Soil and the process of soil formation

Detail

What soil is

• Soil is a dynamic medium: a mix of minerals, organic matter, water and air. "Dynamic" means it is always changing.
• Three kinds of activity go on in soil all the time:
• Chemical: minerals dissolve and new compounds form.
• Physical: water soaks in, the soil swells and shrinks, and air moves through it.

• Biological: roots, bacteria, fungi and animals live, feed and decay in it.

• Soil plays two roles:

• It is a result of decay. It forms from broken-down rock and the rotted remains of plants and animals.

• It is a medium for growth. Plants take water, air and nutrients from it.

• Soil is home to nearly 59% of Earth's species. A tablespoon of soil holds more living organisms than there are people on Earth (FAO, World Soil Day) [2].

• Soils supply 15 of the 18 naturally occurring chemical elements that plants need [2].
• About 95% of our food comes from soils [2].

Soil changes with the seasons

• Soil is a changing, developing body. Its character shifts as the seasons move between cold and warm, and between dry and moist.

Condition	What happens in the soil
Too cold (e.g. winter in the Himalaya, Ladakh)	Biological activity slows down or stops
Too dry (e.g. summer in the Thar, Rajasthan)	Biological activity slows down or stops
Leaves fall or grasses die (autumn, end of the growing season)	Organic matter in the soil goes up
Warm and moist (e.g. south-west monsoon months)	Microbes and roots are most active and decay is fast

Pedology: the science of soils

• Pedology is the scientific study of soils: how they form, what they are like, and how they are spread over the Earth.
• A pedologist is a soil scientist.
• V.V. Dokuchaev (Russia, 1880s) is regarded as the founder of soil science. He was among the first to treat soil as a natural body with its own origin and history, not just as broken rock. He studied the chernozem (black, humus-rich grassland soils) of the Russian steppe.

Soil formation (pedogenesis): weathering comes first

• Soil formation (pedogenesis) is the slow process by which rock material turns into living, layered soil.
• Weathering is the breaking down of rocks in place by weather, chemical action and living things, without the pieces being carried away.
• The weathering mantle is the layer of loose, broken rock left on the surface by weathering. It is the basic input for soil.
• Soil can also form on transported deposits: material carried and dropped by rivers, wind or glaciers. In India this includes:
• the alluvium (river-laid silt) of the Indo-Gangetic plain;
• the wind-blown sand of the Thar.

Stages of soil formation (cause → process → result)

1. First colonisers arrive - Bacteria and lower plants such as mosses and lichens settle on the weathered or transported material. - Lichens are a partnership of a fungus and an alga. They can live on bare rock and give off weak acids that break the rock down further.

2. Small organisms find shelter - Minor organisms (tiny insects, mites, worms) start living in the loose mass.

3. Humus builds up - The dead remains of these organisms and plants pile up and rot. - Humus is the dark, decomposed organic matter in soil. It holds water and nutrients. - Result: the material slowly gains organic matter.

4. Bigger plants follow - First grasses and ferns grow. - Then bushes and trees grow from seeds brought in by birds and wind.

5. Soil becomes porous - Plant roots push into the material. - Burrowing animals (earthworms, ants, termites, rodents) bring particles up to the surface. - Result: the mass becomes porous and sponge-like. It can hold water and let air in.

6. A mature soil forms - The end product is a complex mixture of mineral and organic products.

Weathering is necessary but not enough on its own

• Weathering only produces broken rock (regolith). Broken rock with no life in it is not soil.
• Living things are essential. They add humus, mix the layers and open up pore spaces.
• Exam point (NCERT exercise 2(iv)): "Is weathering solely responsible for soil formation?" The answer is no. Weathering supplies the raw material, while biological activity turns it into soil.
• Other soil-forming factors also act alongside weathering: parent material, topography (relief), climate, biological activity and time.

Soil horizons and the soil profile

• Soil horizon: a layer in the soil, lying roughly parallel to the surface, that differs from the layers above and below it.
• Soil profile: the vertical sequence of horizons, from the surface down to bedrock. It is different for each soil type [4].
• A mature soil profile typically shows A, E, B and C horizons in that order from top to bottom [4].

Horizon	Name	What it is
O	Organic layer	Undecomposed or partly decomposed litter (leaves, needles, twigs, moss, lichens) lying on the surface [3]
A	Topsoil	Mineral surface layer mixed with humus; darkest and most biologically active [3]
E	Eluvial (leached) layer	Has lost silicate clay, iron and aluminium, leaving sand and silt behind; usually lighter or bleached in colour [3][4]
B	Subsoil (zone of accumulation)	Little of the original rock structure remains; collects clay, iron, aluminium, humus or carbonates washed down from above (illuviation) [3]
C	Weathered parent material	Loose material, little changed by soil-forming processes; can still be dug with a spade [3]
R	Bedrock	Hard rock such as granite or basalt; too firm to dig by hand [3]

• Eluviation: material is washed out of a layer (E). Illuviation: material is washed into a layer (B).
• Horizons form mainly through downward processes: water moves down and carries material from the upper layers to the lower ones [3].
• Not every soil has every horizon. Many soils show only A, B and C [4].

Soil forms very slowly, so it is effectively non-renewable

• FAO: it can take up to 1,000 years to produce just 2–3 cm of soil [2]. An often-quoted figure from FAO's International Year of Soils (2015) campaign gives up to 1,000 years for 1 cm of topsoil.
• FAO: 33% of the world's soils are degraded [2].
• Result: soil lost to erosion cannot be replaced within a human lifetime. On human timescales it is a non-renewable resource.
• World Soil Day falls on 5 December. The UN General Assembly endorsed it in 2014, with Thailand leading the effort [2].

Prelims Hooks

• Pedology is the science of soils. V.V. Dokuchaev (Russia, 1880s) is regarded as its founder.
• The first colonisers of weathered material are bacteria, mosses and lichens, not grasses or trees.
• Trap: "Weathering alone forms soil" is false. Biological activity is essential.
• Horizon order from top to bottom: O → A → E → B → C → R.
• E horizon = zone of eluviation (leaching, bleached look). B horizon = zone of illuviation (accumulation of clay, iron, aluminium) [3].
• C horizon = little-changed parent material. R = hard bedrock such as granite or basalt [3].
• FAO: up to 1,000 years for 2–3 cm of soil. 95% of food comes from soils. 33% of soils are degraded [2].
• World Soil Day: 5 December. The UNGA endorsed it in 2014, led by Thailand [2].
• Soil organic matter rises when leaves fall. Biological activity stops when the soil is too cold or too dry.

Mains Points

• Soil as a non-renewable resource (GS-III):
• Soil takes up to 1,000 years to form 2–3 cm, and 33% of the world's soils are already degraded [2].
• So erosion from deforestation, overgrazing and poor tillage is a permanent loss on human timescales.

• This calls for conservation: contour ploughing, terracing, shelter belts and organic manuring.

• Role of living things in soil formation (GS-I):

• Weathering supplies only the mineral skeleton. Microbes, plants and burrowing animals add humus, porosity and water-holding capacity.

• So the loss of soil life (from too much chemical use or burning crop residue) lowers fertility even where weathering continues.

• Seasonal and climatic control (GS-I):

• Cold and dry conditions halt biological activity.

• So soils in the Thar and in high-altitude Ladakh stay thin and poor in humus, while warm, moist regions such as the Western Ghats and the North-East build up organic matter quickly.

• Food security link (GS-III): About 95% of food comes from soils [2]. Protecting soil formation and the soil profile directly supports agricultural productivity and food security.