Soil formation

Also called: Pedogenesis · Topic: Geomorphic Processes: Weathering, Mass Movement, Erosion and Soil Formation · NCERT: Class 7, Ch 1 "The Story of Indian Farming"; Class 11, Ch 1 "Geography as a Discipline"; Class 11, Ch 5 "Geomorphic Processes"

Meaning

Soil formation (pedogenesis) is the slow process that turns loose rock material into living, layered soil. The material is either rock broken down where it lies, or silt and sand dropped by rivers, wind or glaciers. Tiny living things settle on it first. Humus then builds up, and roots and burrowing animals make it porous. The end result is a mature mixture of mineral and organic matter.

It matters because soil forms very slowly. On human timescales it is a non-renewable resource. About 95% of our food comes from soils [2].

Explanation

Weathering supplies the raw material

• Weathering means rocks breaking down where they lie, by weather, chemical action and living things. The broken pieces are not carried away.
• It leaves a weathering mantle (regolith). This is a layer of loose, broken rock on the surface, and it is the basic input for soil.
• Soil can also form on transported deposits. These are materials carried and dropped by rivers, wind or glaciers.
• Weathering alone is not enough. Broken rock with no life in it is not soil.
• Weathering gives only the mineral "skeleton".
• Living things add humus, mix the layers and open up air spaces.
• So the NCERT answer to "Is weathering solely responsible for soil formation?" is no.

Stages of soil formation (cause → process → result)

1. First colonisers arrive. Bacteria, mosses and lichens settle on the loose material. - Lichens are a partnership of a fungus and an alga. They can live on bare rock. - They give off weak acids that break the rock down further.

2. Small organisms find shelter. Tiny insects, mites and worms start living in the loose mass.

3. Humus builds up. - The dead remains of plants and animals pile up and rot. - Humus is the dark, rotted organic matter in soil. It holds water and nutrients.

4. Bigger plants follow. Grasses and ferns grow first. Then bushes and trees grow from seeds brought by birds and wind.

5. Soil becomes porous. - Roots push into the material. - Earthworms, ants, termites and rodents dig and bring particles to the surface. - Result: a sponge-like mass that holds water and lets air in.

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

Soil-forming factors and seasonal control

• Five factors act together:
• parent material (the rock or deposit the soil forms from)
• topography (relief, meaning the slope and height of the land)
• climate
• biological activity

• time

• Soil is a dynamic medium (always changing). It is a mix of minerals, organic matter, water and air. Three kinds of activity go on in it 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

• Seasons change how fast soil forms:

• Too cold or too dry: biological activity slows down or stops.
• Leaves fall or grasses die: organic matter in the soil goes up.
• Warm and moist: microbes and roots are most active, and decay is fast.

Soil horizons and the soil profile

• A soil horizon is a layer lying roughly parallel to the surface. It is different from the layers above and below it.
• A soil profile is the stack of horizons from the surface down to bedrock. It is different for each soil type [4].

Horizon	Name	What it is
O	Organic layer	Fresh or half-rotted litter (leaves, twigs, moss, lichens) lying on the surface [3]
A	Topsoil	Mineral layer mixed with humus. It is the darkest layer and has the most life [3]
E	Eluvial (leached) layer	Has lost clay, iron and aluminium, so only sand and silt are left. It is usually pale or bleached [3][4]
B	Subsoil	Collects clay, iron, aluminium, humus or carbonates washed down from above [3]
C	Weathered parent material	Loose material, little changed by soil formation. It can still be dug with a spade [3]
R	Bedrock	Hard rock such as granite or basalt. It is too firm to dig by hand [3]

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

Where it occurs

• On transported deposits in India:
• Indo-Gangetic plain: soil forms on alluvium (silt laid down by rivers).

• Thar Desert, Rajasthan: soil forms on wind-blown sand.

• Where it is slowed by cold: winter in the Himalaya and Ladakh. Biological activity stops, so soils stay thin and poor in humus.

• Where it is slowed by dryness: summer in the Thar, Rajasthan. Life in the soil slows down, so humus stays low.
• Where it is fast: during the south-west monsoon months, in warm, moist regions such as the Western Ghats and the North-East. Organic matter builds up quickly there.
• World example: the Russian steppe, where V.V. Dokuchaev studied chernozem (black, humus-rich grassland soils) in the 1880s.

Don't confuse with

• Weathering: only breaks rock into loose pieces (regolith). Pedogenesis needs living things to turn that regolith into soil.
• Pedology: the science that studies soils. Pedogenesis is the process by which soils form.
• Eluviation vs illuviation: eluviation removes material from the E horizon. Illuviation adds material to the B horizon [3].
• C horizon vs R horizon: C is loose, little-changed parent material that can be dug with a spade. R is hard bedrock such as granite or basalt [3].

Prelims Hooks

• 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 = zone of eluviation (leached and bleached). B = zone of illuviation (clay, iron and aluminium build up) [3].
• Pedology is the science of soils. Its founder is V.V. Dokuchaev (Russia, 1880s), who studied chernozem.
• FAO: it can take up to 1,000 years to form 2–3 cm of soil. World Soil Day is on 5 December. The UNGA endorsed it in 2014, led by Thailand [2].

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 soil lost to erosion from deforestation, overgrazing and poor tillage is a permanent loss on human timescales.

• Response: contour ploughing, terracing, shelter belts and organic manuring.

• Living things drive soil formation (GS-I):

• Microbes, plants and burrowing animals add humus, air spaces and water-holding capacity.
• Too much chemical use or burning of crop residue kills this soil life.

• So fertility falls even where weathering continues. This weakens food security, since about 95% of food comes from soils [2].

• Climate controls soil depth and quality (GS-I):

• Cold and dry conditions halt biological activity. So soils in the Thar and in Ladakh stay thin and low in humus.
• Warm, moist regions such as the Western Ghats and the North-East build up organic matter quickly.

Related concepts

• Soil
• Pedology

Read more

• Soil and the process of soil formation