Soil

Lessons

Senior 3 · Agriculture

Soil

4 lessons

Lesson 4 of 4

Preventing Soil Erosion: Soil Conservation Measures

25 min

Every conservation measure on a Rwandan hillside, from a mulch of banana leaves to a flight of bench terraces, works by attacking one of the factors from Lesson 2. Learn the five principles first, and you will be able to explain any measure, even one you have never seen.

Cover the soil, so raindrops cannot strike it.
Help water soak in, by keeping the soil rich in organic matter and well structured.
Slow the runoff by breaking a long slope into short sections.
Lead surplus water away safely in drains and waterways.
Feed the soil, so it stays fertile and well covered (this also prevents exhaustion).

Definition

Soil conservation: all the practices that protect soil from erosion and exhaustion so that it stays productive.

Biological (agronomic) measures: practices that use plants and good farming to protect the soil. Example: mulching, cover crops, grass strips.

Mechanical (structural) measures: earth or stone structures that slow or divert runoff. Example: terraces, cut-off drains, check dams.

Contour: a line joining points of the same height on a slope. Work done "on the contour" runs across the slope, never up and down it.

Terrace: a level or nearly level step cut or built across a slope. A bench (radical) terrace is dug level in one go (in Kinyarwanda, amaterasi y'indinganire); a progressive terrace forms gradually as soil collects behind a grass strip or bank on the contour.

Biological and cultural measures

Measure	How it is done	Principle
Mulching	Spread dry grass, crop residues or banana leaves over the soil between plants.	Cover; water soaks in
Cover crops	Grow low, spreading crops such as sweet potato, beans or Mucuna to cover the ground.	Cover
Contour farming	Dig, ridge and plant in rows along the contour, not downhill.	Slow runoff
Grass strips and hedgerows	Plant strips of Napier, Setaria or shrubs such as Calliandra along the contour, every 1 to 1.5 m of vertical drop.	Slow runoff; trap soil; give fodder
Strip cropping	Alternate strips of a row crop (maize) with a dense crop or grass along the contour.	Cover; slow runoff
Agroforestry	Grow trees such as Grevillea or Alnus on field edges and terrace banks.	Cover; roots bind soil; leaf litter
Afforestation	Keep hilltops and very steep slopes under forest.	Cover; water soaks in
Rotation, manure, compost, residues	Rotate cereals with legumes; return residues; add manure and compost.	Feed the soil; stable crumbs
Controlled grazing	Zero-grazing or rotational grazing; no grazing on bare slopes.	Cover

Mechanical measures

Cut-off (diversion) drains: ditches dug across the slope above a field to catch runoff from higher land and lead it to a safe waterway.
Contour trenches (in Kinyarwanda, imirwanyasuri): ditches along the contour that hold water so it soaks in; the soil is thrown downhill to form a bank. If the soil is thrown uphill, it is a fanya juu bank (Kiswahili for "throw it up").
Bench terraces: level steps dug by cutting soil from the upper part of each step and filling the lower part. Each riser is planted with grass. They suit deep soils, and topsoil must be put back on the bench.
Progressive terraces: grass strips or banks on the contour trap soil each season until the strips between them level out.
Stone lines and walls: stones laid along the contour, common where fields are stony, as in the volcanic areas of the north-west.
Gully control: stop water entering the gully head with a cut-off drain; build check dams of stones, sticks or gabions (wire baskets of stones) across the gully to slow water and trap soil; then plant grass and trees in it.
Waterways and storm-water control: grassed channels carry surplus water downhill; tanks collect water from roofs so it does not run onto fields.

Figure 4. (a) Bench terraces are dug level in one go; (b) progressive terraces build up slowly behind grass strips.

Marking contours with an A-frame

Contour work starts with finding the contour. An A-frame is a cheap level that any farmer can make from three poles, a string and a stone (Figure 5).

Figure 5. The A-frame level.

Calibrate: stand the frame on a piece of ground, mark where the string crosses the crossbar, turn the frame round so the feet swap places, and mark again. The point halfway between the two marks is the centre mark.
At one edge of the field, put a peg at the first foot.
Move the other foot up or down the slope until the string hangs exactly on the centre mark. Both feet are now at the same height: put a peg there.
Swing the frame round the second foot and repeat across the field.
Join the pegs with a smooth line; this is the contour. Dig, plant or build along it.

Worked example 1: spacing terraces

Problem. A field has a slope of 25% and is 48 m long, measured horizontally. Terraces are to be built with a vertical interval (height between one terrace and the next) of 1.5 m. Find the horizontal spacing and the number of terraces.

Solution.

A 25% slope rises 25 m in every 100 m horizontally. The horizontal interval is $HI = \frac{VI \times 100}{S} = \frac{1.5 \times 100}{25} = 6\,\mathrm{m}$
Number of terraces $= \frac{48}{6} = 8$ .

Answer: one terrace every 6 m, 8 terraces in all. On a steeper slope the same vertical interval gives narrower benches, which is why bench terraces become impractical on very steep land.

Worked example 2: what Rwandan farmers use

Problem. In 2024 Season A, NISR recorded the main anti-erosion measure on plots that had one. Sort the measures into biological and mechanical, and find the share of each group.

Measure	Share of plots (%)
Cover plants	60.3
Progressive terraces	8.2
Water channels	7.4
Trees, windbreaks, shelterbelts	6.5
Ditches	5.2
Bench terraces	5.2
Beds and ridges	5.1
Mulching	1.7
Water drainage	0.5

Source: NISR, Seasonal Agricultural Survey 2024 Annual Report, Table 70 (national row; "others" under 0.1% left out).

Solution. Biological: cover plants, trees, mulching: $60.3 + 6.5 + 1.7 = 68.5\%$ . Mechanical: progressive terraces, water channels, ditches, bench terraces, beds and ridges, water drainage: $8.2 + 7.4 + 5.2 + 5.2 + 5.1 + 0.5 = 31.6\%$ .

Comment: about two-thirds of protected plots rely on plants, which are cheap and also feed the soil. Terraces protect fewer plots because they cost much more labour, but they are needed on the steepest land.

Warning

Common mistakes. (1) A ditch or terrace built without a safe outlet makes things worse: the collected water breaks out and cuts a gully. (2) When digging bench terraces, the fertile topsoil must be set aside and spread back on the bench; otherwise the new bench is infertile subsoil and needs heavy manuring and lime for several seasons.

Tip

Exam technique: for any measure, say how it is done and which principle it uses: "Mulching covers the soil, so raindrops cannot detach particles, and it slows runoff so more water soaks in." Group your answer into biological and mechanical measures.

TerrAfrica: "Fanya juu Terraces". Watch how East African farmers dig the trench, throw the soil uphill to make a bank, and plant grass on it, and how the bank turns into a terrace over a few seasons, as in Figure 4(b).

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Practice

State the five principles of soil conservation.
Classify each measure as biological or mechanical: mulching, cut-off drain, grass strips, check dam, crop rotation, bench terrace.
Explain how a grass strip planted on the contour turns, over several years, into a progressive terrace.
Describe how to calibrate and use an A-frame to mark a contour.
A slope of 20% is to be terraced with a vertical interval of 1.2 m. Calculate the horizontal spacing of the terraces and how many fit on a field 54 m long horizontally.
Describe three steps to reclaim a field cut by a gully.
A farmer on a 45% slope with shallow, stony soil wants to dig bench terraces. Evaluate this plan and suggest a better option.

Show answersHide answers

Cover the soil; help water soak in; slow runoff by shortening the slope; lead surplus water away safely; keep the soil fertile.
Biological: mulching, grass strips, crop rotation. Mechanical: cut-off drain, check dam, bench terrace.
The strip slows runoff, which drops its soil on the uphill side of the strip. Season by season the soil builds up behind the strip while the upper part of the zone is lowered, so the land between strips becomes less steep, forming a terrace.
Mark where the string crosses the crossbar, turn the frame so the feet swap, mark again, and take the midpoint as the centre mark. Peg the first foot, move the second until the string hangs on the centre mark, peg it, swing the frame round, and repeat; join the pegs.
$HI = \frac{1.2 \times 100}{20} = 6\,\mathrm{m}$ ; $\frac{54}{6} = 9$ terraces.
Divert water from the gully head with a cut-off drain; build check dams of stones, sticks or gabions across it; plant grass, shrubs and trees in and around it; keep animals out.
Bench terraces need deep soil: cutting into shallow, stony soil would expose infertile subsoil or rock, and on 45% the benches would be very narrow and costly. Better: grass strips or hedgerows on the contour to form progressive terraces, stone lines, agroforestry, or keeping the slope under trees or permanent fodder.

Key points

Conservation works by covering the soil, increasing infiltration, slowing runoff, draining surplus water safely and keeping the soil fertile.
Biological measures (mulch, cover crops, grass strips, agroforestry, rotation) are cheap and cover most protected plots in Rwanda (NISR, 2024).
Mechanical measures (cut-off drains, contour trenches, bench and progressive terraces, check dams) are needed on steep land; each needs a safe outlet.
Contours are marked with an A-frame; terrace spacing is $HI = \frac{VI \times 100}{S}$ .

1Biological and cultural measures

2Mechanical measures

3Marking contours with an A-frame

Biological and cultural measures

Mechanical measures

Marking contours with an A-frame