Grade 8 Agriculture & Nutrition Study Notes

Strand 1.0 — Conservation of Resources

This strand is about protecting two of the most important farming resources: soil and water. Without healthy soil, crops cannot grow. Without stored water, crops cannot survive dry spells.

What this strand covers

• Sub-strand 1.1 — Soil Conservation Measures: How to stop soil from being washed away, and how to carry out conservation activities at school.
• Sub-strand 1.2 — Water Harvesting and Storage: How to collect and store rainwater for use on the farm.

Why Soil Needs to Be Conserved

Soil is the top layer of the earth. It holds the nutrients and water that crops need to grow. For a farm to be productive, the soil must stay in place and stay healthy.

When rain falls on a bare or sloping field, water that does not soak in flows across the surface. This is called surface run-off. As it moves downhill, it picks up soil particles and carries them away. The loss of topsoil this way is called soil erosion.

The faster run-off moves, the more soil it carries. One heavy rainstorm on a bare slope can remove topsoil that took hundreds of years to form. Learning how to stop this loss matters for anyone who grows food — at school, at home, or on a farm.

The five methods of soil conservation

1. Practising strip cropping
2. Making grassed waterways
3. Making stone lines
4. Making trash lines
5. Forming soil bunds

Practising Strip Cropping

Strip cropping means growing crops in alternate strips across the same sloping field.

One strip is planted with a dense ground-cover crop such as sweet potatoes, wheat or soybeans. The next strip is planted with a taller crop like maize or beans. They alternate across the slope.

How it conserves soil: Ground-cover crops spread low and cover the soil. When run-off flows downhill, the dense vegetation at each strip slows the water and holds soil particles together. This is repeated strip by strip all the way down the slope.

Making Grassed Waterways

A grassed waterway is a continuous channel through which rainwater flows off a farm. Grasses are planted or allowed to grow in the channel. The grasses hold the soil particles together and prevent soil erosion.

Note: Animals must not graze in a grassed waterway. Their hooves break up the grass and loosen the soil, causing the very erosion the waterway was built to stop.

Making Stone Lines

Stone lines are made by heaping stones across a slope. The stones are arranged along the contour — an imaginary line running across the slope at the same height, neither going uphill nor downhill.

How stone lines conserve soil: When run-off reaches a stone line, the heap of stones slows it down. Slower water drops the soil it was carrying. The soil settles on the uphill side of the line. Over time, soil builds up there and the slope becomes more level.

Stone lines are built from stones collected on or near the farm — no cost, no purchased materials. This makes them very practical for small-scale farmers.

Making Trash Lines

Trash lines are heaps of crop residues — such as maize stalks, wheat straws or dry grass — piled across a slope. "Trash" here means dry plant leftovers from the harvest, not household rubbish.

Strong sticks are pushed into the ground first. Dry plant material is packed between them to make a continuous heap across the slope.

How trash lines conserve soil: The heap slows run-off and prevents soil erosion. As the plant material decomposes, it becomes organic matter that improves soil fertility. Trash lines use materials the farm already has after harvest — zero cost, zero waste.

Spacing: At least 1.5 m to 2 m apart to leave room for planting between them.

Forming Soil Bunds

Soil bunds are raised banks of compacted soil built across a slope. They are the most solid of the five methods — large enough to stop run-off completely, not just slow it down.

How soil bunds conserve soil: Run-off hits the bund and stops. The water ponds behind it and slowly soaks into the ground. This prevents erosion and also captures and retains water, which helps plants grow. Bunds must be well compacted and sealed — any gap lets run-off burst through.

Spillways are low gaps built into the bund to let excess water out safely. Grass is planted on the spillway to stop it eroding.

Comparison of the Five Methods

All five methods slow or stop surface run-off. The table below shows how they differ.

How Strip Cropping Works in Detail

The field is divided into strips running across the slope. Run-off flows downhill, so a strip crossing the slope sits directly in its path.

Step by step

1. Rain falls and run-off starts moving downhill through a tall-crop strip such as maize, picking up some soil.
2. The run-off reaches a ground-cover strip — sweet potatoes, soybeans or wheat.
3. The dense low leaves and roots slow the water and hold the soil particles.
4. The slowed water drops the soil it was carrying.
5. Weaker run-off enters the next tall-crop strip and the process repeats, strip by strip.

Erosion is reduced at every strip, not just at the bottom of the slope.

Crops Suitable for Strip Cropping

The most important choice is the ground-cover crop. It must spread low and cover the soil well so run-off cannot flow freely through it.

Strip cropping works using crops the farmer already grows — no extra materials needed.

What a Grassed Waterway Is

On a sloping farm, rain collects in natural low points and flows downhill. If those channels are left bare, water cuts into the soil season after season until a large gully forms.

A grassed waterway solves this. The channel is shaped into a smooth path and grass is planted inside it. The channel still carries water, but the grass protects the soil beneath.

Look at any sloping field after rain — bare channels cut into unprotected land, while grass-lined channels stay intact. That difference is what a grassed waterway achieves.

How a Grassed Waterway Conserves Soil

The grass inside the waterway works in four ways:

• Root grip — roots hold the channel soil firmly so water cannot dislodge it.
• Speed reduction — the dense grass mat creates friction that slows the water. Slower water carries less soil.
• Infiltration — because the water slows down, more of it soaks into the ground instead of rushing off.
• Filtration — the grass traps soil particles and pollutants, cleaning the water before it leaves the field.

Result: water leaves the farm through a safe grass-lined path, carrying far less soil than it would through a bare channel.

What Stone Lines Are and How They Conserve Soil

Stone lines are rows of stones heaped across a slope along the contour. A contour line runs across the slope at the same height — not going up or down.

Stones are collected from the farm or nearby — no cost. This makes stone lines practical for any small-scale farmer in a rocky area.

How stone lines conserve soil: When run-off reaches a stone line, the stones break its speed. Slower water drops the soil it was carrying. That soil settles on the uphill side of the line. Over many seasons the slope becomes more level.

What Trash Lines Are

Trash lines are heaps of crop residues piled across a slope. "Crop residues" means dry plant leftovers from the harvest — maize stalks, wheat straws, dry leaves, grass, vines. The word "trash" here does not mean rubbish.

Strong sticks are pushed into the ground along the contour first. Then the dry materials are packed between the sticks to form a solid heap. Lines should be at least 1.5 m to 2 m apart to leave planting space between them.

How Trash Lines Conserve Soil — and Do More

Like stone lines, trash lines are a physical barrier that slows run-off. The water drops the soil it was carrying, and that soil settles on the uphill side.

But trash lines have an extra benefit: decomposition. Over time the crop residues rot into organic matter that improves soil fertility. The farm already produces these materials after harvest, so trash lines cost nothing and turn waste into a useful resource.

Stone lines vs trash lines