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2.1.1 Soil composition

When dry soil is crushed in the hand, it can be seen that it is composed of all kinds of particles of different sizes.

Most of these particles originate from the degradation of rocks; they are called mineral particles. Some originate from residues of plants or animals (rotting leaves, pieces of bone, etc.), these are called organic particles (or organic matter). The soil particles seem to touch each other, but in reality have spaces in between. These spaces are called pores. When the soil is “dry”, the pores are mainly filled with air. After irrigation or rainfall, the pores are mainly filled with water. Living material is found in the soil. It can be live roots as well as beetles, worms, larvae etc. They help to aerate the soil and thus create favourable growing conditions for the plant roots (Fig. 26).

Fig. 26. The composition of the soil

2.1.2 Soil profile

If a pit is dug in the soil, at least 1 m deep, various layers, different in colour and composition can be seen. These layers are called horizons. This succession of horizons is called the profile of the soil (Fig. 27).

Fig. 27. The soil profile

A very general and simplified soil profile can be described as follows:

a. The plough layer (20 to 30 cm thick): is rich in organic matter and contains many live roots. This layer is subject to land preparation (e.g. ploughing, harrowing etc.) and often has a dark colour (brown to black).

b. The deep plough layer: contains much less organic matter and live roots. This layer is hardly affected by normal land preparation activities. The colour is lighter, often grey, and sometimes mottled with yellowish or reddish spots.

c. The subsoil layer: hardly any organic matter or live roots are to be found. This layer is not very important for plant growth as only a few roots will reach it.

d. The parent rock layer: consists of rock, from the degradation of which the soil was formed. This rock is sometimes called parent material.

The depth of the different layers varies widely: some layers may be missing altogether.

2.1.3 Soil texture

The mineral particles of the soil differ widely in size and can be classified as follows:

The amount of sand, silt and clay present in the soil determines the soil texture.

In coarse textured soils: sand is predominant (sandy soils).
In medium textured soils: silt is predominant (loamy soils).
In fine textured soils: clay is predominant (clayey soils).

In the field, soil texture can be determined by rubbing the soil between the fingers (see Fig. 28).

Farmers often talk of light soil and heavy soil. A coarse-textured soil is light because it is easy to work, while a fine-textured soil is heavy because it is hard to work.

The texture of a soil is permanent, the farmer is unable to modify or change it.