The Soil, how the soil is made up


Twenty-three titles have been published in this series, designed as handbooks for intermediate level agricultural education and training. They may be purchased as a set or as individual documents.

1. The way to work. The living plant

2. The plant – the root

3. The plant – the stem

4. The plant – the leaf

5. The plant – the flower

6. The soil – man and the soil

7. The soil – how the soil is made up

8. The soil – the living soil – working the soil

9. The soil – working the soil (continued)

10. The soil – conserving the soil – improving the soil

11. Animal husbandry – introduction

12. Animal husbandry – feeding animals

13. Animal husbandry – looking after animals – how cattle reproduce

14. Animal husbandry – what cattle produce

15. Keeping chickens

16. Food crops

17. Market gardening

18. The oil palm

19. Groundnuts

20. Upland rice

21. Wet paddy or swamp rice

22. Cocoa

23. Coffee

Better Farming Series    7

The Soil

— How the Soil is Made Up

Published by arrangement with the Institut africain pour le développement économique et social


Rome 1970

© French edition, lnstitut africain pour le développement économique et social (INADES) 1967

© English edition, FAO 1970


  1. Preface
  2. Outline Of Course
  3. How is the soil made up?
    1. It is made up of several layers
    2. The first layer cultivable land
    3. The second layer
    4. The third layer: savanna
    5. The third layer: in the forest
  4. To cultivate a soil, you must know it
  5. The soil elements
    1. Clay
    2. Sand
    3. Silt
    4. An experiment
  6. Humus
    1. Organic matter mixed with soil becomes humus
    2. The function of humus
    3. Humus and soil structure
    4. Humus improves soils
  7. Air in the soil
  8. Water in the soil
    1. Circulation of water in the soil
  9. Suggested question paper


The text of this manual is a translation and adaptation of “Le sol – comment est fait le sol,” published by the Agri-Service-Afrique of the Institut africain pour le développement économique et social (INADES), and forms part of a series of 23 booklets. Grateful acknowledgement is made to the publishers for making available this text, which it is hoped will find widespread use at the intermediate level of agricultural education and training in English-speaking countries.

It should be noted that the original texts were prepared for an African environment and this is naturally reflected in the English version. However, it is expected that many of the manuals of the series — a list of which will be found on the inside front cover — will also be of value for training in many other parts of the world. Adaptations can be made to the text where necessary owing to different climatic and ecological conditions.

Applications for permission to issue this manual in other languages are welcomed. Such applications should be addressed to: Director, Publications Division, Food and Agriculture Organization of the United Nations, Via delle Terme di Caracalla, 00100 Rome, Italy.

The cover illustrations were prepared by Asun Balzola.


• How is the soil made up?

It is made up of several layers

The first layer cultivable land

The second layer

The third layer: savanna

The third layer: in the forest

• To cultivate a soil, you must know it

• The soil elements




An experiment

• Humus

Organic matter mixed with soil becomes humus

The function of humus

Humus and soil structure

Humus improves soils

• Air in the soil

• Water in the soil

Circulation of water in the soil

• Suggested question paper




• Read pages 4 to 11.

• Is there some big hole in your village?

If there is, observe it carefully.

If there is not, dig one.

• Compare what you see with the drawings.

Perhaps you don’t see all the layers shown in the drawings.

• Take a good look at the part of the soil where the roots are.



• Read pages 12 to 21.

• Do the experiment described on page 19. Take a good look at it.

• Learn to see what the soil is like where you live.

Is it clay soil? Sandy? Silty?

• Do carefully all the experiments shown.



• Read pages 22 to 27.

• To help your memory, reread pages 4 to 21.

• Look carefully at the drawing on page 24. It will help you to understand.

• Find out what crops make the soil poor in humus. How is humus added to the soil?



• Read pages 28 to 31.

• Do the two experiments described on pages 29 and 30.

• Take a good look at the drawing on page 31. It shows you all the movements of water in the soil.

• Reread the whole booklet.

• Read the question paper.

When you have understood fully, try to answer the questions.



1. In the village maybe a hole has been dug, in order to make a well, or to get earth to make bricks.

Or maybe a ditch has been dug between two fields.

Let us dig a hole ourselves.

Let us dig it deep, and quite straight.

What do we see?

The soil is made up of different kinds of earth, or different layers.

2. These layers

• are not of the same colour;

that on top is often darker.

• are not of the same thickness;

the first layer is often less thick than the second.

• are not made of the same materials;

for instance, there can be a layer of earth on a layer of pebbles.

Sometimes you can see very clearly the difference between layers.

Sometimes these layers look similar and you cannot see the difference very clearly.

What do you see on the sides of the hole we have dug?


3. This first layer is what we work. It can be turned over with the hoe or plough. It can be heaped in mounds or ridges.

4. In this layer are found living roots. Creeping roots (see Booklet No. 2, paragraphs 10 and 11) and fibrous roots (see Booklet No. 2, paragraph 7) get their food here. Here too are dead roots, rotting leaves and stems.

The thicker and richer this layer is, the more fertile is the soil, and the stronger the plants.


5. Beneath the cultivable layer, there is often a layer lighter in colour, containing more sand.

Tap-roots go through this layer when it is not too thick.

There are often little pebbles below this layer.


6. Beneath the second layer there is a redder and thicker layer.

It is difficult to make a hole in this layer.

This is where the tree roots get their water during the dry season.

7. This layer is very unstable.

It is easily transformed; it may become a layer of pebbles, and may also become very hard.

Then a hardpan forms and roots cannot penetrate it to get food.

The savanna parent rock.

8. Beneath this third layer, there is soil which has not been transformed.

It is called parent rock.

In the mountains there is often no first, second or third layer.

The parent rock can be seen and nothing grows.

If there is a little soil on the parent rock, a few small trees and some grass grow among the stones.

There is no cultivable layer, no second layer and the pebble layer can be seen right on top.


9. In the forest this third layer is often very thick.

It does not become hard; it seldom forms a hardpan. It is always very poor; there are not many mineral salts.

In this layer you do not find pebbles, but you often see red, yellow or dark patches.

10. The parent rock in the forest

Beneath this third layer, there is also the parent rock. It is much deeper than in savanna soil and it seldom comes into view.


11. Certain plants look for water and mineral salts in the deep layers.

12. In various fields of the same village the layers of soil can be different. On the slopes, for example, the cultivable soil may have been carried away by the rain. This is why the cultivable soil is thicker in the valleys.


All the layers are mixed; almost always each contains sand, clay, silt, pebbles and humus.


13. Adobe walls and earthenware pots are made of earth. This earth is called clay.

Most often 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.

14. Most often the clay is mixed, which is what gives it a red or sometimes brown or black colour.

15. When the clay is not mixed, it is white. This is kaolin. Kaolin is used to whitewash houses.

16. When it is dry clay forms hard lumps.

If you crush a lump of clay it becomes dust finer than flour.

Clay and water

17. Wet clay takes whatever shape it is given, such as bricks, pots and stoneware jars.

18. 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.

19. If dry clay is made wet again, it becomes soft and sticky.

20. If clay is baked, it becomes very hard.

The stoneware jars keep their shape.

21. Clay is impermeable

Let us make a little bowl with wet clay.

Pour a little water into this bowl.

The water does not go through the wet clay.

We say the clay is impermeable.

22. On the other hand, if you pour water onto sand, the water goes through.

We say the sand is permeable because it lets the water through.

Clay is impermeable.

Sand is permeable.

23. Clay in soils

Almost all soils contain clay.

Soils that contain a lot of clay are called clay soils. Like the clay itself, clay soils are

• impermeable.

24. When it rains on a clay soil, the water does not go through the soil easily. It takes a long time for the water to disappear. So clay soils are

• difficult to work.

25. 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.

Soils with much clay are difficult to work and often too wet.

Soils with little clay are easily carried away by water and by the wind.


26. 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.

27. Let us take a closer look at 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.

You cannot crush a grain of sand with your fingers. Sand is permeable; it allows water to pass through it. Take some sand in your hand, and let it run gently. The grains roll over each other; you cannot make a lump with sand.

28. Sand in the soil.

Almost all soils contain sand.

Soils that contain a lot of sand are called sandy soils. Like sand itself, sandy soils are

• permeable.

29. When it rains on sandy soil, the water passes through easily.

You can walk on sandy soil after rain. The sand does not stick to the feet like clay: it is

• easy to work.

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

Sandy soils are called light soils. They are

• unstable.

31. 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.

32. Groundnuts, cassava, yams, and coconut trees grow very well in sandy soils; their roots easily penetrate them.

But sandy soils do not hold water and mineral salts well; fertilizers must be added.


Certain soils are neither sandy nor clay soils. They are made of silt

33. 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; it is not as permeable as sand.

Silt does not form dust as fine as clay dust; therefore 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 are:

34. 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.

And they are harder to work than sandy soils.

Wind and rain do not carry them away so easily.

35. less heavy than clay soils.

They are less hard to work.

They do not crack when they dry.

[]An experiment

36. Get a little earth from a valley, from a plateau and from the side of a slope.

Put the earth from each plot in a different glass or empty bottle.

• In each glass (or empty 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 five 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 sometimes one of small 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.

Thus the soil is a mixture.

37. All soils are mixtures.

Clay soils also contain sand and silt.

Sandy soils also contain silt and clay.

Let us take another look at the different glasses.

We see that the layers of soils are different.

• If you put in a glass earth that is mainly sand, the sand layer is thick and the water quickly becomes clear.

• If you put in another glass earth that is mainly clay, the sand layer is not so thick, and the water remains dirty for a longer time.

38. What we see and what we do not see.

In studying the soil we have spoken of sand, clay and silt.

These can be seen.

But in the soil there are also mineral salts and humus.

They cannot be seen.


39. The organic matter in the plant

From the soil the plant gets water and mineral salts; that is, mineral matter.

This mineral matter is transformed by light and carbon to elaborated sap (see Booklet No. 4, paragraphs 27 to 33).

The elaborated sap feeds the plant. As blood enables a man to make muscles, hair and bones, so the elaborated sap enables a plant to make leaves, wood and fruits.

The leaves, wood and fruits are organic matter.

The mineral matter has become organic matter.

40. The organic matter in the soil

Leaves, branches and dead trees rot in the soil.

This is called decomposing.

Even big dead trees rot in a few years on damp soil. A lot of worms and insects live in the rotting tree. You can often see them.

But other living creatures cannot be seen.

You need a microscope to see them.

These are microbes.


41. Organic matter decomposes in soil.

The dead plant becomes organic matter in the soil. But soon it is no longer seen. For instance, a few months after putting on manure, you no longer see the manure.

The organic matter of the plant has been decomposed in the soil.

Microbes decompose organic matter.

42. The microbes are so small that they cannot be seen. There are very many of them. In a lump of earth the size of a piece of sugar, there are millions and millions of microbes.

43. They feed on organic matter.

They also need air to breathe, and water.

If air and water are missing, the plant does not rot, the organic matter does not decompose.

Dry seeds keep well.

Wet seeds rot.

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


45. Humus makes soils richer.

Humus returns to the soil the mineral salts absorbed by the plants.

46. It is easier to work soil that contains humus.

Earth that contains humus sticks less to the hands.


47. What is soil structure?

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.

It is the humus which joins together sand, clay, silt and pebbles.

Humus is necessary for soil structure.

48. Bad soil structure

The structure is bad when there is sand, clay and silt, but no humus.

Then air and water circulate badly. Roots penetrate badly, breathe badly and feed badly.

49. Good soil structure

The structure is good, because the sand, clay and silt are joined by a mixture of clay and humus.

Air and water circulate well.

Roots penetrate well, breathe well, feed well.


50. 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.


51. Cassava takes humus from the soil.

In traditional farming, leaves and branches are burned, the fire destroys the organic matter. The quantity of organic matter, to provide humus, is thus reduced. When cassava is harvested, the whole plant is lifted: the roots to eat, the stems to be replanted. Almost no organic matter is left on the soil or in the soil.

The cassava has taken humus from the soil, but the organic matter of the cassava has not been returned to the soil.

52. Beans make the soil richer in humus.

When a bean plant is harvested, only the fruits are taken. The stems, roots and leaves are left to rot on the soil.

They decompose and provide humus.

Beans are a crop that leaves organic matter in the soil.

The humus taken from the soil by certain crops must be returned to it.

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


53. Air must circulate in the soil.

The microbes, which are living things, need air to breathe.

To live, they decompose the organic matter in the soil.

If there is no air, the microbes cannot breathe.

They cannot transform the organic matter into humus. Roots too need air to breathe.

Without air, roots die.

They cannot go on feeding the plant.

54. How can the soil be given air?

When you work the soil, air enters into the soil.

If there is too much water the air does not circulate well.

Water prevents air entering the soil.

So ditches are made to get rid of the surplus water. If the soil structure is good the air circulates well.

To get a good soil structure, there must be humus.

Humus makes it easier for air to circulate in the soil.


55. To live, a plant needs water.

When plenty of rain falls, the harvest may be good.

But when very little rain falls, the harvest is bad.

To get a good harvest, there must be enough water.

56. A plant needs water every day.

But it does not rain every day.

When it does not rain, the plant must be able to find water in the soil.

Thus the soil must build up reserves of water.

Good soil builds up reserves of water and gives it back to the plant when necessary.


57. Water goes from above downward during the rainy season.

Water enters easily into soil which has a good structure.

It also enters easily into light soils.

Water enters with difficulty into a soil which has a bad structure, or into soil that is too dense. It may carry away the soil.

58. Water goes from below upward during the dry season.

Let us make an experiment.

Put a little water in a plate. Take some lumps of earth from three different fields. Put these lumps in the plate. What do we see?

In each of the lumps the water rises higher than it is in the plate.

In the same way, water rises from the damp soil up to the roots, even if they are in dry soil.

In some lumps the water rises more quickly.

In clay soils the water rises more quickly than it will do in silty soils.

In soils with a good structure water rises more quickly than in soils with a bad structure.

59. Water also goes sideways.

Let us make a hole in the earth, and then pour water into the hole.

The water goes into the hole from above downward, but it also makes the sides of the hole wet.

In the soil water also goes sideways.

60. The earth can build up reserves of water. How?

• If the soil structure is good, the water goes in better and remains there: the earth builds up reserves.

A clay soil holds the water better than a sandy soil.

61. When it rains a lot, the water may sink very deep into the earth.

If it meets an impermeable layer of soil, it remains there.

If we dig a well, this water can be found.

62. To have good reserves of water in the soil, it is necessary to:

• Improve the soil structure, by adding organic matter, such as manure and the remains of plants.

The organic matter becomes humus.

• Work the soil, since water does not go into soil that is too dense.

Working the soil at the beginning of the rainy season enables the soil to put a lot of water into reserve.



Clay is used to make pots

Yes or No

Sandy soils are heavy soils

Yes or No

The soil is a mixture

Yes or No

Organic matter decomposes in the soil

Yes or No

Humus destroys the soil structure

Yes or No


Microbes need__________to breathe.

A good__________must build up reserves of water.

A clay soil holds water better than a__________soil.

If the cultivable land is__________the harvest will be good.

The third layer of savanna soil may harden and become a__________


Where is the parent rock?

Why are clay soils called heavy soils?

How do microbes change the soil?

Where does humus come from?

What are the soil layers where you live?

What is humus for?

What is soil structure?


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The Soil, how the soil is made up

Better Farming Series, no.7. This handbook is designed for intermediate level agricultural education and training. This manual is a translation and adaptation of "Le sol: comment est fait le sol," published by the Agri-ServiceAfrique of the lnstitut africain pour le developpement economique et social (INADES), and forms part of a series of booklets. Grateful acknowledgement is made to the publishers for making this text available for English-language agricultural education and training. This illustrated manual explains the layers that compose soil, how to cultivate a soil, go over the elements of clay, sand and silt in soils, and explains how humus, air, and water in the soil enrich it.

  • Author: FAO
  • Published: 2017-01-16 12:35:12
  • Words: 5137
The Soil, how the soil is made up The Soil, how the soil is made up