Soil
Senior 1 · Agriculture
Soil
Lesson 1 of 4
20 min
Let's begin
The farms around Musanze grow some of Rwanda's best Irish potatoes on dark soils formed from the lava and ash of the Virunga volcanoes. On many other hills the soil is red, and in valley bottoms it is often grey and wet. All of these soils began as solid rock.
Soil takes a very long time to form and can be lost in a single rainy season. Knowing how it forms explains why soils differ from place to place, and why a farmer must treat the soil as something that cannot be quickly replaced.
Soil is the thin, loose upper layer of the earth's crust in which plants grow. It is a mixture of mineral particles (broken rock), organic matter (dead and decaying plants and animals), water, air and living organisms. A lump of rock is not soil: soil must be loose enough for roots to enter and must hold water, air and nutrients.
Soil formation (pedogenesis) happens in two overlapping stages:
Weathering is not the same as erosion. Weathering breaks rock down where it is; erosion carries soil or rock away.
| Type | How it works | Examples |
|---|---|---|
| Physical (mechanical): rock breaks into smaller pieces without any change in its chemicals | Heating by day and cooling at night make the outer layers of rock expand and contract until they crack and peel off (exfoliation). Rain drops, running water and wind-blown sand wear rock away. On very high, cold peaks, water freezing in cracks expands and splits the rock. | Peeling rock surfaces on bare hilltops; rounded pebbles in rivers |
| Chemical: the minerals in the rock are changed into new substances | Solution: some minerals dissolve in rain water, which is a weak acid because it contains carbon dioxide. Hydrolysis: water reacts with minerals such as feldspar and turns them into clay. Oxidation: iron in rock reacts with oxygen and forms red and yellow iron oxides (rust). Hydration: minerals take up water, swell and weaken. | The red colour of many Rwandan hill soils comes from oxidised iron |
| Biological: living things break rock physically or chemically | Roots grow into cracks and widen them; lichens and roots release acids that attack rock; earthworms, termites and moles mix and move soil; people dig, quarry and cultivate. | Tree roots splitting a rock; termite mounds built of fine subsoil |
In a warm, rainy climate like Rwanda's, chemical weathering is especially fast, because chemical reactions speed up with warmth and water.
Question: Suppose soil forms at 0.1 mm per year (an illustrative rate; real rates are often slower). (a) How many years does it take to form a 20 cm layer of topsoil? (b) A farmer loses 5 mm of topsoil from a bare slope in one year. How many years of soil formation is that?
(a) Convert to the same unit: 20 cm = 200 mm. Time = 200 mm ÷ 0.1 mm per year = 2,000 years.
(b) 5 mm ÷ 0.1 mm per year = 50 years of soil formation lost in one year.
Interpretation: soil is, in practice, not renewable within a human lifetime. Keeping it in place is far easier than waiting for new soil to form.
Tip
Memory hook: the five soil-forming factors spell ClORPT: Climate, Organisms, Relief (topography), Parent material, Time.
Exam technique: for each type of weathering, name the agent (heat, water, oxygen, roots) AND the process (expansion and contraction, hydrolysis, oxidation, root wedging). A bare list of names earns few marks.
Try each question before reading the solution.
Define soil.
Solution: Soil is the thin, loose upper layer of the earth's crust in which plants grow, made of mineral particles, organic matter, water, air and living organisms.
Distinguish between weathering and erosion.
Solution: Weathering is the breakdown of rock into smaller particles where it lies. Erosion is the removal and transport of soil or rock by agents such as running water and wind.
Explain how daily temperature changes break rock.
Solution: By day the outer layer of the rock heats and expands more than the inside; at night it cools and contracts. Repeated day after day, this stress cracks the outer layer, which peels off (exfoliation).
Why are soils on steep hilltops usually thinner than soils in valley bottoms?
Solution: On steep slopes, runoff washes soil downhill almost as fast as it forms, so it stays thin. The washed soil collects at the foot of the slope and in the valley, where it builds up deep.
Soil forms at 0.05 mm per year. How many years are needed to form 1 cm of soil?
Solution: 1 cm = 10 mm. 10 mm ÷ 0.05 mm per year = 200 years.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
