Soil Formation and Types
~10 min read
- Soil: Top layer of Earth's crust — minerals + organic matter + water + air + organisms.
- Formation: Weathering of rock + organic decay over time, influenced by climate, parent rock, relief, biota, time.
- India types: Alluvial (covers most), Black (Deccan), Red (peninsular interior), Laterite (rainy plateaus), Mountain, Arid, Saline, Peaty.
Soil is the thin layer where agriculture happens. India's soil distribution determines its crop pattern. NDA tests soil types and the specific crops/regions associated with each.
Soil Formation Factors
Five classical factors (CLORPT):
- Climate — rainfall and temperature control weathering rates.
- Organisms — plants, animals, microbes add organic matter.
- Relief — slope determines drainage and soil retention.
- Parent material — rock type that weathered to form the soil.
- Time — older soils are deeper and more developed.
Soil profile horizons: O (organic), A (topsoil), B (subsoil), C (weathered parent), R (bedrock).
Alluvial Soil
Most widespread Indian soil — covers ~40% of land. Formed by deposition by rivers from the Himalayas. Highly fertile due to potash, lime, phosphoric acid.
- Coverage: Northern Plains (Punjab to West Bengal), coastal plains, river deltas.
- Crops: Wheat, rice, sugarcane, jute, oilseeds.
- Types: Khadar (new, renewed by floods) and Bhangar (older, lime-rich nodules).
Black and Red Soils
Black soil (regur):
- Formed from Deccan basalt weathering.
- Rich in iron, lime, magnesium, alumina; poor in nitrogen and phosphorus.
- Highly moisture-retentive; self-ploughing (cracks when dry).
- Found in Maharashtra, MP, Gujarat, Telangana, Karnataka.
- Ideal for cotton — also called "black cotton soil."
Red soil:
- Found in southeastern peninsular India — Tamil Nadu, Karnataka, Odisha, eastern MP.
- Reddish due to iron oxides (oxidation).
- Poor in nitrogen, phosphorus, humus; responds well to fertilisers.
- Crops: groundnut, millets, potato, tobacco.
Other Indian Soils
| Soil | Region | Note |
|---|---|---|
| Laterite | Western Ghats, Eastern Ghats, Meghalaya, parts of Odisha | Heavy rainfall leaches silica, leaves iron-aluminium rich crust; poor for crops without fertiliser; used as building blocks |
| Mountain | Himalayas, hills | Thin, coarse; used for tea, coffee, spices, fruit |
| Arid / Desert | Rajasthan, north Gujarat | Sandy, saline, low organic matter; needs irrigation |
| Saline / Alkaline | Indo-Gangetic plains, coastal areas | Salt accumulation due to poor drainage; unsuitable for most crops |
| Peaty / Marshy | Kerala, Sundarbans | High organic content; waterlogged |
NDA PYQ Examples
Q: Which soil is best for cotton cultivation?
(a) Alluvial (b) Red (c) Black (regur) (d) Laterite
Answer: (c) Black soil — high moisture retention.
Q: Laterite soil is found where rainfall is:
(a) Very low (b) Moderate (c) Heavy with alternate wet-dry seasons (d) Snowfall
Answer: (c) Heavy with alternate wet-dry seasons — silica leaches out, iron and aluminium remain.
Q: Khadar and Bhangar are two types of which soil?
(a) Black (b) Red (c) Alluvial (d) Laterite
Answer: (c) Alluvial — Khadar is newer, Bhangar is older.
Drill Soil Formation and Types for NDA
NDA-pattern items on Soil Formation and Types with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is black soil called regur?
'Regur' is a Telugu word for the dark, calcareous black cotton soil. It is locally called regur in the Deccan and 'kali mitti' in Hindi-speaking regions.
What is the difference between Khadar and Bhangar soils?
Both are alluvial. Khadar is newer, deposited by recent floods, lighter in colour, more fertile. Bhangar is older, found on higher ground, contains lime nodules (kankar), less fertile.
Which Indian state has the most diverse soils?
Maharashtra — has black, red, laterite and alluvial soils in different parts of the state. Karnataka and Andhra Pradesh are similarly diverse.
Why is laterite soil poor for agriculture?
Heavy leaching has removed bases and silica, leaving acidic soil low in nitrogen and humus. It needs heavy fertilisation. Better use: as building blocks (lateritic stone).
How is soil different from sand?
Sand is just weathered rock fragments (mostly quartz). Soil is sand + clay + organic matter + water + air + microorganisms — a biologically active medium.