Soils
Senior 1 · Geography
Soils
Lesson 4 of 4
25 min
When a new latrine pit or a road cutting is dug, look at its side. The soil is not the same all the way down: a dark band at the top, then a thicker red or yellow layer, then broken, crumbly rock, and at the bottom hard rock. Each band is a horizon, and together they tell the history of the soil.
Definition
Soil profile: a vertical section through a soil, from the surface down to the parent rock, showing its layers (horizons). Example: the side of a road cutting between Kigali and Musanze.
Definition
Soil horizon: a layer of soil, roughly parallel to the surface, with its own colour, texture and structure. The main horizons are named O, A, B, C and R.
Figure 1. A soil profile. Depths are typical of a Rwandan hillside and vary from place to place.
Definition
Leaching: the washing down of dissolved minerals and nutrients from the upper layers of a soil by water soaking through it. Heavy rain causes strong leaching, which makes topsoil poorer and more acidic.
Walk from the top of a Rwandan hill down to the marsh in the valley and the soil changes, even though the rock underneath is the same. On the steep slopes soil is washed away; at the bottom it piles up and stays wet. This regular sequence of soils down a slope is called a catena (Latin for "chain").
Figure 2. A soil catena: the same rock, but different soils from hilltop to valley because of slope and drainage.
Definition
Soil catena: a sequence of different soils down a slope, formed from the same parent rock but differing because of slope, drainage and the movement of soil and water downhill. Example: thin, eroded soils on the upper slopes of a hill in Huye District and dark, waterlogged soils in the marsh below.
Worked example: how much soil does a bare slope lose?
Studies on steep, bare farmland in the tropics often measure soil losses of 50 tonnes per hectare in a year. Soil weighs about 1.3 tonnes per cubic metre. How deep a layer is that? (1 hectare = 10,000 m²)
Step 1. Volume lost.
Step 2. Depth over one hectare.
So what? 4 mm a year sounds small, but in 25 years it is 10 cm, nearly half of the topsoil of Figure 1, and the topsoil is where the fertility is. With terraces and ground cover, losses can fall to a small fraction of this.
| Importance | How | Rwandan example |
|---|---|---|
| Food | Almost all food grows in soil | About two-thirds of Rwandans work in farming |
| Water | Soil soaks up rain, stores it, filters it and releases it to springs and rivers | Springs at the foot of hills; wetlands such as Rugezi act as sponges |
| Building | Clay for bricks and tiles; earth for walls and roads | Brick-making in valley bottoms |
| Home for life | Earthworms, termites, fungi, bacteria and roots | A handful of healthy topsoil holds billions of microbes |
| Climate | Soils and peat store huge amounts of carbon | Peat in marshlands such as Rugezi |
| Income and exports | Soil grows cash crops | Tea and coffee exports |
Important
Case study: saving the soil on Rwanda's hills
Rwanda has steep slopes, heavy rain and dense farming, three conditions that make soil erosion likely. Farmers and the government fight it together:
Soil Profile and Soil Horizons, by MooMooMath and Science (3 min). As you watch, match each horizon described to its place in Figure 1.
Look at the catena in Figure 2. A family owns one field on the upper slope and one in the valley bottom. Suggest the best use for each field, and one way to protect the soil in each.
Tip
Horizons from the top: "Only Ants Build Castles on Rock" (O, A, B, C, R). And a catena is a chain of soils linked by the slope.
Practice
Remember
Further reading
Books and resources for this unit
Sources and credits
The lesson text, profile and catena diagrams, table, worked example and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
