Soil
Senior 3 · Agriculture
Soil
Lesson 3 of 4
25 min
Let's begin
Erosion does damage in two places. On the hill, the field loses its richest soil. Down in the valley, the same soil arrives as unwanted mud in rivers, marshes and lakes. And even a field that is not eroding can still wear out if it gives more nutrients to every harvest than it gets back. This lesson looks at both problems: the effects of erosion, and soil exhaustion.
Definition
On-site effect: damage done on the field where the soil is lost. Example: falling yields on an eroded hillside.
Off-site effect: damage done where the eroded soil ends up. Example: silt filling a reservoir.
Siltation (sedimentation): the build-up of eroded soil (silt) in rivers, lakes, marshes, dams and drains.
Soil exhaustion: the decline of a soil's fertility, its nutrients, organic matter and structure, through repeated use without replacing what is removed, until it can no longer give good yields.
Leaching: the washing of dissolved nutrients down through the soil, below the reach of plant roots, by water that drains through it.
| On the farm (on-site) | Downstream (off-site) |
|---|---|
| The topsoil, which holds most of the humus and nutrients, is lost, so yields fall each season. | Rivers carry heavy loads of silt; water costs more to treat for drinking. |
| Seeds, seedlings, manure and fertiliser are washed away soon after planting. | Silt fills dams, reservoirs and lakes, so they store less water and hydroelectric plants lose capacity. |
| The soil becomes shallower and holds less water, so crops suffer sooner in a dry spell. | Silt raises river beds and blocks channels, so floods become more frequent. |
| Roots are exposed and plants fall over. | Silt buries crops and fills drains in valley-bottom marshlands. |
| Rills and gullies cut fields, block paths and make work slow. | Nutrients carried with the soil feed weeds and algae in lakes and slow-moving water. |
| Landslides destroy crops, houses and roads, and can kill people and animals. | Roads, bridges and culverts are damaged and must be cleared. |
One effect is sometimes useful: silt deposited on valley floors over centuries built the deep, fertile soils of some marshlands. But on a farm the gain in the valley never repays the loss on the hill, and fresh silt usually arrives as damage, burying a crop.
The 2022 national assessment by the Rwanda Water Resources Board (RWB) and IUCN estimated that:
Think of the nutrients in the root zone as a bank account. Every harvest is a withdrawal; manure, compost, residues and fertiliser are deposits. Figure 3 shows the main entries. When losses are larger than gains, season after season, the soil is exhausted.
Figure 3. The nutrient budget of a field.
Signs of an exhausted soil: yields fall although the farmer works as hard as before; crops are stunted, with pale or discoloured leaves; the soil is pale, hard and crusts after rain; few earthworms are found; and weeds that tolerate poor soil spread.
Worked example 1: what a maize harvest takes away
Problem. Assume each tonne of maize grain carries away about of nitrogen (illustrative). A farmer harvests . How much urea (46% N) would replace the nitrogen in the grain from one hectare?
Solution.
Answer: about 130 kg of urea, a little over two and a half 50 kg bags, just to replace the grain's nitrogen. If the stalks are also removed and nothing is returned, the deficit grows each season.
Worked example 2: a nutrient budget
Problem. In one year a field gains from manure and from bean roots. It loses in harvested crops, by leaching and with eroded soil. Find the balance and comment. (Illustrative values.)
Solution. Gains . Losses . Balance per year.
Comment: the soil is being exhausted by 70 kg of nitrogen a year. Stopping the erosion would remove 30 kg of the loss; more manure, residues or fertiliser would add to the gains.
Warning
Common mistake. Soil exhaustion and soil erosion are not the same thing. Erosion moves the soil away; exhaustion empties the soil that stays. A flat valley field that never erodes can still be exhausted by years of continuous cropping, and erosion is only one of the causes of exhaustion.
Tip
Exam technique: sort effects into on-site and off-site before you write, and give each a consequence: "Silt fills the reservoir, so it stores less water for power." For exhaustion, think of the budget: every cause is either a loss that is too large or a gain that is missing.
Earth Science Classroom: "Soil Science: What Is Soil Leaching?". As you watch, note which nutrients are washed down and why leaching makes the topsoil poorer and more acid.
Practice
Key points
Sources and credits
Written by Ganzaa: the text, the tables, Figure 3 and the worked examples. Nutrient figures in the worked examples are illustrative.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
