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Introduction to Soil Science

14 min read

Soil

Soil is the loose upper layer of the Earth’s crust in which plants grow. It is made of mineral particles from weathered rock, organic matter (humus), water, air and living organisms. A good topsoil is about 45% minerals, 25% water, 25% air and 5% organic matter.

A soil profileA vertical slice through soil showing layers called horizons. The top is the O horizon of dead plant matter. Below it is the dark A horizon or topsoil rich in humus, then the B horizon or subsoil with clay washed down from above, then the C horizon of broken rock, and finally solid bedrock.O horizondead leaves and humusA horizon (topsoil)dark, rich in humus; roots and organismsB horizon (subsoil)clay and minerals washed downC horizonbroken rockBedrocksolid rockdepth

Soil formation

Soils form by the weathering of rocks (physical, chemical and biological), mixed with organic matter. The type of soil depends on the parent rock, the climate, relief, living things and time.

Soil texture and structure

ParticleSizeFeel
Sand0.05–2 mmGritty; drains quickly
Silt0.002–0.05 mmSmooth like flour
ClayBelow 0.002 mmSticky when wet; holds water

Texture is the mixture of sand, silt and clay. Loam is a mixture with about equal effects of sand, silt and clay and is the best for most crops. Structure is how the particles are grouped into crumbs. A crumb structure gives good drainage and aeration.

PropertySandy soilClay soilLoam
DrainageFastSlow, waterloggedGood
Water holdingLowHighModerate to high
AerationGoodPoorGood
FertilityLowOften highHigh
Ease of workingEasyHeavyEasy

Soil pH and water

Soil pH measures acidity. Most crops grow best between pH 6 and 7. Acid soils (below 6) are corrected by adding lime; alkaline soils are corrected by adding sulphur or organic matter. Water in the soil is held as gravitational water (drains away), capillary water (available to plants) and hygroscopic water (held tightly, unavailable).

Minerals make up almost half; water and air share the pore spaces.
Try itSoil texture

Choose the percentage of sand and silt. The rest is clay. A rough rule gives the texture class.

Check yourself The best soil for most crops is
Check yourself Which particles are the smallest?
Check yourself Acid soil is improved by adding

Key terms

Soil
The upper layer of the Earth in which plants grow.
Humus
Dark decayed organic matter.
Loam
A balanced mixture of sand, silt and clay.
pH
A measure of acidity or alkalinity.

Summary

  • Soil has minerals, organic matter, water, air and organisms.
  • Texture depends on sand, silt and clay.
  • Loam is the best soil; crumb structure is the best structure.
  • Most crops need pH 6–7.
Mechanisation in Agriculture