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close this bookBetter Farming Series 04 - The Soil: How the Soil is Made up (FAO - INADES, 1976, 37 p.)
close this folderWhat is soil made of ?
View the document(introduction...)
Open this folder and view contentsSand
Open this folder and view contentsClay
Open this folder and view contentsSilt
Open this folder and view contentsHumus
View the documentSome practical advice


Soil is a mixture.

Get a little earth from a valley, from a plateau and from the side of a slope.
Put the earth from each field in a different glass. (If you haven't got glasses, use empty bottles.)

Earth from first and second field

· In each glass or bottle put two fingers of earth.
Fill up with water.

· Stir the mixture well in each glass.
Put it down and do not touch it for Jive minutes.

· Stir the mixture well in each glass once more.
Put it down and do not touch it for an hour.

An hour later, what do we see?

The earth has dropped to the bottom of the glass and the water is clear.

Look at the earth: several layers have formed.

· At the bottom of the glass there is a layer of sand and some little pebbles.

· In the middle is a layer of silt.

· Above is a thin layer of clay.
If the water is not clear, that is because clay is still mixed with the water.

· On top of the water float pieces of leaves and roots. The leaves and roots rot and form humus.

Thus the soil is a mixture.


Sand is found everywhere.
Rain carries sand into hollows and streams. It is then white- or yellow and shining because it is clean.
In the soil, sand is grey and does not shine, because it is mixed with earth.

Let us take a closer look at some sand.

It is made up of little grains.
These grains are not all alike.
They are very hard. If you rub a piece of iron with them, they scratch the iron because sand is harder than iron.

Grains of sand

· Sand is permeable.

Take a can.
Make a hole in the bottom of it.
Fill the can with sand.
If you pour water on to the sand, it goes through.

If you pour water on to the sand, it goes through

We say sand is permeable because it lets water through.

· Sand is unstable.

Take some dry sand in your hand and let it run gently.
The grains slide over each other; you cannot make a ball of sand.

Sand runs like water

· Sand in the soil.

Almost all soils contain sand.
Soils that contain a lot of sand are called sandy veils.

Sandy soils

Like sand itself, sandy soils are:

· permeable.

When it rains on sandy soil, the water passes through easily.
You can walk on sandy soil after rain.
Sand does not stick to the feet like clay.
Sand does not make mud. It is

· easy to work.

After the first rains, sandy soils are easy to work; they do not stick to tools like clay.
Sandy soils are called light soils.

· unstable.

The grains of sand do not stick together.
In the rainy season, water easily carries them away.
In the dry season, the wind can lift them up and carry them a long way. In the northern savannas people speak of a sand wind.
Groundnuts, cassava, yams and coconut trees grow very well in sandy soils; the roots easily penetrate them.
But sandy soils hold water and mineral salts badly.


Adobe walls and pots are made of earth.
This earth is called clay.
Usually the clay is found in the third soil layer: the red layer.
Because of this, a hole has often to be dug to get earth for bricks.
Usually the clay is mixed, which is what gives it a red or sometimes a brown or black colour.
When the clay is not mixed, it is white. This is kaolin. It is pure clay. Kaolin is used to whitewash houses.

· Clay and water

Wet clay takes whatever shape it is given, such as bricks, pots and stoneware Jars.
Wet clay sticks to the fingers; it makes mud.
When it is dry, clay forms hard lumps.
If you crush a lump of clay, it becomes dust finer than flour.
When it is dry, brown or black clay loses its water and cracks.
Red clay also loses its water, but does not crack.
Red clay can be used to make bricks and stoneware jars.
If dry clay is made wet again, it becomes soft and sticky.
If clay is baked, it becomes very hard. The stoneware jars keep their shape.

Clay is impermeable.

Take a can.
Make a hole in the bottom of it.
Fill the can with clay.
If you pour water on to the clay, it does not go through.
We say clay is impermeable because it does not let water through.

Water does not go through

Clay in the soil

Almost all soils contain clay.
Soils that contain a lot of clay are called clay soils,

Clay soils

Like clay itself, clay soils are:

· impermeable.

When it rains on a clay soil the water does not go through the soil easily. The water takes a long time to disappear. You can see the mud. So clay soils are

· difficult to work.

Wet clay sticks to the hands. It also sticks to tools.
For this reason clay soils are very difficult to work after the rains.
Clay soils are called heavy soils.
During the dry season clay soils become very hard. Cracks form in them.
The lumps are difficult to break.

Water remains in the soil; Cracks form in the soil

Soils with much clay are difficult to work and are often too wet.
Soils with little clay are easily carried away by water and by the wind.


Certain soils are neither sandy nor clay soils. They are made of silt.
Silt is made up of grains much smaller than sand grains.
Because of this, silt does not let water through as easily as sand does.
Silt does not form dust as fine as clay dust; because of this it is not impermeable like clay.
Wet silt does not stick like clay. However, silt can be made into lumps.
Soil that contains a lot of silt is called silty soil.

Silty soils

Like silt itself, silty soils are:

· not as light as sandy soils.

The silt grains are closer together than the grains of sand.
Thus water does not go through so easily; silty soils do not dry quickly.
They are harder to work than sandy soils.
Wind and rain do not carry them away so easily.

· Iess heavy than clay soils.

Silty soils stick less than clay soils.
They are less hard to work.
They do not crack when dry.


In the soil there are dead leaves and roots.
They rot and change into humus.
You cannot see humus as you see sand, clay and silt.

· Dead plants change into humus.

Leaves, branches and dead trees rot in the soil.
We say they decompose.
Even big trees rot in a few years on wet soil.
Many worms and insects live in a rotting tree. You can often see them.
But other living things cannot be seen. They are too small.
These are called microbes.
There are very, very many of them. In a lump of earth as big as a lump of sugar there are millions and millions of microbes.
They feed on leaves, on dead branches, on organic matter (see Booklet No. 2, page 23).
They also need air to breathe, and water.
If there is no air and water, the plant does not rot. The organic matter does not decompose.

Organic matter decomposed by the microbes in the soil is humus.

What humus does

· Humus makes soils richer.

From the soil the plant gets mineral salts (see Booklet No. 1, page 19).
The leaves turn the raw sap into elaborated sap (see Booklet No. 2, page 20).
The plant feeds on elaborated sap.
Wood and leaves are organic matter rich in mineral salts and carbon.
Organic matter rots and makes humus.
Humus returns to the soil the mineral salts used by the plant.

Humus returns the mineral salts to plant

Humus improves soils

Heaps of bricks, cement and sheet iron do not make a house.
To make a house they must be arranged, must be joined together.
Sand, clay, silt and pebbles without humus do not make a good soil.
They must be arranged, must be joined together to make a good soil.
The way in which sand, clay, silt and pebbles are joined together is called the soil structure.
It is the humus which joins together sand, clay, silt and pebbles.
Humus is necessary for soil structure.

Bad soil structure

This structure is bad because there is no humus.
The sand, clay and silt are not joined together.
Air and water circulate badly.
Roots penetrate badly, breathe badly and feed badly.
This is a bad soil structure.

Good soil structure

The structure is good, because the sand, clay- and silt are joined together by a mixture of clay and humus.
Air and water circulate well.
Roots penetrate well, breathe well and feed well.

Good soil structure

· Humus improves sandy soils.

Sandy soils with humus hold water better.
They are less easily carried away by wind and rain.

· Humus improves clay soils.

Clay soils with humus are less hard.
Air and water circulate better.
Soil without humus is not good soil.

Some practical advice

· Brush fires

In traditional farming; leaves and branches are burned.
The fire destroys the organic matter, so that no organic matter is left to make humus.
The brush fire destroys the organic matter and changes the soil structure.
Modern farmers do not make brush fires.

· Fallow

Cassava gives an example.

When cassava is harvested, the whole plant is lifted: the root for eating and the stems for replanting.

Almost nothing is left either on the soil or in the soil.

The cassava has taken humus from the soil, but the organic matter of the cassava is not returned to the soil. So the soil is less rich.

After growing, cassava farmers let the soil rest. They let it lie fallow.

During the fallow, the soil gets all the dead plants; it gets the organic matter from the dead plants, and the soil improves.

Soil must be given organic matter. The remains of the crop, grass, manure, provide organic matter.

· Some crops improve the soil.

When you harvest a bean plant, you take only the fruits.
The stems, roots and leaves are left to rot on the soil.
They decompose and give humus.
Beans are a crop that leaves organic matter in the soil.
The beans take humus from the soil but return organic matter to it.

· Manure

Modern farmers use animal manure.
Manure returns to the soil the organic matter taken out by the crop.

· Do not mix the soil and the subsoil.

The soil is rich in humus.
The subsoil has no humus.
In tilling, the farmer should never mix the soil and the subsoil, so as to keep his soil rich.
In planting a tree, do not mix the layers of soil.

Do not mix the layers of soil