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Challenges of Food Supply » Managing Desertification and Soil Erosion

What you'll learn this session

Study time: 30 minutes

Cambridge spec: 10.3.5

What desertification and soil erosion are, and their main causes
The strategies and techniques used to manage them
The strengths, weaknesses and sustainability of each one
How to write an evaluation with a clear judgement

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Two threats to farmland

Good soil takes hundreds of years to form, but it can be lost in a few seasons. This lesson looks at how people manage that loss. Remember from Threats to Food Supply and Food Insecurity that losing soil puts food supply at risk. Here we ask: what can be done about it?

Key terms:

Desertification: the process by which dry land becomes more like a desert and can no longer grow crops or feed animals.
Soil erosion: the removal of the fertile topsoil by wind or water faster than new soil can form.
Stone lines (bunds): low lines of stones laid along the contours of a slope to slow rainwater.
Terracing: cutting steps into a hillside to make flat strips for farming.
Contour ploughing: ploughing across a slope, along lines of equal height, instead of up and down it.
Crop rotation: growing different crops in a field in a planned order from year to year.
Agroforestry: remember, this means growing crops among trees. Here it means trees and crops or animals sharing the same land.
Zai pits: small planting holes dug in hard dry ground and filled with manure to catch water.

Causes in brief

Desertification happens mostly in dry regions such as the Sahel, the dry belt south of the Sahara in Africa. Soil erosion can happen almost anywhere that soil is bare. The causes overlap.

Cause	How it damages the land
Overgrazing	Too many animals eat the plants. Bare soil is blown or washed away.
Overcultivation	Land is farmed too hard without rest, so nutrients run out and the soil turns to dust.
Deforestation	Roots no longer hold the soil, and there are no leaves to shield it from rain.
Drought and climate change	Less rain means fewer plants. Higher temperatures dry the soil further.
Population pressure	More people need more food and fuel, so land is overworked and trees are cut for firewood.

Managing the problem

A strategy is the overall action plan. A technique is the method used on the land. Most of the techniques below are cheap and use local skills, which is why they are called appropriate technology.

Techniques for water and soil

💧 Stone lines

Farmers lay lines of stones across a slope. Rainwater slows down, soaks in and drops the soil it carries. Used widely in Burkina Faso and Mali.

⛰ Terracing

Steps are cut into a hillside so water cannot rush down. It is used in many hilly places, such as the Andes and monsoon Asia.

🌾 Contour ploughing

Furrows run across the slope, so each one holds back water like a small dam instead of acting as a channel for it.

🔧 Zai pits

Small pits are dug before the rains and filled with manure. They trap water and feed seeds. They are common in Burkina Faso and Niger.

Techniques for plants and animals

🔄 Crop rotation

Different crops use different nutrients. Growing a bean or pea crop adds nitrogen back, so the soil is not exhausted.

🌳 Agroforestry and tree planting

Tree roots bind the soil, leaves shade it and cut wind speed. Falling leaves feed the soil and the trees give fuel, fruit and animal feed.

🐐 Reducing herd sizes

Fewer animals on the land lets plants grow back. Herders may move animals in rotation so each area can rest.

📚 Education

Teaching farmers and herders about soil care helps the other methods last, because people understand why they matter.

The Great Green Wall

The Great Green Wall is a strategy led by African countries across the Sahel, starting in 2007. The aim is to restore degraded land by planting trees and using land restoration techniques across a belt of about 8,000 km, from Senegal in the west to Djibouti in the east. The target is to restore about 100 million hectares by 2030.

Strengths: it brings together many countries, gives jobs, and improves food and water supply where it works.
Weaknesses: it is very large and costly, progress has been slower than planned, and young trees need water and care or they die. Conflict in parts of the Sahel makes the work harder.
Lesson learned: it works best as a mosaic of local projects, such as farmland trees and zai pits, rather than a single line of trees.

Evaluating the methods

Method	Strengths	Weaknesses
Stone lines, zai pits	Very cheap, local materials, farmers can do it themselves, yields can rise	Hard work, a lot of labour, small scale, does not help in a severe drought
Terracing	Strong protection on steep slopes, more land for crops	Costly and slow to build, needs repair
Contour ploughing, crop rotation	Almost no cost, easy to learn	Less effective on steep land, results are slower
Agroforestry, tree planting	Several benefits at once, long-lasting	Trees take years to grow, young trees need water and protection
Reducing herd sizes	Cheap, plants recover quickly	Herders lose income and animals mean wealth, so it is often resisted
Education	Cheap, and helps every other method last	Slow results, needs teachers and time
Great Green Wall	Large scale, international support	Expensive, slow, depends on funding and peace

What does sustainable mean here?

Sustainable land management protects the soil so that farmers can keep using it, without damaging the environment, and so that future generations can do the same. Methods that use local labour, cost little and bring a benefit to farmers are the most likely to last. Methods that rely on outside money stop when the money does.

Worked example

A village loses soil from a bare slope each rainy season. Stone lines are cheap and slow the water at once, but only protect a small area. Terraces protect more, but cost more and take longer. A sensible plan starts with stone lines and adds tree planting, so the village gains quick results now and long-term protection later.

Common mistakes

Writing "plant trees" with no explanation. Say how roots bind soil and slow wind. Another mistake is calling every method sustainable. Always say who pays, who gains and whether it can last.

Exam-style question

Fig. 1.1 shows made-up data for a farming village in the Sahel before and after farmers used stone lines and zai pits.

Year	Land used for crops (hectares)	Millet yield (kg per hectare)
Year 1 (before)	40	300
Year 5 (after)	60	600

(a) Define the term desertification. [1 mark]

(b) Give two causes of soil erosion. [2 marks]

(d) Evaluate the use of stone lines and zai pits to manage desertification and soil erosion. [6 marks]

Model answer

(a) Dry land turning into desert, so that it can no longer support crops or animals (1).

(b) Any two from: overgrazing, overcultivation, deforestation, drought and climate change, population pressure (1 + 1).

(d) Level 3 (5-6 marks): Stone lines slow the rainwater and trap soil, and zai pits catch water and feed seeds with manure. Fig. 1.1 suggests they work, as yield doubled and more land could be farmed. They are cheap and use local materials and skills, so farmers can do the work themselves, which makes them sustainable. However, they are hard work, only cover small areas and cannot help in a severe drought, so they would not fix a whole region on their own. Overall they are a good, sustainable method for villages, especially when combined with tree planting and fewer animals on the land.

Exam tip

In an "Evaluate" answer, use the figures from Fig. 1.1, give a strength and a weakness, and end with a judgement about how sustainable the method is.

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