Soil formation, properties and fertility: Agricultural Science made simple

Soil is the foundation of crop production, so soil science is one of the most heavily examined areas of Agricultural Science. Questions on soil formation, the soil profile, texture, fertility and conservation appear regularly in WAEC, NECO and JAMB, in objective, theory and practical papers alike.

What soil is

Soil is the loose, weathered top layer of the earth's crust in which plants grow. It is made up of four main components:

Soil also contains living organisms, such as earthworms, bacteria, fungi and insects, which break down organic matter and improve soil structure.

How soil is formed

Soil forms from rock through weathering, the breakdown of rocks into smaller particles. There are three types:

• Physical (mechanical) weathering: rock breaks down without a change in its chemical composition. Causes include temperature changes (expansion by day and contraction by night), water freezing in cracks, and the action of wind and running water.
• Chemical weathering: the rock's minerals are changed by chemical reactions such as hydrolysis, oxidation, carbonation and hydration. It is especially active in hot, wet climates like southern Nigeria.
• Biological weathering: living things break rocks down, for example plant roots growing into cracks, burrowing animals, and acids produced by lichens.

Factors affecting soil formation

1. Parent material: the rock the soil forms from.
2. Climate: rainfall and temperature control the speed of weathering.
3. Living organisms: plants, animals and micro-organisms.
4. Topography (relief): slope affects drainage and erosion.
5. Time: soil formation is very slow.

The soil profile

A soil profile is a vertical section through the soil showing its layers, called horizons:

The process of minerals being washed down from the topsoil into lower layers is called leaching. It is a major cause of low fertility in high-rainfall areas.

Soil texture

Soil texture describes the relative proportions of sand, silt and clay particles.

Loamy soil is usually the best agricultural soil because it balances drainage, aeration, water retention and nutrient supply.

Soil structure

Soil structure is how soil particles are grouped together into clumps (aggregates or crumbs). A good crumb structure allows air and water to move through the soil and roots to grow easily. Organic matter, earthworms and good management improve structure; heavy machinery and overworking the soil can damage it.

Soil pH

Soil pH measures how acidic or alkaline the soil is, on a scale of 0–14:

• Below 7: acidic
• 7: neutral
• Above 7: alkaline

Most crops grow best in soils that are slightly acidic to neutral (around pH 6–7). Very acidic soils are common in high-rainfall areas because of leaching, and they can be corrected by liming, adding lime (calcium carbonate) to raise the pH.

Soil fertility

A fertile soil can supply the nutrients crops need, in the right amounts and proportions, for good growth and yield.

Nutrients plants need

• Macronutrients (needed in large amounts): nitrogen (N), phosphorus (P), potassium (K), calcium, magnesium and sulphur.
• Micronutrients (trace elements, needed in small amounts): iron, zinc, copper, manganese, boron, molybdenum and others.

The three major nutrients are N, P and K:

Causes of loss of soil fertility

• Leaching of nutrients by heavy rain.
• Erosion removing the fertile topsoil.
• Continuous cropping without replacing nutrients.
• Bush burning, which destroys organic matter and soil organisms.
• Crop removal: harvesting takes nutrients away with the crop.

Maintaining soil fertility

• Crop rotation: growing different crops in sequence, including legumes, which fix nitrogen with the help of Rhizobium bacteria in their root nodules.
• Adding organic manure: farmyard manure, compost and green manure.
• Applying inorganic fertilisers such as NPK, urea and single superphosphate.
• Bush fallowing: leaving land to rest and recover.
• Cover cropping and mulching to protect the soil and add organic matter.
• Liming acidic soils.
• Preventing erosion and avoiding bush burning.

Fertilisers

An NPK 15-15-15 fertiliser contains 15% nitrogen, 15% phosphorus (as P₂O₅) and 15% potassium (as K₂O) by weight.

Methods of fertiliser application include broadcasting, band placement, ring application (around trees), side dressing, top dressing and foliar application (spraying on leaves).

Soil erosion and conservation

Soil erosion is the removal of topsoil by water or wind. In Nigeria, water erosion causes serious gully erosion in parts of the South-East, while wind erosion is a problem in the far North.

Conservation methods include:

• Contour ploughing: ploughing across the slope along contour lines.
• Terracing: cutting step-like level areas on steep slopes.
• Strip cropping: alternating strips of different crops across a slope.
• Cover cropping: keeping the soil covered with low-growing plants.
• Mulching: covering the soil with organic materials.
• Afforestation and windbreaks: planting trees to protect soil.
• Avoiding bush burning and overgrazing.

Common exam mistakes

• Confusing texture and structure. Texture is particle size proportions; structure is how particles are grouped.
• Mixing up the horizons. The A horizon (topsoil) is the most fertile.
• Getting N, P and K roles wrong. Nitrogen is for leaves, phosphorus for roots and fruiting, potassium for disease resistance and strength.
• Confusing similar conservation methods, such as strip cropping and contour ploughing.

Practise soil science with explanations

Soil science is the focus of Levia's Agricultural Science Level 2 (Land, Soil and Soil Management). Every question explains the concept with Nigerian farming context.