Soil Conservation, Management and the Importance of Soils – Soils in Rwanda (Senior 4 Geography) | Ganzaa

Soils in Rwanda

Lessons

Senior 4 · Geography

Soils in Rwanda

3 lessons

Lesson 3 of 3

Soil Conservation, Management and the Importance of Soils

25 min

Rebuilding a hillside

The photograph below was taken in northern Rwanda in November 2012. Dozens of people are reshaping a whole hillside into level steps. They were paid through a national social-protection programme, and the work they were doing, terracing, is the most visible way Rwanda fights soil erosion.

Definition

Soil conservation: methods that protect soil from erosion and keep it in place. Soil management: methods that maintain or improve the soil's fertility and structure so it stays productive.

Bench (radical) terrace: a level step cut into a slope, held up by a steep riser. Progressive terrace: a terrace that forms gradually as soil builds up behind a contour bund or grass strip.

Conservation and management methods

Group	Method	How it works
Mechanical (shaping the land)	Bench (radical) terraces	Level benches stop run-off; recommended where risk is high to extremely high (RWB, in REMA 2021)
	Contour bank terraces, progressive terraces and ditches	Banks and ditches along the contour slow and store run-off; soil builds up into terraces over the years
	Grassed waterways	Carry surplus water safely downhill; needed on terraces built without them, which otherwise develop gullies (REMA, 2021)
	Gully control	Check dams, stone or bamboo plugs; REMA notes bamboo is recommended to close gullies and for riverside buffers
Biological and agronomic (cover and roots)	Agroforestry	Trees on farmland hold soil with roots, add leaf litter and give fodder and wood
	Mulching and cover crops	Cover protects bare soil from splash erosion and keeps it moist
	Grass strips and contour planting	Strips across the slope trap soil and slow run-off
	No-till for perennial crops	Recommended on extremely high-risk land (REMA, 2021)
	Forests on the steepest land	Recommended for extremely high-risk areas (REMA, 2021)
Fertility management	Manure, compost and fertilisers	Replace nutrients removed by crops
	Liming acid soils	Raises the pH so nutrients become available; proposed for Nyaruguru, for example (REMA, 2021, citing Bizimana, 2018)
	Crop rotation, fallow, intercropping	Rests the soil and keeps it covered

Figure 5: bench (radical) terraces. Each bench is nearly level, so rain soaks in instead of running off; the riser is protected by grass, and a drain carries surplus water to a grassed waterway.

Worked example 1: designing bench terraces

Problem. A field slopes at 30% ( $\tan\theta = 0.30$ ) over a horizontal distance of 48 m. The farmer wants benches 4 m wide. (a) How high is each riser? (b) How many benches fit? (c) If each bench is made by cutting soil from its upper half and filling the lower half, what volume of soil is moved per metre length of terrace?

Solution. (a) Over a horizontal width $W$ , the ground rises $H = W \tan\theta$ :

H = 4 \times 0.30 = 1.2\ \text{m}

(b) $\frac{48}{4} = 12$ benches. (c) The cut is a triangle with legs $\frac{W}{2}$ and $\frac{H}{2}$ :

V = \frac{1}{2} \times \frac{W}{2} \times \frac{H}{2} = \frac{W H}{8} = \frac{4 \times 1.2}{8} = 0.6\ \text{m}^3 \text{ per metre}

A 100 m terrace means moving about 60 m³ of soil by hand, which is why terracing is organised as paid community work, and why benches are kept narrow on very steep slopes, where $H$ becomes too high to stay stable.

Worked example 2: Rwanda's national target

Data. Rwanda's Land Degradation Neutrality targets aim to increase land protected against soil erosion from 1,034,509 ha in 2018 to 1,495,624 ha in 2024 (REMA, 2021).

Problem. Find the increase, the percentage increase, and the average area to be protected each year.

Solution.

1\,495\,624 - 1\,034\,509 = 461\,115\ \text{ha}, \qquad \frac{461\,115}{1\,034\,509} \times 100 = 44.6\%

\frac{461\,115}{6\ \text{years}} = 76\,853\ \text{ha per year}

That is roughly 770 km² a year, about the area of Kicukiro District four and a half times over (Kicukiro: 16,671 ha, RWB, 2021).

The practices are already widespread: in Season A of 2024, 90.6% of farmers practised anti-erosion activities (NISR, Seasonal Agricultural Survey 2024). The challenge is quality and maintenance as much as coverage.

Warning

Terraces can fail. Benches without drains or grassed waterways collect water that breaks through risers and cuts gullies, and on very steep land, deep cutting exposes infertile subsoil. Good answers name the conditions a method needs: drainage, grass on risers, manure for exposed subsoil, and forest instead of terraces on the steepest slopes.

Why soils matter to Rwanda

Importance	Example
Food and farming	Almost all food crops, coffee and tea grow in soil; valley soils grow rice and vegetables
Building and crafts	Clay for bricks, tiles and pottery (REMA, 2009)
Water	Soil stores rain water, feeds springs and rivers, and filters it
Carbon store	Soil organic carbon averages about 130 t/ha under trees and 102 t/ha under crops (REMA, 2021, citing Bizimana, 2018)
Fuel	Peat from marsh soils is used as fuel (REMA, 2009)
Habitat and biodiversity	Billions of organisms live in soil and recycle nutrients
Foundations	Every building and road stands on soil (REMA, 2021)

RwandaTV: "Government reiterates the necessity for farmers to design terraces to reduce soil erosion". A Rwandan television news report on the Ministry of Agriculture's call for radical terraces. Note the reasons given and compare them with this lesson.

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Practice

Distinguish soil conservation from soil management, with two methods of each.
Explain how bench terraces reduce soil erosion, and the conditions they need to work well.
A slope of 20% is terraced with benches 5 m wide. Find the riser height and the soil cut per metre of terrace.
Why are forests, not terraces, recommended on extremely high-risk land?
Explain why liming is proposed for the acid soils of districts such as Nyaruguru.
Give five reasons why soils are important to Rwanda.
Evaluate paid public works as a way to build terraces in Rwanda.

Show answersHide answers

Conservation keeps soil in place (terraces, grass strips, mulching); management keeps it fertile (manure, liming, crop rotation).
Level benches stop run-off gathering speed so rain soaks in; risers held by grass resist collapse. They need drains and grassed waterways, maintenance, and manure on exposed subsoil.
$H = 5 \times 0.20 = 1.0$ m; $V = \frac{5 \times 1.0}{8} = 0.625\ \text{m}^3$ per metre.
On the steepest slopes risers would be too high and unstable and cutting would expose infertile subsoil and could trigger landslides; trees protect the soil with permanent cover and deep roots.
Leaching has made the soils acidic, so nutrients such as phosphorus are locked up and aluminium can harm roots; lime raises the pH and improves yields.
Food and farming; clay for bricks and pottery; water storage and filtering; carbon storage; habitat for organisms; fuel (peat); foundations.
Advantages: large areas protected quickly; cash income for poor households; community ownership. Disadvantages: costly; quality varies; terraces need long-term maintenance and fertility management, which paid work alone may not ensure. A balanced conclusion supports it when combined with training and maintenance.

Quick check

Match each method to its main purpose.

Tap one on the left, then its partner on the right.

Key points

Conservation keeps soil in place (bench and progressive terraces, contour banks, grassed waterways, gully control, agroforestry, mulch, grass strips, forest on the steepest land).
Management keeps it fertile (manure, compost, fertiliser, liming acid soils, rotation).
Riser height $H = W\tan\theta$ ; terraces need drainage and maintenance.
Rwanda aimed to protect 1,495,624 ha against erosion by 2024 (REMA, 2021); soils give food, building clay, water, carbon storage, habitat and fuel.

1Rebuilding a hillside

2Conservation and management methods

3Why soils matter to Rwanda

Rebuilding a hillside

Conservation and management methods

Why soils matter to Rwanda