Soil science and soil fertility

Soil formation

Factors: parent material, climate, organisms, topography and time (CLORPT). Processes: weathering, humification, leaching, eluviation and illuviation. Zimbabwe's soils range from infertile granite-derived sands (much of the communal areas) to fertile red clays derived from dolerite and basalt.

Physical properties

Texture (sand, silt, clay proportions – the textural triangle), structure (crumb, blocky, platy), bulk density, porosity, water-holding capacity (field capacity, permanent wilting point, available water).

Chemical properties

• Soil colloids (clay and humus) carry negative charges and hold cations.
• Cation exchange capacity (CEC): the soil's ability to hold exchangeable cations (Ca²⁺, Mg²⁺, K⁺, Na⁺, H⁺). Clay and organic matter raise CEC.
• Soil pH controls nutrient availability; most nutrients are most available at pH 6–7. Acid soils are limed; aluminium toxicity occurs in very acid soils.

Nutrients

Macronutrients (N, P, K, Ca, Mg, S) and micronutrients (Zn, B, Mn, Cu, Fe, Mo). Deficiency symptoms, e.g. N – yellowing of older leaves; P – purple leaves; K – leaf-edge scorch; Zn – white bands in maize leaves.

The nitrogen cycle and fertilisers

Fixation, mineralisation, nitrification, denitrification, volatilisation and leaching. Fertiliser recommendations are based on soil tests and yield targets.

Check yourself

1. Name the five soil-forming factors.
2. Why do sandy soils have a low CEC?
3. Give the deficiency symptom of phosphorus in maize.