Soils in Africa and the World
Senior 3 · Geography
Soils in Africa and the World
Lesson 1 of 4
25 min
Let's begin
Dig a pit in the Congo Basin and you find soil that is red or yellow and many metres deep. Dig one in the Sahara and you meet sand and stones a few centimetres down. Dig one in the grasslands of Ukraine and the top metre is almost black. All three began as broken rock. In S1 you learned what soil is made of and how to describe a soil profile, and in S2 you studied the soils of Rwanda. This unit asks a bigger question: why are soils so different from one part of the world to another?
Definition
Soil formation (pedogenesis): the slow process by which rock and dead plant and animal matter are changed into soil with distinct layers.
Parent material: the rock, or the loose material such as river mud or volcanic ash, from which a soil forms.
Soil horizon: a layer in a soil, roughly parallel to the surface, with its own colour, texture and make-up. The layers seen together in a pit make the soil profile.
Soil scientists since Vasily Dokuchaev in the 1880s have explained soil differences with five factors. A useful way to remember them is the "equation" below: soil (S) depends on climate (cl), organisms (o), relief (r), parent material (p) and time (t).
Figure 1. The five factors of soil formation. Drawn by Ganzaa.
Climate is the most important factor over large areas. Rainfall controls how much water soaks down through the soil, carrying minerals with it; temperature controls how fast rock breaks down chemically and how fast dead matter rots. In hot, wet places such as the Congo Basin, rock weathers to great depth and the soil is deep but heavily washed out. In hot deserts such as the Sahara, there is too little water for deep weathering or for plants, so soils are thin, sandy or stony and may have salt crusts.
Plants supply dead leaves and roots, which decay into dark, fertile humus. Grassland roots add humus deep into the soil, while forest litter adds it mainly at the surface. Earthworms, termites and burrowing animals mix and air the soil; bacteria and fungi break down dead matter. People are organisms too: they clear forest, plough, irrigate and add fertiliser, and can improve or ruin a soil in a few years.
On steep slopes, rain washes soil away almost as fast as it forms, so soils are thin. On gentle lower slopes, material collects and soils are deep. On flat valley floors, water collects and the soil becomes waterlogged. A sequence of different soils down one slope is called a catena (you met the word in S1). Relief also affects climate: high ground is cooler and often wetter.
Figure 2. A catena: how soils change from hilltop to valley floor on the same rock. Drawn by Ganzaa.
The rock supplies the soil's minerals and much of its texture. Soils on volcanic rock such as basalt or ash are often dark and rich in plant nutrients, as on the slopes of the East African volcanoes and the Ethiopian Highlands. Soils on granite tend to be sandy and less fertile. Soils on limestone are rich in calcium. Young soils look most like their parent rock; old soils in hot, wet climates may look very little like it.
Soil forms very slowly. A young soil, for example on a fresh lava flow or a new river deposit, is thin and has few horizons. Over thousands of years horizons become clearer and deeper. Much of Africa is old, stable plateau, so many African soils have been weathered and washed for an extremely long time and have lost many of their nutrients.
Food and Agriculture Organization of the United Nations (FAO): Soil 101, a short animation on what soil does for us and what threatens it. Notice how long soil takes to form compared with how fast it can be lost.
Worked example
Worked example 1: which factor? For each observation, name the main factor at work.
| Observation | Main factor |
|---|---|
| A soil on a steep hillside is only 10 cm deep, but the soil at the foot of the same hill is 2 m deep. | Relief |
| Two fields with the same climate have dark, fertile soil on old lava and pale, sandy soil on granite. | Parent material |
| A soil on a lava flow from the 1900s is thin and stony, while one on older lava nearby is deep. | Time |
| Soils become thinner and saltier from the forest belt northwards into the desert. | Climate |
Worked example
Worked example 2: explaining a contrast. Explain why soils in the hot, wet Congo Basin are deep but poor in nutrients.
Conclusion: deep, because weathering is fast and long-lasting; poor, because nutrients are washed out and little humus collects.
Warning
Do not list the factors, link them. "Climate, organisms, relief, parent material and time" earns little on its own. Examiners want each factor tied to a result: "heavy rain washes nutrients down out of reach of roots, so the soil is infertile".
Tip
Memory hook: "CLORPT". CLimate, Organisms, Relief, Parent material, Time. Say it as one word, "clor-pt", and you will never forget a factor.
Practice
Sources and credits
Video: Soil 101, Food and Agriculture Organization of the United Nations, embedded from YouTube.
Figures 1 and 2, the worked examples and the practice exercises are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
