Soil Science, Soil Management and Conservation
This domain tests your ability to describe soil properties, interpret soil tests and recommend management that conserves Namibia's fragile soils. Most Namibian soils are sandy, low in organic matter and vulnerable to wind and water erosion.
Core soil properties
- Texture is the sand, silt and clay proportion; structure is how particles aggregate. Sandy soils drain quickly but hold few nutrients.
- Cation exchange capacity (CEC) is measured in cmol(+)/kg. Kalahari sands often have CEC below 5, so leaching losses are high.
- pH below 5.5 indicates acidity, common in Kavango sands; alkaline and calcareous soils occur in parts of the north-central region.
- Organic matter is usually below 1%; it supplies nutrients, holds water and stabilises aggregates.
- Saline and sodic soils occur in some irrigation schemes and are diagnosed with electrical conductivity and sodium levels, not pH alone.
Conservation measures
- Protect the surface with crop residues, mulching and cover crops to reduce wind erosion.
- Use contour banks, grass strips, check dams and gully control where runoff concentrates.
- Rip or deep-rip to break plough pans and improve infiltration.
- Recycle manure, rotate with legumes and band phosphorus near the seed on low-P sands.
Worked example
A soil test reports pH 4.9, phosphorus 3 mg/kg (low), potassium 60 mg/kg, CEC 2.5 cmol(+)/kg and organic matter 0.5%. The reasoning is: acidity limits nutrient availability, phosphorus is deficient, but the very low CEC means heavy liming can raise pH too far and lose bases to leaching. The recommendation is a laboratory-recommended lime rate split over seasons, banded phosphorus rather than broadcast, manure or compost to build organic matter, and a legume rotation. Re-test before adding more lime.
Traps
- Treating a pH drop of one unit as a small change; it is a tenfold change in acidity.
- Confusing CEC units or ignoring texture when interpreting nutrient levels.
- Over-liming low-CEC sands.
- Calling all pale soils infertile; depth and water holding also matter.