Classification of Soils and Soil Formation

Lessons

Senior 6 · Geography

Classification of Soils and Soil Formation

3 lessons

Lesson 3 of 3

Soil Erosion, Conservation and the Value of Soils

25 min

Soil that takes centuries to make and a season to lose

A few centimetres of topsoil can take many centuries to form, yet one heavy storm on a bare, steep field can wash away more than a year's worth of it. Rwanda knows this problem well: its steep slopes and heavy rainfall make soils vulnerable to erosion and land degradation, and terraced fields now dominate much of the farmed land, while the government has run large reforestation, terracing and wetland restoration programmes. This lesson explains the causes, types and effects of soil erosion, the methods used to conserve soil, and why soils matter so much to the economy.

Definition

Soil erosion: the removal of soil, faster than it forms, by water, wind or people. Soil conservation: methods that protect soil from erosion and maintain or improve its fertility.

Types and causes of soil erosion

Type	What happens
Splash erosion	Raindrops strike bare soil and throw particles into the air, breaking up crumbs
Sheet erosion	A thin film of runoff removes a uniform layer of topsoil, often unnoticed
Rill erosion	Runoff concentrates into small channels a few centimetres deep
Gully erosion	Rills grow into deep channels that cannot be ploughed over and can cut into roads and buildings
Wind erosion	Dry, loose soil is blown away in dry lands, as in dust storms
Mass movement	Landslides and slumps carry soil downslope (unit 4)

Physical causes: heavy, intense rainfall; steep and long slopes; erodible soils such as silts and sands; and strong winds in dry areas. Human causes: deforestation and clearing of vegetation; cultivating steep slopes without protection; ploughing up and down the slope; overgrazing; bush burning; and poorly drained roads and paths that concentrate runoff.

Areas most affected include steep, densely farmed highlands such as those of Rwanda, Burundi and Ethiopia; semi-arid lands at the edge of deserts such as the Sahel, where wind erosion is strong; and cleared tropical forest land.

Figure 4: how terraces reduce runoff on a slope.

Worked example 1: estimating soil loss

Problem. The Universal Soil Loss Equation estimates average annual soil loss as $A = R \times K \times LS \times C \times P$ , in tonnes per hectare per year. For a maize field on a steep slope take (illustrative values) rainfall erosivity $R = 500$ , soil erodibility $K = 0.2$ , slope factor $L S = 6$ , cover factor $C = 0.3$ and practice factor $P = 1.0$ (no conservation). Find $A$ . Then find it with bench terraces ( $P = 0.15$ ).

Solution.

A = 500 \times 0.2 \times 6 \times 0.3 \times 1.0 = 180\,\mathrm{t\,ha^{-1}\,yr^{-1}}

A_{\text{terraced}} = 500 \times 0.2 \times 6 \times 0.3 \times 0.15 = 27\,\mathrm{t\,ha^{-1}\,yr^{-1}}

Terracing cuts the estimated loss by $1 - 0.15 = 85\%$ . Adding a mulch that lowers $C$ to 0.1 would cut it further, to $9\,\mathrm{t\,ha^{-1}\,yr^{-1}}$ .

Worked example 2: loss compared with formation

Problem. Soil with a bulk density of $1.3\,\mathrm{t\,m^{-3}}$ is lost at 180 t per hectare per year. How many millimetres of soil is that? Compare it with a formation rate of about 0.1 mm a year (illustrative).

Solution. One hectare is $10\,000\,\mathrm{m^2}$ .

\text{depth} = \frac{180\,\mathrm{t}}{1.3\,\mathrm{t\,m^{-3}} \times 10\,000\,\mathrm{m^2}} = 0.0138\,\mathrm{m} \approx 14\,\mathrm{mm\,yr^{-1}}

That is about $\frac{14}{0.1} = 140$ times faster than the soil forms; in 20 years about 28 cm, the whole of a typical topsoil, would be gone. With terraces (27 t, about 2 mm a year) the loss is still about 20 times the formation rate, which is why terraces are usually combined with mulch, cover crops and trees.

Effects of soil erosion

On farms: loss of fertile topsoil and organic matter, falling yields, exposed roots, gullies that cut fields into pieces.
Downstream: silt fills rivers, lakes, reservoirs and irrigation canals, reduces the life of hydroelectric dams, muddies drinking water and smothers wetlands and fish breeding grounds; fertiliser washed off fields feeds algae.
On people: food insecurity, poverty and migration from degraded land; damage to roads and buildings from gullies and landslides.

Soil management and conservation

Method	How it protects soil
Bench (radical) terraces	Level steps cut into the slope (Figure 4) stop runoff gaining speed and let water soak in; risers are protected by grass
Progressive terraces	Hedges or grass strips planted along the contour trap soil, which builds up behind them into terraces over several years
Contour ploughing and ridging	Furrows along the contour hold water instead of channelling it downhill
Agroforestry and afforestation	Tree roots bind soil; leaves break the force of rain and add organic matter
Mulching and cover crops	Keep the surface covered, preventing splash and sheet erosion
Strip cropping and crop rotation	Alternate strips of crops and grass slow runoff; rotation with legumes restores nitrogen
Controlled grazing	Stocking within the land's capacity keeps grass cover; zero grazing (stall feeding) protects slopes
Check dams and gully plugging	Small barriers of stone or vegetation slow water in gullies and trap sediment
Manure, compost and lime	Restore organic matter and nutrients; lime reduces the acidity of leached soils

Economic importance of soils

Agriculture: soils support food crops, cash crops such as tea and coffee, and pasture, and so most rural livelihoods.
Building and industry: clay for bricks, tiles and pottery; murram (laterite) for roads; sand for construction; kaolin for paper and ceramics.
Forestry: soils support timber and fuelwood.
Water: soils store and filter rainwater, feeding springs and rivers and reducing floods.
Climate and wildlife: soils store large amounts of carbon and are home to a vast number of organisms.

Warning

Common mistakes

"Erosion is caused by rain." Rain is the agent; the cause is usually bare soil on a slope, often the result of human activity.
Listing methods without explaining them. Say how each method reduces the speed or volume of runoff or protects the surface.

Tip

Exam technique. For "suggest measures", match each method to a cause: terraces and contour ploughing to steep slopes, mulch and cover crops to bare soil, controlled grazing to overgrazing, afforestation to deforestation.

Food and Agriculture Organization of the United Nations (FAO): why soil erosion matters and how farmers can keep soil in place.

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Practice

Distinguish between sheet, rill and gully erosion.
Give three human causes of soil erosion.
Explain how bench terraces reduce soil erosion.
Using $A = R \times K \times LS \times C \times P$ with $R = 400$ , $K = 0.25$ , $L S = 5$ , $C = 0.2$ and $P = 1$ , calculate $A$ . What is $A$ if contour strips give $P = 0.4$ ?
Describe two effects of soil erosion downstream of the eroding fields.
Explain three ways in which soils are economically important.
Evaluate terracing as a method of soil conservation on steep slopes.

Show answersHide answers

Sheet: a thin, even layer removed by a film of water; rill: small channels a few centimetres deep; gully: deep channels that cannot be ploughed over.
Any three: deforestation, cultivating steep slopes without protection, ploughing up and down slopes, overgrazing, bush burning, badly drained roads.
The level steps break the slope into short sections, so runoff cannot gain speed; water soaks into the level benches instead of running off; grass on the risers holds the soil.
$A = 400 \times 0.25 \times 5 \times 0.2 \times 1 = 100\,\mathrm{t\,ha^{-1}\,yr^{-1}}$ ; with $P = 0.4$ , $A = 40\,\mathrm{t\,ha^{-1}\,yr^{-1}}$ .
Silting of rivers, lakes, reservoirs and irrigation canals; muddy drinking water; damage to fish breeding grounds; algal growth from fertiliser washed off fields.
Any three: farming of food and cash crops; materials such as clay for bricks and laterite for roads; forestry; storing and filtering water; storing carbon.
Strengths: large cuts in soil loss, more water for crops, more land usable on steep slopes. Weaknesses: high labour or cost to build; benches can expose infertile subsoil, so yields may fall at first unless manure and lime are added; they need maintenance. Terracing works best combined with fertility management, trees and cover.

Summary

Soil erosion by splash, sheet, rill and gully removes soil far faster than it forms.
Its causes are mostly bare soil on slopes, through deforestation, poor farming and overgrazing.
Terraces, contour farming, agroforestry, mulch, controlled grazing and manure conserve soil.
Soils underpin farming, building materials, water supply and carbon storage.

1Soil that takes centuries to make and a season to lose

2Types and causes of soil erosion

3Effects of soil erosion

4Soil management and conservation

5Economic importance of soils

Soil that takes centuries to make and a season to lose

Types and causes of soil erosion

Effects of soil erosion

Soil management and conservation

Economic importance of soils