Soils in Rwanda

Lessons

Senior 4 · Geography

Soils in Rwanda

3 lessons

Lesson 1 of 3

Soil Types of Rwanda: Characteristics and Location

25 min

One country, many soils

Soil scientists at Ghent University analysed Rwanda's national soil profile database and summed up the country's soils in one sentence: they range from Andosols in the volcanic highlands, through Cambisols, Luvisols, Alisols and Acrisols in the sub-humid, hilly western highlands, to strongly weathered Ferralsols in the semi-arid eastern lowlands (Van Ranst, Verdoodt and Baert, 2010). This lesson explains what those names mean, why each soil is where it is, and how one hill can carry several soils. (S2 introduced how soil forms; here we use the international soil names that Rwandan soil maps use.)

Figure 1: the horizons of a typical well-drained hill soil (a sketch).

Definition

Soil: the loose upper layer of the land surface, made of mineral particles, organic matter (humus), water and air, in which plants grow. Soil profile: a vertical section through the soil showing its layers, called horizons.

Leaching: the washing of dissolved nutrients down through the soil by rain water. Laterisation (ferralitisation): intense weathering and leaching in warm, wet climates that leaves soils rich in iron and aluminium oxides and poor in nutrients.

The soil groups of Rwanda

Soil group	Main characteristics	Where in Rwanda
Andosols (volcanic soils)	Formed on volcanic ash and lava; dark, light, porous, rich in organic matter; hold water well; fertile but bind phosphate	Volcanic highlands of the north-west (Van Ranst and others, 2010); REMA notes the fertile volcanic soils near the Volcanoes National Park
Acrisols and Alisols	Strongly leached, acidic, low in nutrients; clay washed down into a red or yellow B horizon	Hilly western and central highlands, on schists and granites (Van Ranst and others, 2010)
Cambisols and Luvisols	Younger, moderately developed soils; Luvisols have a clay-enriched B horizon but are less leached and more fertile than Acrisols	Slopes of the hilly western highlands (Van Ranst and others, 2010)
Ferralsols	Very deep, strongly weathered, red or yellow; rich in iron and aluminium oxides, poor in plant nutrients; well drained	Semi-arid eastern lowlands (Van Ranst and others, 2010)
Histosols (peat soils)	Thick layers of partly decayed plant matter in waterlogged places; very high organic matter	High-altitude marshes; REMA lists peaty swamps such as Kamiranzovu in Nyungwe (REMA, 2009)
Vertisols	Dark, heavy clays that swell when wet and crack deeply when dry	REMA lists vertisols in the swamps of the Bugarama plain (REMA, 2009)
Alluvial soils (Fluvisols)	Layered sands, silts and clays laid down by rivers; usually fertile; often waterlogged	Valley floors and floodplains, largest along the Akagera (BGS alluvium, 2018)

Warning

Acid soils are the rule, not the exception. REMA reports that, besides being acidic, most of Rwanda's soils are exhausted from continuous farming (REMA, 2009). An exam answer that calls all Rwandan soils "fertile" will lose marks; the fertile exceptions are the volcanic soils, young alluvium and well-managed valley soils.

Why the soils are where they are

The five soil-forming factors explain the pattern:

Parent material: volcanic ash and lava give Andosols in the north-west; schists, quartzites and granites give acid, clay-rich soils elsewhere; river sediments give alluvial soils.
Climate: rain and warmth drive weathering and leaching; the drier, warmer east has long-weathered Ferralsols.
Relief: on a single hill the soil changes from crest to valley floor (Figure 2). This sequence is called a catena.
Organisms: forests and grasses add humus; termites and worms mix the soil.
Time: old, stable surfaces carry deeply weathered soils; young lava and alluvium carry young soils.

Figure 2: a soil catena. On one Rwandan hill the soil changes from crest to valley floor because of slope, drainage and the movement of material downhill.

Definition

Catena: a sequence of different soils down a slope, from the same parent rock and climate, which differ because of drainage, erosion and deposition along the slope.

Example: in Figure 2, the valley-floor soil is grey because waterlogging removes oxygen, so iron is reduced instead of forming red oxides.

Worked example 1: what controls organic carbon in Rwanda's topsoils?

Analysing 1,463 soil profiles, Van Ranst and colleagues (2010) found that altitude, the clay and fine silt content of the soil, and population density together explained 48% of the variation in topsoil organic carbon. Their equation is

\ln(\text{SOC}) = -1.748 + 0.001\,\text{alt} + 0.015\,\text{cfs} - 0.001\,\text{pd}

where SOC is the topsoil organic carbon content (%), alt the altitude (m), cfs the clay and fine silt content (%) and pd the population density (people per km²).

(a) A highland site: alt = 2,200, cfs = 50, pd = 500.

\ln(\text{SOC}) = -1.748 + 2.2 + 0.75 - 0.5 = 0.702 \;\Rightarrow\; \text{SOC} = e^{0.702} = 2.0\%

(b) An eastern lowland site: alt = 1,400, cfs = 40, pd = 400.

\ln(\text{SOC}) = -1.748 + 1.4 + 0.6 - 0.4 = -0.148 \;\Rightarrow\; \text{SOC} = e^{-0.148} = 0.86\%

Cooler highland soils keep more than twice the organic carbon of warm lowland soils, and every extra person per km² lowers it: decomposition is faster where it is warm, and dense farming removes plant material. (The authors note that the equation does not apply to the volcanic soils of the Volcanoes National Park.)

Tip

Reading a soil in the field: colour (dark = humus; red = well-drained iron oxides; grey or mottled = waterlogged), texture (rub moist soil: gritty = sand, silky = silt, sticky = clay), depth to rock, and stones. Record them for each horizon with a sketch of the profile.

Purdue Extension: "Soil Basics: Soil Profiles". A soil scientist explains the horizons of a real profile. As you watch, match each horizon to Figure 1.

✦

Practice

Draw and label a soil profile, describing each horizon.
Name the soil group of (a) the volcanic north-west, (b) the semi-arid eastern lowlands, (c) high-altitude marshes. Give one characteristic of each.
Explain why most soils of Rwanda's hills are acidic.
Describe and explain the changes in soil along the catena in Figure 2.
Use the equation in Worked example 1 to estimate SOC for alt = 1,800, cfs = 45, pd = 600.
According to the equation, by how much does ln(SOC) change if population density rises by 300 people per km²? What does this suggest?
"Vertisols are easy soils to farm." Evaluate this statement.

Show answersHide answers

O (litter), A (dark topsoil with humus), B (subsoil where clay and iron collect), C (weathered rock), R (bedrock). See Figure 1.
(a) Andosols: dark, porous, rich in organic matter, fertile. (b) Ferralsols: deep, red, strongly weathered, poor in nutrients. (c) Histosols: peat, very high organic matter, waterlogged.
Heavy rain over long periods leaches basic nutrients (calcium, magnesium, potassium) out of the soil, leaving acidic aluminium and iron behind; the parent rocks (schists, quartzites, granites) are themselves poor in bases; continuous farming removes nutrients without replacing them.
Crest: thin and stony because soil is eroded and the quartzite resists weathering. Upper and middle slopes: deep, red and well drained. Lower slope: deeper because soil washed from above collects. Valley floor: grey and waterlogged, with alluvium or peat, because water collects there.
$\ln(\text{SOC}) = -1.748 + 1.8 + 0.675 - 0.6 = 0.127$ ; SOC $= e^{0.127} = 1.14\%$ .
It falls by $0.001 \times 300 = 0.3$ , so SOC is multiplied by $e^{-0.3} = 0.74$ , a fall of about a quarter: dense farming tends to reduce soil organic matter.
Partly false. Vertisols are rich in nutrients and hold water, but they are very sticky when wet and crack and set hard when dry, so they are hard to plough and drain; they suit crops such as rice in well-managed swamps (as in Bugarama), not all crops.

Quick check

Match each soil group to where it is found in Rwanda.

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

Key points

Rwanda's soils range from Andosols (volcanic north-west), through Cambisols, Luvisols, Alisols and Acrisols (hilly west and centre), to Ferralsols (semi-arid east) (Van Ranst and others, 2010); Histosols and Vertisols occur in some swamps, alluvial soils in valleys.
Most hill soils are acidic and leached (REMA, 2009).
Parent rock, climate, relief, organisms and time explain the pattern; a catena shows soils changing down one slope.
Topsoil organic carbon rises with altitude and clay content and falls with population density.

Sources and credits

Sources: Soil Mapping in Africa at the Crossroads: Work to Make up for Lost Ground, E. Van Ranst, A. Verdoodt and G. Baert, Bulletin des Séances de l'Académie Royale des Sciences d'Outre-Mer, 56(2), 2010, pp. 147–163 (soil groups of Rwanda, organic carbon equation); Rwanda State of Environment and Outlook Report, REMA, 2009 (acid soils, peat and vertisol swamps, volcanic soils); Africa Groundwater Atlas: Hydrogeology of Rwanda, British Geological Survey, 2018 (alluvium). Facts only; the text was written by Ganzaa.

Figures 1 and 2, the worked example and the exercises were drawn and written by Ganzaa.

Video: "Soil Basics: Soil Profiles", Purdue Extension, embedded from YouTube.

1One country, many soils

2The soil groups of Rwanda

3Why the soils are where they are

One country, many soils

The soil groups of Rwanda

Why the soils are where they are