Soil Science and Land Management: Fertility, Water, Erosion
Soil Science and Land Management tests soil formation, texture, structure, pH, organic matter, nutrient supply and water, then the land practices that keep a field productive: erosion control, liming, fertiliser placement, rotation, mulching and minimum tillage.
Core rules
- Texture (sand, silt, clay) controls water holding and nutrient retention; structure controls drainage and root growth. Most Kalahari sands are deep, well drained, low in organic matter and low in cation exchange capacity, so they hold little fertiliser.
- pH governs nutrient availability. Lime raises pH and neutralises exchangeable aluminium on acid soils, and the rate comes from a soil test.
- Erosion begins when soil is bare and raindrops detach particles; sheet, rill and gully erosion form a sequence, and wind erosion dominates on dry sandy land.
- Control measures: contour ridges, grass strips, mulching, cover crops, reduced tillage, and never cultivating straight down a slope.
- Rotation and manure maintain organic matter, the cheapest water-holding investment on sand.
Worked example: fertiliser for a small plot
A soil test recommends 60 kg N and 30 kg P₂O₅ per hectare. The plot is 40 m × 100 m = 0.4 ha. Urea is 46% N, so 60 ÷ 0.46 = 130 kg urea/ha; for 0.4 ha that is 52 kg. Single superphosphate is 20% P₂O₅, so 30 ÷ 0.20 = 150 kg/ha; for 0.4 ha that is 60 kg. Always convert the recommendation to product, then scale to field area.
Traps
- Mixing kg/ha with kg per plot.
- Assuming sandy soils retain applied nutrients.
- Applying lime and nitrogen at the same time.
- Leaving land bare through the rainy season.