Last reviewed: 25 October 2026
Soil is the foundation of farming, and it is a core topic in CBC Agriculture from upper primary into junior school. Questions ask learners to compare soil types, describe simple experiments and explain how to stop soil erosion. This guide covers each part with examples from Kenyan farms.
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How Soil Forms
Soil forms slowly as rocks are broken down by weathering. Physical weathering happens when rocks heat and cool repeatedly, or when plant roots and running water break them. Chemical weathering happens when rainwater reacts with minerals in the rock. Living things add organic matter as plants and animals die and decay. It can take hundreds of years to form a few centimetres of soil, which is why protecting it matters.
Components of Soil
Soil is a mixture of:
- Mineral particles from weathered rock (sand, silt and clay).
- Organic matter (humus) from decayed plants and animals, which adds nutrients and improves structure.
- Water, which dissolves nutrients for plant roots.
- Air, which roots and soil organisms need for respiration.
- Living organisms such as earthworms, insects, bacteria and fungi.
Main Soil Types
- Sandy soil: large particles, feels gritty, drains water very quickly and holds little water or nutrients. It warms up fast and is easy to dig.
- Clay soil: very small particles, feels sticky when wet and hard when dry. It holds a lot of water but drains poorly and can become waterlogged. It is often rich in nutrients.
- Loam soil: a balanced mixture of sand, silt and clay with plenty of humus. It drains well, holds enough water and nutrients and is the best soil for most crops.
Simple Soil Experiments
Drainage and water retention: place equal amounts of sand, clay and loam in three funnels lined with filter paper or cotton wool. Pour the same amount of water into each and measure the water collected after a set time. Sandy soil drains most water fastest; clay drains least.
Soil components: shake soil with water in a clear jar and leave it to settle. Layers form, with heavy stones and sand at the bottom, then silt, then clay, and organic matter floating on top.
Air in soil: add water to a tin of dry soil and watch the bubbles rise, showing that soil contains air.
Soil Erosion
Soil erosion is the removal of topsoil by water or wind. Topsoil is the most fertile layer, so erosion lowers crop yields. Main causes include:
- clearing vegetation and cutting down trees;
- overgrazing by livestock;
- ploughing up and down slopes;
- heavy rain on bare land;
- strong winds in dry areas.
Soil Conservation Methods
- Terracing: building step-like levels on slopes, common in hilly areas such as parts of the central highlands.
- Contour farming: ploughing and planting across the slope rather than up and down.
- Cover crops and mulching: keeping the ground covered to protect it from rain and sun.
- Planting trees and grass strips: roots hold soil, and windbreaks reduce wind erosion.
- Controlled grazing: keeping livestock numbers suited to the land.
- Gabions and cut-off drains: slowing and redirecting running water.
Improving Soil Fertility
Farmers add manure and compost, rotate crops, plant legumes such as beans that add nitrogen, and use fertilisers carefully. Acidic soils may be treated with lime to raise the pH, which links to the science topic of acids and bases.
Revision Tips
Make a comparison table of the three soil types and draw the funnel experiment with labels. List five causes of erosion with a matching conservation method for each. The Grade 9 Agriculture page and Agriculture revision papers give exam practice, and our acids and bases guide explains liming. Full papers with answers are in the Monthly Pass (KSh 500 by M-Pesa).