What is soil made of?
Soil is the loose top layer of the Earth's land where plants can root. It is where the rock sphere, water, air and life all meet.
- Mineral grains (about 45%): bits of weathered rock.
- Organic matter (about 5%): living things and humus, the dark, rotted remains of plants and animals.
- Water (about 25%): holds dissolved nutrients that roots drink.
- Air (about 25%): fills the pores so roots and soil life can breathe.
These numbers are for a good farm soil. A waterlogged soil has more water and less air.
How soil forms: processes and factors
Soil forms in two big jobs that go on together:
- Weathering breaks rock. Physical weathering cracks it (heating and cooling, frost, roots). Chemical weathering changes its minerals (water, oxygen, weak acids).
- Living things add organic matter. Bacteria and fungi turn dead leaves into humus. Earthworms and termites mix it in.
Water then moves things around: leaching carries minerals down; in dry places water can rise and leave salts on top.
Five soil-forming factors (CLORPT)
- Climate: heat and rain speed up weathering. Hot, wet tropics make deep, leached red soils; cold places make thin soils.
- Organisms: grassland makes thick dark topsoil; forest makes a litter layer.
- Relief (slope): steep slopes lose soil downhill; valleys collect deep soil.
- Parent rock: sets the minerals, colour and texture (basalt gives dark clay soil, granite gives sandy soil).
- Time: about 1 cm of soil can take 100 to 400 years to form.
The soil profile and its horizons
A vertical cut through soil is a soil profile. Its layers are horizons:
- O: fallen leaves and humus on the surface.
- A (topsoil): dark, rich in humus and life; most roots are here.
- B (subsoil): lighter or reddish; collects clay, iron and minerals washed down from A.
- C: partly broken parent rock.
- R: solid bedrock.
Main soil types (examples)
Soils are grouped by their profile and climate: deep dark grassland soils (chernozem-type, as in the steppes), leached forest soils, red and yellow tropical soils (laterite), black clay soils on basalt (as in India's Deccan), river alluvium (Ganga and Nile plains), and thin mountain and desert soils.
Texture, structure and why soil matters
Texture is the mix of grain sizes: sand (0.05–2 mm, gritty), silt (0.002–0.05 mm, smooth like flour) and clay (under 0.002 mm, sticky). Loam, a balanced mix, is best for most crops.
- Sandy soil drains fast and warms quickly but holds few nutrients.
- Clay soil holds water and nutrients but can waterlog and crack.
Structure is how grains stick together into crumbs. Humus and earthworms make a crumbly structure with many pores.
Adsorption: tiny clay and humus particles carry negative charges and hold nutrient ions such as potassium and calcium on their surface, so rain does not wash them away. This is why clay and humus make soil fertile.
Soil's jobs in nature
Soil grows our food, filters and stores rainwater, recycles dead matter, stores carbon and is home to a quarter of all species. It links the rock, water, air and living spheres.
Threats and conservation
Erosion by water and wind, salt build-up from bad irrigation, compaction by machines, pollution and sealing under concrete all damage soil. We protect it with contour ploughing, terraces on slopes, cover crops, crop rotation, mulching, shelter belts of trees, planting on bare land and adding compost.
Key formulas and definitions
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Worked examples
1. A soil sample is 40% sand, 40% silt and 20% clay. What is it called and is it good for crops?
This balanced mix is loam. It holds enough water and nutrients and still drains, so it is very good for most crops.
2. Why are soils in hot, rainy tropical forests often red and poor in nutrients, even though the forest is lush?
Heavy rain leaches most nutrients down and out of reach; iron oxides left behind make the soil red (laterite). The forest survives by quickly recycling nutrients from fallen leaves in the thin O layer.
3. A farmer on a hillside loses topsoil every monsoon. Suggest two ways to stop it.
Build terraces or plough along the contours so water cannot rush downhill, and keep the ground covered with crops, grass or mulch so raindrops do not hit bare soil.
Common mistakes
- Thinking soil is just "dirt" or crushed rock. It also contains humus, water, air and living organisms.
- Mixing up horizon order. A (topsoil) is above B (subsoil); C is broken rock, R is bedrock.
- Believing soil forms in a few years. It takes centuries, so lost topsoil is effectively lost for our lifetime.
- Saying clay soil is best because it holds water. Too much clay waterlogs; loam is best for most crops.