Soil Properties and Fertility

Why soil matters

Soil is the natural medium in which crops grow. Under the Kenyan Senior School CBC (KICD) Agriculture pathway, you learn how to read soil so that you can choose the right crop and management practice for your farm or school garden.

Components of soil

Soil is made up of:

• Mineral particles from weathered rock (about 45%)
• Organic matter or humus from decayed plants and animals (about 5%)
• Water (about 25%) and air (about 25%)
• Living organisms such as earthworms, bacteria and fungi

Physical properties

Soil texture refers to the relative proportions of sand, silt and clay.

• Sand (2.0 - 0.05 mm): feels gritty, drains fast, holds few nutrients.
• Silt (0.05 - 0.002 mm): feels smooth like flour.
• Clay (below 0.002 mm): sticky when wet, holds water and nutrients.
• Loam has a balanced mixture and is the best for most crops.

Soil structure is how particles clump into crumbs. Good crumb structure allows air and water movement and root growth. Porosity is the amount of pore space, and soil temperature affects germination and microbial activity.

Chemical properties

Soil pH measures acidity. pH 7 is neutral, below 7 acidic, above 7 alkaline. Most crops prefer pH 6.0 to 7.0. Tea, however, prefers acidic soils (pH 4.5 to 5.5), which is why it thrives in Kericho and Nandi. Cation exchange capacity is the soil's ability to hold nutrients such as potassium and calcium.

The soil profile

A soil profile is a vertical section showing layers called horizons:

1. O/A horizon - topsoil rich in humus and roots.
2. B horizon - subsoil where leached minerals collect.
3. C horizon - weathered parent rock.
4. R - bedrock.

Soil fertility

A fertile soil supplies nutrients in adequate amounts and balance. Farmers maintain fertility by:

• Applying farmyard manure, compost and inorganic fertiliser
• Crop rotation and growing legumes (beans, desmodium) that fix nitrogen
• Liming acidic soils
• Controlling erosion through terraces and cover crops
• Mulching to add organic matter and reduce evaporation

Worked example 1

A soil sample has 40% sand, 40% silt and 20% clay. Which texture is it? Answer: None of the three dominates strongly, so it is a loam, suitable for maize and vegetables.

Worked example 2

A farmer in Kiambu finds soil pH 5.0 but wants to grow beans (pH 6.0 to 7.0). What should he do? Answer: Apply agricultural lime (calcium carbonate) to raise the pH, then retest after a season.

Try it

• Collect a handful of moist soil, roll it into a ribbon and describe its texture.
• Dig a small pit and sketch the horizons you see.
• List three practices used in your community to maintain soil fertility.
• Explain why legumes are used in crop rotation.