various types of soil and each carries with itself different nutrients, which assists in the growth of various plants and crops.

Sandy Soil: Sandy soil is light and dry in nature. It does not have moisture content and warms up quickly in the spring. Thus, it is good for the production of early crops. Sandy soil is fit for cultivation any time of the year if moisture is available. Since it absorbs water quickly, the plants rooted in it need to be watered frequently.

Clay Soil: Clay soil is also called ‘late’ soil, because its wet nature makes it apt for planting seeds in late autumn. The soil serves as an excellent resort for the dry season, as it has a high water retention quality. It is necessary to drain clay soil frequently, for improving its texture. The soil becomes unmanageable during rainy season, as it becomes ‘sticky’. On the other hand, during draught, it becomes ‘rock solid’.

Silt Soil: Slit soil is considered to be the one of the most fertile soils present on earth. It is high in nutrient content and is comparatively heavier than the sandy soil, as it retains moisture easily. However, unlike clay soil, silt soil has a good drainage facility, which makes it very productive.

Loam Soils: Given the tag of being the perfect soil, It is suitable for any and every kind of crops. An amalgamation of three soils, loam soil has best of the characteristic of all.  It has high nutrients content, warms up quickly in summers and rarely dries out in the dry weather. It has become the ideal soil for cultivation.

Peaty Soil: Peaty soils are acidic in content, which makes them sour. This is the most exceptional feature of Peaty soils. Usually found in low-lying areas, these soils require proper drainage, as the place is accustomed to a lot of water clogging. Though peaty soils have less nutrient content, they warm up quickly in the spring, making them excellent if right amount of fertilizers are added.

Chalky Soils: Chalky soil is alkaline in nature and usually poor in nutrients. It requires nourishment, in the form of additional nutrients and soil improvers, for better quality. The soil becomes dry in summers, making it very hard, and would require too much of watering for the plants to grow. The only advantage which such a soil has is its lime content. When deep-rooted, Chalky soil becomes excellent for plant growth and favors good growing conditions as well.

Soil Profile

A soil can be described in terms of its soil profile, that is, a vertical cross-section from ground-level to the bedrock on which the soil sits. The profile is divided into layers called horizons.

O Horizon - The top, organic layer of soil, made up mostly of leaf litter and humus.

A Horizon - The layer called topsoil; seeds germinate and plant roots grow in this dark-colored layer. It is made up of humus mixed with mineral particles.

B Horizon - It contains clay and mineral deposits (like iron, aluminum oxides, and calcium carbonate) that it receives from layers above it when mineralized water drips from the soil above.

C Horizon - Also called regolith: the layer beneath the B Horizon and above the R Horizon. It consists of slightly broken-up bedrock. Plant roots do not penetrate into this layer; very little organic material is found in this layer.

R Horizon – It is the un-weathered rock (bedrock) layer that is beneath all the other layers.

Soil Formation

Factors affecting soil formation

Parent material:

This refers to the mineral material or organic material from which the soil is formed. Soils will carry the characteristics of its parent material such as color, texture, structure, mineral composition and so on. For example, if soils are formed from an area with large rocks (parent rocks) of red sandstone, the soils will also be red in color and have the same feel as its parent material.

Time:

Soils can take many years to form. Younger soils have some characteristics from their parent material, but as they age, the addition of organic matter, exposure to moisture and other environmental factors may change its features. With time, they settle and are buried deeper below the surface, taking time to transform. Eventually, they may change from one soil type to another.

Climate:

This is probably the most important factor that can shape the formation of soils. Two important climatic components, temperature and precipitation are key. They determine how quickly weathering will be, and what kind of organic materials may be available on and inside of the soils. Moisture determines the chemical and biological reactions that will occur as the soils are formed. A warmer climate with more rainfall means more vegetative cover and more animal action. It also means more runoff, more percolation and more water erosion. They all help to determine the kind of soils in an area.

Relief:

This refers to the landscape position and the slopes it has. Steep, long slopes mean water will run down faster and potentially erode the surfaces of slopes. The effect will be poor soils on the slopes, and richer deposits at the foot of the slopes. Also, slopes may be exposed to more direct sunlight, which may dry out soil moisture and render it less fertile.

Organisms:

The source and richness of organic matter are down to the living things (plants and animals) that live on and in the soils. Plants, in particular, provide lots of vegetative residues that are added to soils. Their roots also hold the soils and protect them from wind and water erosion. They shelter the soils from the sun and other environmental conditions, helping the soils to retain the needed moisture for chemical and biological reactions. Fungi, bacteria, insects, earthworms, and burrowing animals help with soil aeration. Worms help break down organic matter and aid decomposition. Animal droppings, dead insects and animals result in additional decaying organic matter. Microorganisms also help with mineral and nutrient cycling and chemical reactions.

Leaching

It is the process by which water soluble substances are washed through or out of the soil. In tropical areas, leaching of the soil due to heavy rains removes scarce nutrients and can lead to a dramatic loss of soil fertility. Destruction of forests aids the process of leaching. The leaching of soluble minerals in soils can lead to the formation of distinct soil horizons as different minerals are deposited at successively lower levels.

Chemical Fertilizers leached out of the soil drain into rivers, lakes, and ponds and cause water pollution.

Soil Classification

Geographers have classified soil in terms of their areal distribution over the earth's land surfaces which are linked with the climates, parent materials etc. As we know, under similar climatic conditions similar soil types develop.

1. Zonal Soils

2. Intrazonal Soils

3. Azonal soils

Zonal soils are by far the most important and widespread soils. These are exhibiting a mature and well-developed soil profile indicating the fullest play of various soil-forming factors.

They develop on well- drained areas, on parent material which has ramained in the original place for a sufficiently long time to have been affected by various soil- forming processes.

Intrazonal soils are developed under conditions of poor drainage, on regolith where soluble salt contents are high. Soils of bog areas and alkali flats are examples of intrazonal soil. All of them have distinct profile characteristics.

Azonal soil lack well-developed soil profiles which may be due to non-availability of sufficient time for them to develop fully or due to the location on very steep slopes which prohibits profile development. Alluvial soils, dune sands, lithosols (i.e. mountain soils on steep slopes) and organic soils, which develop on peat bogs are examples of azonal soils.

These soils from the point of view of area distribution are found in similar latitudes. Soils influence the type of agriculture employed in a particular region - light well-drained soils favour arable farming, whereas heavy clay soils give rise to lush pasture land.

Charactersitics of Major Soils

Chernozem Soils

These are dark coloured soils found in grassland regions, where evaporation exceeds precipitation. The A horizon is rich in humus due to decomposition of a thick litter of dead grass at the surface. The soil is also rich in phosphorous compounds. Minerals and moisture migrate upward due to evaporation, leaving the B and A horizons enriched. The organic content of soil is widely variable, ranging from zero in some arid regions to almost 100% in peats.

Lateritic Soils

These are red residual soils characteristic of tropical rainforests. It is formed by the weathering of basalts, granites, and shales and contains a high percentage of aluminium and iron hydroxides[1]. It may form an impermeable and infertile layer that hinders plant growth.

Podzol Soils

Type of light-coloured soil found predominantly under coniferous forests and on moorlands in cool regions where rainfall exceeds evaporation. The constant downward movement of water leaches nutrients from the upper layers, making podzols poor agricultural soils. The leaching of minerals such as iron, lime, and alumina leads to the formation of a bleached zone, often also depleted of clay. These minerals can accumulate lower down the soil profile to form a hard, impermeable layer which restricts the drainage of water through the soil.

Desert Soils

Desert soils are infertile, lacking in humus and generally grey or red in colour. The few plants capable of surviving such conditions are widely spaced, scrubby and often thorny. Long-rooted plants (phreatophytes) such as the date palm and mesquite commonly grow along dry stream channels. Salt-loving plants (halophytes) such as saltbushes grow in areas of highly saline soils and near the edges of playas (dry saline lakes).

Alluvial Soils

These consist of layer of broken rocky matter, or sediment, formed from material that has been carried in suspension by a river or stream and dropped as the velocity of the current decreases. River plains and deltas are made entirely of alluvial deposits, but smaller pockets can be found in the beds of upland torrents. Alluvial deposits can consist of a whole range of particle sizes, from boulders down through cobbles, pebbles, gravel, sand, silt, and clay. The raw materials are the rocks and soils of upland areas that are loosened by erosion and washed away by mountain streams. Much of the world's richest farmland lies on alluvial deposits. These deposits can also provide an economic source of minerals. River currents produce a sorting action, with particles of heavy material deposited first while lighter materials are washed downstream.

Volcanic soils

Volcanic soils are formed from volcanic ash and cinder deposits. They are rich in metallic minerals and being provided with adequate moisture and humus, are extremely fertile.