Soil

What is Soil?

Soil is the thin, uppermost layer of Earth's surface that supports plant life and is home to enormous numbers of living organisms. It is one of our most vital natural resources — it takes around 1,000 years to form just 1 cm of topsoil.

Soil is a complex mixture of five components:

1. Mineral particles — fragments of weathered parent rock (sand, silt, clay)
2. Humus — dark, decomposed organic matter from dead plants and animals
3. Water — held in pore spaces between particles; dissolves and transports minerals
4. Air — essential for root respiration and aerobic decomposition by microorganisms
5. Living organisms — bacteria, fungi, earthworms, insects, nematodes, and plant roots

The ideal soil for farming contains roughly 45% mineral particles, 25% water, 25% air, and 5% organic matter.

Soil Formation

Soil forms extremely slowly through the interaction of five factors:

• Parent material — the underlying rock that weathers to provide mineral particles
• Climate — temperature and rainfall control weathering rate and organism activity
• Organisms — plants, animals, and microbes add organic matter and speed up weathering
• Topography — slope affects drainage and erosion
• Time — soil formation takes hundreds to thousands of years

The process begins when bare rock is colonised by pioneer organisms (lichens and mosses) that slowly break down the surface through biological and chemical weathering. As they die, the first thin layer of organic matter accumulates.

Soil Profile (Horizons)

A vertical cross-section through soil reveals distinct layers called horizons:

Horizon	Name	Description
O	Organic layer	Surface litter; leaves, twigs, partly decomposed material
A	Topsoil	Darkest layer; most organic matter and organisms; where plant roots grow
B	Subsoil	Less organic matter; mineral deposits washed down from A horizon
C	Parent material	Partially weathered rock
R	Bedrock	Solid unweathered rock

Topsoil (A horizon) is the most important layer for agriculture — it is dark, rich in humus, and full of living organisms.

Types of Soil

Soil texture is determined by the proportion of mineral particle sizes:

Soil type	Particle size	Key properties	Advantages	Disadvantages
Sandy	Largest (0.05–2 mm)	Large pores; free-draining	Warms quickly; easy to work	Poor water and nutrient retention
Clay	Smallest (<0.002 mm)	Tiny pores; holds water	High water and nutrient retention	Poor drainage; compacts easily; slow to warm
Silt	Medium (0.002–0.05 mm)	Moderate drainage	Reasonably fertile	Can be easily eroded; can form a crust
Loam	Mixed sand, silt, clay	Balanced properties	Best for most crops	Can be expensive to purchase commercially

Loam is considered the ideal agricultural soil because it combines the drainage of sand with the water and nutrient retention of clay.

The Role of Organisms in Soil

Soil is one of the most biodiverse environments on Earth. A single teaspoon of healthy soil contains billions of microorganisms.

Earthworms:

• Burrow through soil, creating channels that aerate the soil (allows roots and decomposers to access oxygen)
• Mix organic matter from the surface into deeper layers
• Their casts (faeces) are rich in nutrients
• Improve drainage and water-holding capacity
• Charles Darwin estimated earthworms pass all topsoil through their bodies every few years

Bacteria and fungi:

• Primary decomposers — break down dead organic matter, releasing mineral nutrients
• Nitrogen-fixing bacteria (e.g., Rhizobium in root nodules) convert atmospheric N2 into nitrates that plants can absorb
• Nitrifying bacteria convert ammonium ions into nitrates in the nitrogen cycle

Other organisms:

• Beetles and millipedes fragment organic matter, speeding up decomposition
• Moles and burrowing animals aerate soil
• Plant roots stabilise soil and add organic matter when they die

Soil and Nutrient Cycles

Soil is central to biogeochemical cycles — the recycling of minerals through ecosystems.

The nitrogen cycle in soil:

1. Decomposers break down dead matter, releasing ammonium ions
2. Nitrifying bacteria convert ammonium → nitrites → nitrates
3. Plants absorb nitrates through roots for protein synthesis
4. Denitrifying bacteria convert nitrates back to N2 gas (nitrogen loss from soil)
5. Nitrogen-fixing bacteria replenish soil nitrates from atmospheric nitrogen

Farmers maintain soil fertility by:

• Adding compost or manure (organic matter)
• Applying fertilisers (adding N, P, K minerals)
• Crop rotation — alternating crops, including nitrogen-fixing legumes
• Ploughing — aerates soil and incorporates crop residues

Soil Erosion

Soil erosion — the removal of topsoil by water or wind — is a major environmental threat.

Causes:

• Removal of vegetation (farming, deforestation, overgrazing)
• Heavy rainfall on bare soil
• Wind over exposed land

Effects:

• Loss of fertile topsoil reduces agricultural productivity
• Sediment clogs waterways
• Downstream flooding increases

Prevention: planting cover crops, terracing slopes, maintaining hedgerows, reducing tillage.

Key Terms

• Soil — complex mixture of mineral particles, humus, water, air, and living organisms
• Humus — decomposed organic matter that darkens soil and improves structure
• Loam — ideal agricultural soil with balanced proportions of sand, silt, and clay
• Decomposer — organism that breaks down dead organic matter, releasing nutrients
• Nitrogen fixation — conversion of atmospheric N2 into usable nitrates by bacteria
• Soil erosion — removal of topsoil by water or wind, reducing soil fertility
• Horizon — a distinct layer within a soil profile

Common Mistakes

• Confusing humus (decomposed organic matter) with hummus (the food) — different spelling and meaning
• Thinking clay soil is always bad — it holds water and nutrients well; the problem is drainage
• Saying sandy soil is fertile — it drains too quickly, leaching nutrients before plants can absorb them
• Forgetting that air in soil is essential — waterlogged soil suffocates roots and kills decomposers

Tips and Tricks

• Remember soil components with the acronym MHWAO: Minerals, Humus, Water, Air, Organisms
• Soil horizon order from surface down: O-A-B-C-R (remember "Old Age Brings Cold Rest")
• Sandy soil = fast drainage = nutrients leach away; Clay soil = slow drainage = waterlogging
• Loam = best of both worlds — the target for most agricultural land