This is the surface of the earth's crust which supports plant life. Soil is formed by gradual breaking down of the parent rock - a process called weathering.

Causes of weathering:

Rain water also washes off particles from rocks.

COMPONENTS OF SOIL:

These include:

• Inorganic particles of sand, silt and clay;
• Organic matter (humus);
• Water;
• Air;
• Dissolved salts;
• Living organisms.

Inorganic particles:

These are particles of:

- Clay: which has very tiny particles.

- Silt: which has medium particle size.

- Sand: which has large particle size

- Gravel: which has very large stone particle sizes.

Silt and clay are made of silicon and aluminum oxides whereas sand is silicon oxide alone.

Iron oxide gives the particles a red/brown coating.

Inorganic particles form the framework of aggregate with humus to produce crumbs of soil.

Organic matter:

When plants and animals decay, humus is formed. This is especially in the top soil.

For complete decomposition to form humus, oxygen is required.

Incomplete decomposition leads to formation of peat which is acidic.

Humus is important in:

- Giving soil colour;

- Glueing soil particles together;

- Decaying to produce nitrates and other nutrients;

- Absorbing and retaining water.

Water:

This occurs as a thin moisture film which adheres on soil particles by capillary attraction and by chemical forces.

The maximum amount of water a soil can hold by capillary attraction and chemical forces is called field capacity. Above field capacity, any more water would drain away by gravity - a process called leaching.

Water is important in dissolving mineral salts so that they are available to plants.

Air:

This is found in spaces between soil particles. In water-logged soils these spaces are covered up with water.

Air contains oxygen which is for respiration of plant roots and of soil microorganisms.

Air also has nitrogen which is fixed into the soil.

Mineral salts:

These make a dilute solution with soil water. They are vital for plant growth/

They contain elements like:

Nitrogen which is a major component of protein and helps in proper leaf growth.

Phosphorus for enzyme functioning and proper root growth.

Potassium for controlling the rate of photosynthesis and respiration. It also helps in proper stem growth.

Sulphur is also a component of protein.

Magnesium for the formation of chlorophyll.

Calcium for cell wall formation.

Soil organisms:

Examples of soil organisms include the micro-organisms like bacteria and fungi and the macro-organisms like earthworms, ants, millipedes and termites.

Bacteria help to:

Break down organic matter by decomposing it to humus.

Convert humus to nitrites in the nitrogen cycle for example nitrosomonas.

Convert nitrites to nitrates in the nitrogen cycle for example nitrobacter.

Fix nitrogen in the soil for example rhizobium, azotobacter.

Convert soil nitrogen into free atmospheric nitrogen for example denitrifying bacteria.

The macro-organisms help in:

- Improving soil drainage and aeration for example burrowers.

- Forming part of food chains and food webs.

TYPES OF SOIL:

Clay soil:

This has very tiny particles.

When wet, it is hard to plough because it is sticky. When dry, it is hard to break up during cultivation.

It is poorly aerated and is also poorly drained.

It has a high water retention capacity and does not allow leaching.

It has small but numerous air spaces.

It has got good capillarity.

To improve on clay soil add organic matter or lime so that flocculation takes place i.e. the tiny clay particles glue themselves together to form crumbs.

Clay soil can also be improved by growing grass on it.

Sandy Soil:

- This has large particle size mainly of sand which give a better aeration and drainage.

- It has low water retention capacity.

- It is easy to dig both when dry and when wet.

- It is easily leached.

- It has a low capillary.

Sand soil can be improved by use of farmyard manure or compost manure, mulching or addition of peat.

Differences between clay soil and sand soil:

The texture of sandy soil has larger particles than that of clay soil.

Clay soil is hard to plough but sandy soil is easy to plough.

Clay soil is poorly aerated whereas sandy soil is well aerated.

Clay soil has poor drainage whereas sandy soil has good drainage.

Clay soil has high water retention capacity whereas sandy soil has low water retention capacity.

Clay soil does not allow leaching but sandy soil easily allows leaching.

Clay soil contains more mineral salts than sandy soil.

Clay soil takes long to warm but sandy soil easily warms up.

Loam Soil:

This is the soil with a balanced mixture of particle sizes and is the most suitable for plant growth.

It has good humus content.

It is easy to plough, is well aerated and has good drainage.

It has good capillarity.

Soil pH:

A soil can become acidic when:

There is water-logging which prevents air for the aerobic bacteria to completely decompose plant remains fully to humus; but instead incompletely to peat;

There is over-application of fertilizers of ammonium sulphate which causes acidity. Such fertilizers should be accompanied by lime.

To improve on an acid soil, add lime.

To improve on an alkaline soil, add peat i.e. half-decomposed humus.

Soil profile:

These are changes in appearance and constitution of soil from the surface downwards.

Top soil is dark because of presence of humus.

Soil structure and soil texture:

Soil structure is the arrangement of soil particles in crumbs.

Soil texture refers to the size of particles of soil for example sand, silt, clay.

Soil texture affects aeration, drainage, capillarity, water absorption and ease of tillage.