Soil
Senior 3 · Agriculture
Soil
Lesson 1 of 4
25 min
Let's begin
In S1 you learnt that soil forms very slowly from weathered rock, and in S2 that land pressure pushes farming onto steep slopes. Now put the two together. A heavy afternoon storm in Season A can carry away, in one hour, a layer of soil that took decades to form. Where does that soil go, what carries it, and how can a farmer tell how badly a field is being eroded? This lesson answers those three questions.
Definition
Soil erosion: the detachment (loosening) and transport of soil particles from one place to another by an agent such as water, wind or gravity. Example: rain washing topsoil from a bean field into the valley below.
Agent of erosion: the force or thing that moves the soil: water, wind, gravity, or living things (people and animals).
Geological (natural) erosion: the slow erosion that happens under natural vegetation, roughly balanced by the formation of new soil.
Accelerated erosion: erosion made faster than soil formation by human activity, such as clearing, digging and overgrazing. This is the erosion farmers must control.
Runoff: rainwater that flows over the ground surface because it cannot soak into the soil fast enough.
Every kind of erosion follows the same three stages:
The finest and most valuable particles, clay and humus, are the easiest to carry, so eroded soil is always poorer than the soil it came from. That is why erosion is measured by soil lost, not just soil moved.
| Agent | How it moves soil | Where you see it in Rwanda |
|---|---|---|
| Water | Raindrops detach particles; runoff and streams carry them away. | On almost every cultivated slope during the rainy seasons; the main agent nationally. |
| Wind | Lifts and rolls dry, loose particles from bare ground. | Bare, dry fields and overgrazed land in the lower, drier east (for example Bugesera and Kayonza) in the dry season. |
| Gravity | Pulls soaked soil and rock downhill as landslides, mudflows or slow soil creep. | Very steep slopes of the Western and Northern Provinces after days of heavy rain. |
| Living things | People dig, clear and make paths; animals trample, overgraze and make tracks that channel runoff. | Footpaths that turn into channels; cattle tracks down to a water point. |
Water erosion gets worse in a fixed order as runoff gathers into bigger and bigger channels. Figure 1 shows the four types as cross-sections of the soil surface.
Figure 1. The four types of water erosion. Channels grow deeper from left to right as runoff concentrates.
Two other forms complete the list. Stream-bank erosion is the cutting away of the banks of rivers and streams by fast water. Landslides (mass movement) happen when a slope is so soaked that a whole block of soil slips downhill under gravity.
Worked example 1: how much soil does a hectare lose?
Problem. A 2022 study by the Rwanda Water Resources Board and IUCN estimated that eroding land in Rwanda loses on average about of topsoil per hectare per year. Topsoil has a bulk density of about (illustrative). What depth of soil is lost from a hectare in one year, and in 20 years?
Solution.
Answer: about 1.9 mm a year, or nearly 4 cm in 20 years. FAO notes that forming just 2 to 3 cm of soil can take up to 1,000 years, so this loss cannot be replaced in a farmer's lifetime.
Worked example 2: soil lost from a family farm
Problem. A family farms of unprotected slope that loses a year. How many 50 kg sacks of soil is that?
Solution. Mass lost . Number of sacks .
Answer: 200 sacks of the farm's richest soil leave the field every year.
Warning
Common mistakes. (1) Erosion is not weathering: weathering breaks rock down where it lies; erosion moves the soil. (2) Do not judge a field by its gullies alone. By the time rills and gullies appear, sheet erosion has usually removed centimetres of topsoil unseen. Look for pale soil, stones left on the surface and exposed roots.
Tip
Memory hook: "Drops, sheets, rills, gullies": each stage carries more water in fewer, deeper channels. In an exam, define each type by what you see and how deep.
geocy: "Soil Erosion; Sheet erosion, Rill erosion and Gully erosion". As you watch, note how each type differs from the one before it in depth and in how much the runoff has concentrated.
Practice
Key points
Sources and credits
Written by Ganzaa: the text, Figure 1, the table, the worked examples and the practice questions. The soil density is an illustrative value.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
