THE COMPOSITION OF SOIL
The soil consists of three main components: soil, liquid and gaseous. Solid mineral matter, derived from rocks and organic material, constitutes the bulk of the soil. The liquid and gaseous components, water and air, fill a network of channels which are formed by the interspaces between the solid materials. It contains as well small living organisms such as bacteria, earthworms, fungi and algae; and finally organic matter in different stages of decomposition.
A good agricultural soil is made up of a mixture of the following substances, the number representing percentage by volume:
- Solid particles ............... 45%
- Water .............................. 25%
- Air .................................. 25%
- Organic matter ................ 5%
(i) Solid particles (mineral matter)
They constitute the greater part of the soil or the bulk of soil. They consist of stones, gravel, and other soil particles.
(ii) Water
Soil water is the soil solution. This helps to supply food materials to growing plants. Most of the water needed by plants comes from the soil. Water enters and leaves the soil in a way which is referred to as the water cycle.
(iii) Air
Air is present in the spaces between soil particles. It consists of a mixture of gases just like the atmospheric air. But oxygen and carbon dioxide in the soil are the most important components of soil air. The more the soil air in a particular soil sample, the less the soil water in the same sample.
(iv) Organic matter
Organic material is formed by the decaying of plant and animal tissues. Decayed organic matter is known as humus and when this is mixed with the mineral particles of rocks a humus type of soil is formed. Humus is dark brown in colour.
(v) Small living organisms
These organisms bring about the breaking down of organic matter. Examples are fungi, bacteria, algae and earthworms.
TYPES OF SOIL
A type of soil is determined by the rock from which it has been formed. The different types of soil are sand, silt, loam and clay.
(a) Sandy soils
Chiefly sand and silt with about 6% clay is called a sandy soil. These soils are free-draining and do not retain any appreciable amount of soil water. It contains gravel, coarse sand, fine sand and limited amount of silt. It is brown, loose, dry, can be easily worked, poor in food and air spaces. They need humus for binding up.
(b) Silt
The size range of silt is from 0.002 to 0.2 mm in diameter. Silt particles are more micro-sand grains usually irregular fragmental but have a more surface area per gram than sand. Silt particles feel floury, have moderate temperature, and are slightly sticky when wet. Silt holds more available water than sand and is better aerated than clay. It however has a better mineral holding capacity than sand.
(c) Clay soils
Clay ranges in size from 0.002 mm and lower. These contain more than 40% of clay particles. They hardly contain any of the coarser materials. They are sticky in texture, plastic, and easily mouldable into shapes when wet. Air and water do not move easily within them. They are heavy to work, drain poorly and tend to crack, leaving deep crevices in the dry season.
(d) Loamy soils
Loamy soils contain a fair balance of clay, silt and sand particles. Loam is therefore, the best soil for agriculture as it has a high proportion of organic matter or content. They have the advantage of clay soils in that they retain plant nutrients yet they also have the drainage of sandy soils. They are easy to cultivate, can withstand moderate periods of drought.
When the proportion of sand particles is high the soil is said to be sandy-loam, and when the clay content is high and sand content is low the soil is referred to as clay-loam.
Soil particles
Soil particles are classified on the basis of their size groups and particle size diameter as follows:
Table 12: Soil particles and Diameter
Types of soil particles
Particle size Diameter (mm)
Stones and Gravel
2.00 and above
Coarse sand
2.00–0.20
Fine sand
0.20–0.02
Silt
0.02–0.002
Clay
0.002 and below
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