Soil
Senior 2 · Agriculture
Soil
Lesson 2 of 5
20 min
Let's begin
Rwanda is called the land of a thousand hills, and most of its farms sit on slopes. When a heavy rain falls on a bare hillside, the water that runs off turns brown. That colour is topsoil, the most fertile layer of the farm, travelling down to the Nyabarongo and on towards the Nile.
Fertility can be built up or run down. Some factors are natural (rainfall, slope, parent rock), and many are caused by the way land is farmed. Knowing them lets you explain why a field is getting poorer and choose the right remedy.
Question: Topsoil has a bulk density of about 1,300 kg per cubic metre. (a) What mass of soil is lost from one hectare when erosion removes a layer just 1 mm thick? (b) A bare slope loses 50 tonnes of soil per hectare in one year. How thick a layer is that?
(a) One hectare = 10,000 m². 1 mm = 0.001 m.
Volume = 10,000 m² × 0.001 m = 10 m³.
Mass = 10 m³ × 1,300 kg/m³ = 13,000 kg = 13 tonnes.
(b) Each millimetre is 13 tonnes, so 50 ÷ 13 ≈ 3.8 mm of topsoil in one year.
Interpretation: 3.8 mm sounds small, but soil forms from rock extremely slowly, far more slowly than a few millimetres a year. Over 10 years that slope loses almost 4 cm of its richest layer. This is why Rwanda has invested heavily in terracing through community work and public works programmes.
Tip
Memory hook: the six thieves of fertility are E-L-M-B-A-W: Erosion, Leaching, Monocropping, Burning, Acidity, Waterlogging.
Exam technique: for every factor you give, pair it with a control measure. "Erosion: build terraces and plant grass strips along the contour" scores more than "erosion" alone.
Distinguish between soil erosion and leaching.
Solution: Erosion removes the soil itself (mainly topsoil) from the surface, carried by water or wind. Leaching removes dissolved nutrients by carrying them downwards through the soil, below the root zone; the soil stays in place.
Give two ways in which burning crop residues lowers soil fertility.
Solution: Any two: it destroys organic matter; it kills useful soil organisms; nitrogen and sulphur escape as gases; it leaves the soil bare and easily eroded.
A farmer has grown maize on the same plot for eight seasons and yields keep falling. Suggest two causes and two remedies.
Solution: Causes: continuous cropping has removed the same nutrients each season; maize pests and diseases have built up. Remedies: rotate with a legume such as beans or soybean; add compost or farmyard manure (and fertiliser if a soil test shows it is needed).
A 0.5 ha plot loses a 2 mm layer of topsoil in one rainy season. Using a bulk density of 1,300 kg/m³, find the mass of soil lost.
Solution: Area = 0.5 × 10,000 = 5,000 m². Depth = 0.002 m. Volume = 5,000 × 0.002 = 10 m³. Mass = 10 × 1,300 = 13,000 kg = 13 tonnes.
Explain how grass strips on progressive terraces reduce soil loss.
Solution: The strips run along the contour. They slow the runoff so it drops the soil it carries, and their roots bind the soil. Over time the trapped soil builds up behind each strip and the slope between strips becomes flatter, forming a terrace.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
