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Unit 8: SOILS IN RWANDA
Key unit competence
At the end of this unit, you should be able to appreciate the measures of soil conservation and soil control in Rwanda.Introduction
Soil is the top layer of loose materials that lies on top of the crustal rocks. Soils form after weathering of the parent rock into small particles. The soils are made up of gases, water, mineral matter, remains of plant and living organisms. It is therefore important to learn about soil so that we can protect it for future generations.
8.1 Types of soils in Rwanda
1.Kaolisols
They form the main type of soils in Rwanda. These soils developed on weathered parent material which mainly had clay mixed with iron and aluminium oxides. They appear reddish due to the presence of iron and aluminium oxides. The profile of kaolisols reveals an upper humus horizon of low humus while the structure of the middle section differs for different subtypes, but it generally shifts gradually from the humus horizon to the parent rock. Kaolisols are found in the central area of Rwanda. They are usually deep.
In Rwanda, these soils are further divided into xerokaolisols and humus-bearing kaolisols.
a) Xerokaolisols are characterised by a thin layer of humus. Beneath the humus lies a layer resulting from the accumulation and cementation of iron and aluminium oxides thicker than the humus layer. Patches of xerokaolisols are witnessed stretching from the central part of Rwanda, though they become more prevalent as one moves towards the eastern part.
b) Humus-bearing kaolisols are largely composed of elements and compounds of aluminium, iron and, in smaller quantities, calcium, magnesium, sodium, and potassium. Factors determining the nature of these soils are vegetation type and climate. Other factors are parent rock material and geographic relief. Geologically young soils resemble their parent material more than older soils, which have been altered over time by climate and vegetation.
2. Valley Soils
These are mainly found along river valleys. They are mainly of alluvial origin. The two main categories of valley soils are vertisols and organosols (or histosols) and vertisols.
a) Vertisols have a high clay content and are often subject to marked shrinking and swelling with changes in water content. They are generally low in organic content. Vertisols typically form in climates that are seasonally humid or subject to periods of droughtsand floods or that impeded drainage. Depending on the parent material and the climate, they can range from grey or red to the more familiar deep “black cotton” soils. Areas with vertisols include those found in the lower parts of Nyabarongo River.
b) Organosols on the other hand refer to wet soils consisting mostly of high organic matter, popularly called peats and mucks. When they dry, they become hard and ends up cracking.
Organosols form whenever the rate of formation of organic matter is more rapid than its rate of destruction. This occurs because of restricteddrainage and a slow process of decomposition making the remains of plants and animals to remain within the soil. This makes organosols very importantbecause they store large quantities of organic materials. They are generally very difficult to cultivate because they are found in areas of poor drainage. However, organosols can often be very productive when drained. Areas with organosols include those found in Rugezi Swamps.
3. Highland soils or Inceptisols
Inceptisols form quickly through weathering of parent material under the influence of low temperatures and high temperatures. They are more developed than soils that do not show any profile development. They have no accumulation of clays,iron oxide,aluminium oxideor organic matter. Their geographic settings vary widely, but are mainly found in upland forest environments.
Inceptisols are found on the slopes of mountains in the north west of Rwanda, covering most parts of the Congo-Nile peak. They also form a larger part of the soils in the Birunga slopes.
4. Volcanic soils
As their name implies, this grouping of soils is found only on volcanic parent materials. These soils are usually the dominant soil in young volcanic landscapes. This is because of the unique combination of soil properties that result from the weathering of volcanic rocks. Only under strong tropical weathering will lavas weather to finer grained volcanic soils, so it is usually parent materials of a volcaniclastic origin which result in the high-producing volcanic soils in Rwanda.
Many volcanic soils have excellent physical properties that make them highly desirable for a wide range of uses. Generally, these soils are among the most fertile lands in the world and are, therefore, very intensively cultivated
Rwanda’s volcanic soils are fertile although periodic drought, soil erosion and soil exhaustion through over-cultivation on small family plots has led to food insecurity in some parts.
In Rwanda, with the highest population densities in Africa, volcanic soils cover 700 km2, providing a vital food resource. Cultivated crops here include maize, sorghum, potatoes, pyrethrum, and peas, along with bamboo and eucalypts.
Activity 8.1
1.In pairs, study the map showing distribution of soils in Rwanda.
2.List all the types of soils shown.
3.Use your atlas to identify the provinces where these soils are found.
(a) Which provinces share the same type of soils?
(b) Which type of soil covers a larger area in Rwanda?
Soils differ in colour, size of grain particles, the pore spaces and amount of water the soil can hold. All these are referred to as soil characteristics. The activity that follows will help you to learn more about the soil characteristics.
Activity 8.2
Purpose
To find out soil characteristics
Inquiry
What are the characteristics of soil?
Materials needed
• 2 containers/Beaker
• Water
• A handful sample of soil
Procedure
1.Make a copy of the table below to guide your observations.
2.in g roups, examine the soil sample carefully and record your findings.
Share the differences and similarities of soil characteristics observed by your classmates.
Activity 8.3
In pairs, look at the following photographs showing different types of soils.
In pairs, look at the photographs of different types of soils shown above.
With reasons, identify each type of soil shown.
8.2 Soil erosion and impoverishment in Rwanda
Activity 8.4
In pairs, answer the following questions:
1. From the knowledge gained in Unit 5, give the meaning of soil erosion.
2. Use the dictionary, atlas and the Internet to get the meaning of soil impoverishment.
3. Explain the main difference between soil erosion and soil impoverishment.
Soil erosion is the physical removal of topsoil by various agents. These agents includerain, water flowing over land through the soil profile, wind, ice or gravitational pull. It also means the process by which soil is removed from a place by human activities such as construction and farming. The soils are eventually deposited at a new place
Soil erosion and impoverishment may be caused by natural events such as heavy rainfall or drought particularly in areas that receive low rainfall. The impoverishment of the soil may be caused by natural events or by human activities such as poor methods of agriculture. The damage on soil may have long lasting effects on land, people and their way of life.
(a) Causes and effects of soil erosion in Rwanda
There are several reasons why soil erosion happens in a given area. It is either caused by natural events or it is human induced.
Activity 8.5
In groups of four, revisit Unit 5 on the section about causes of soil erosion on landscape.Make brief notes on how the following factors cause soil erosion.
• Influence of climate
• Shape of the landscape
• Removal of plants cover
• Human activities
Prepare a report for class presentation.
Did you know?
During the liberation war in 1994, many people sought refuge in the forests. This increased pressure on land. It also led to excessive cutting of trees for fuel.
(b)Farming practices that lead to soil erosion
i. Where plots are cleared and later abandoned makes them open to agents of erosion.
ii. Ploughing of land in the wrong way that is, up and down the slope results into formation of channels that are gradually enlarged by water erosion to form gullies.
iii. Farming of the same type of crop or mono- cropping reduces important soil nutrients. This weakens the soils making it prone to wind and soil erosion.
iv. Cultivating crops in areas of unpredictable rainfall makes the soil lose during the dry season exposing it to wind erosion.
v. Overgrazing of pasture particularly by domestic animals destroys the vegetation cover exposing the soil to erosion.
vi. Other human activities such as mining, quarrying, road construction make the soil susceptible.
(c) Effects of soil erosion
Activity 8.6
In pairs, discuss the effects of soil erosion in the community where the school is located.Present your findings for class discussion.(d)Effects of soil erosion
i) Soil erosion in parts of Rwanda has made a larger part of the land infertile. This has lowered farm productivity and is a great loss to farmer’s earning and government revenue. There is also rise in poverty.
ii) Soil erosion has led to loss of arable land and spread of aridity.
iii) With the deterioration of land, the farmers are forced to migrate to make a living in other places.
iv) When soil erosion sets in, the vegetation and other important organisms are lost. In turn, the land becomes furrowed with gullies. This makes it to lose its natural beauty.
v) The soil washed by surface flow is deposited in the streams which causes water pollution. This also causes siltation.
vi) Absence of vegetation cover increases chances of flooding.
8.3 Soil conservation methods in Rwanda
You learnt that the soil is an important natural resource. This is because it supports almost all human activities. However, soil is a non-renewable resource because the rate of soil loss through erosion is faster than its rate of formation. Soil should therefore be effectively conserved for it to remain productive.
Soil conservation measures are the careful decisions and practices taken to protect land from erosion. The soil conservation practices are aimed at helping minimise soil loss. These measures are also aimed at rehabilitating waste lands to usable lands for cultivation. The measures include:
a) Crop rotation
b) Use of fertilisers
c) Planting cover crops
d) Mulching
e) Contour ploughing
f) Planting trees
g) Strip cropping
h) Controlled grazing
i) Terracing of land
j) Creating wind breakers
Activity 8.7
In groups of four:
1. Study the photographs below carefully. Explain the method of soil conservation being used in each photograph.
2. With reasons, identify the conservation method used in your local environment.
Each of the method of soil conservation is explained below:
a) Crop rotation
What is crop rotation?
This is the practice of growing different crops at different times on the same piece of land. In contrast, mono-cropping is the continuous cultivation of a single crop on the same piece of land. This impoverishes the soils.
The practice of crop rotation keeps the topsoil covered with plants. Rotation of cereal crops with legumes also keeps the soil enriched with nitrogen (from the legumes). This ensures the maintenance of soil fertility and reduces the possibility of soil erosion.
b) Use of fertilisers
Fertile soil supports a variety of farming practices. Loss of fertility results in decline in productivity. Fertility of soil can be restored by addition of manure or fertilisers.
c) Planting cover crops
Cover crops include the grasses and small grains which form dense cover crop stands. These crops may also include cow peas and beans (collectively called legumes). They help slow down the speed of rain drops before hitting the soil surface. They also prevent surface runoff, which may cause erosion.
d) Mulching
Mulching involves covering the soil with crop remains, grass or artificial materials such as polythene sheets. The purpose is to reduce loss of soil moisture. Mulching also reduces soil erosion by running water. With the decomposition of plant remains, humus is formed and improves soil fertility.
e) Contour ploughing
In this system of cultivation, the ploughing is aligned to the contours. The furrows in contour ploughing help to trap the soil and reduce soil erosion by surface runoff. This way, the soils are able to retain water.
f) Planting trees
Activity 8.8
For you to find out!
a) Which method of soil conservation is shown in the photograph? Give a reason for your answer.
b) Explain the difference between afforestation and reafforestation.
You may have noticed the bad effects of heavy rain mostly on bare ground. To reduce that negative effect, it is important to plant trees. They prevent rain splash erosionby protecting the soil from the direct impact of water droplets. The trees also reduce the amount and speed of surface runoff by improving the rate of water absorption. The decayed plant remains from the trees become humus which increases soil fertility.
g) Strip cropping
Strip cropping involves growing of crops in alternate bands. The strips of crop are planted at right angles to the slope. The strips of crop may also be planted at right angles to the direction of wind to limit wind erosion.Strip cropping reduces the speed of surface runoff. The roots of these crops also hold soil particles together. This prevents soil erosion.
h) Controlled grazing
Large herds of domestic animals such as cattle, sheep and goats graze and expose the topsoil to erosion. Thus, the land should be protected from overgrazing. Farmers are encouraging to control the numbers of their stock to avoid overgrazing.
i) Terracing of land
In this system of cultivation, terraces are cut at right angles to the slope. This slows down the flowing water and reduces soil erosion. In places with deep gullies, barrier terraces are constructed to protect such land. Grasses can also be planted on the slopes to form protective bunds.
j) Creating wind breakers
These are shelter belts whereby trees are planted across the wind direction to protect against the high velocity winds. These rows of trees are wind breaks or wind barriers. They help to control wind erosion.
END OF UNIT REVISION EXERCISE
1. Write brief notes on the three constituents of soil.
2. How does heavy rain cause soil impoverishment?
3. Herbert has a small farm on which he practices mixed farming. Lately, the productivity has gone down.
a) What are some of the probable causes of the declining farm productivity?
b) Suggest appropriate solutions that Herbert could apply to improve soil quality.
4. How does Rwanda stand to benefit from soil conservation?