Soil forms the upper-most layer of the earth's crust and is made up of inorganic and organic matter. The inorganic components are weathered rock, air, water and minerals. The organic components are the decomposing (rotting or decaying) fragments of plants and animals. The spaces between the small particles that make up the soil are filled with air or water.

Living plants (e.g. algae, lichen) and animals (e.g. earthworms, moles, termites) live in the soil and improve aeration and drainage. Some organisms, such as bacteria, play an important role in converting plant foods or nutrients, e.g. nitrogen, into a form that plants can use to grow.

Some important plant foods or nutrients:
Nitrogen - helps leaves and stems to grow
Phosphate - helps roots and fruits to develop
Potassium - stimulates overall plant health

As plants and animals die and decompose, humus is formed from their remains. Humus fertilizes and enriches the soil as it contains nutrients and improves the soil's ability to hold water and air. Thus, nutrients in the soil are used by plants and animals and are returned to the soil when they die and rot. In this way soil plays an important role in the recycling of nutrients.

FORMATION OF SOIL

Soil takes thousands of years to develop from parent rock - 10 mm of soil takes between 100 and 1000 years to form. The exact amount of time taken depends upon the speed at which the parent rock weathers, i.e. is broken down into small particles. Weathering occurs through chemical, physical and biological processes.

* Chemical weathering is caused by the chemical action of water, oxygen, carbon dioxide and organic acids (secreted by lichens). Chemical weathering is very active in the humid tropics as it is accelerated by high temperatures and rainfall.

* Physical weathering is caused by frost, temperature changes and salt crystallization. For example in places where the temperature variation in one day is great, repeated cooling and heating of a rock surface will weaken it and layers sometimes peel off.

* Biological weathering is caused by plants and animals. For example, tree roots in rock crevices grow and widen the crack. Burrowing animals such as rabbits, worms and ants bring soil and rock to the surface where they can be weathered by chemical and physical processes.

Pioneer plants such as grasses, lichens and moss grow on the loose, weathered particles of rock and add organic material to the developing soil. These plants also trap water and wind-blown soil, contributing to more plant growth and soil formation.

THE SOIL PROFILE

As soil is formed it develops layers or horizons which make up the soil profile. There are generally three horizons in soil.

* Topsoil: the upper layer, about 100 - 200 mm deep is where plants get their nutrients so that they are able to grow. Topsoil is often darker than the other layers as it is rich in humus. In addition to releasing nutrients for plants, humus improves the crumbly nature of the soil. When soil is crumbly it allows air to move through it, soaks up water, reduces runoff and erosion, and promotes plant growth. For topsoil to remain productive, humus must be constantly added to soil.

* Sub-soil: generally more clay-like, this layer acts as a reservoir (water store) for plants growing in the topsoil. When the sub-soil is exposed it erodes fairly easily.

* Bedrock or parent rock: this is the underlying layer from which the first two horizons are formed.

TYPES OF SOIL

Soils can be classified according to the proportion of different- sized particles that they contain. Clay soils have a large proportion of the smallest particles and are the least suitable for plant growth. Because the particles are tightly packed, there are no spaces between them to hold enough oxygen for the plant roots and soil micro-organisms. By contrast, sandy soils have a high proportion of the largest particles, with big air spaces. Water drains through sandy soils very easily making them equally unsuitable for plants. Soils which have approximately equal proportions of sand and clay are called loams and are best for plant growth because they contain enough air and can hold moisture.

DID YOU KNOW

* Almost everything we need can be traced back to soil: food, clothing, paper, timber, medicines, shade, and oxygen.

* Soil erosion is one of our worst environmental problems. South Africa loses about 300 to 400 million tonnes of soil each year!

WHAT CAN YOU DO

* Reduce demands on the soil by not wasting anything that comes from soil.

* If you have a garden, care for the soil by adding compost to replace organic matter (see Enviro Facts "Compost").

* Avoid pesticides which might kill soil life.

* Rotate the plants you grow to keep the soil healthy.

FURTHER READING

HANDS-ON: SOIL AND COMPOST LIFE - A FIELD GUIDE. I. van den Berg. Share-Net, address below.

SOIL IS LIFE: A HANDBOOK FOR TEACHERS. M. Roos (ed). Share-Net, address below.

SOIL IS LIFE: A POSTER. Available from Delta Environmental Centre. P/Bag X6, Parkview, 2122. Tel. 011-888 4831.

MADLESUTHE'S FARM: An enviro-picture building game. Shell Education Service. PO Box 2231 Cape Town, 8000.

PEOPLE'S WORKBOOK. Environmental Development Agency. Address below.

WE CARE! PRIMARY. A resource package for teachers of primary school. Available from Environmental Education Programme, Department of Didactics, University of Stellenbosch, Stellenbosch, 7600.

SOIL IS WEALTH. KwaZulu Dept. Nature Conservation, address below.

Enviro Facts: Compost, Soil erosion.

USEFUL CONTACTS

Share-Net. PO Box 394, Howick 3290. Tel. 0332-303931.

University of Natal, PO Box 375, Pietermaritzburg, 3200. Tel. 0331-68385/6/7.

Farmers Support Group. Environmental Development Agency. PO Box 322, Newtown, 2113. Tel. 011-834 1905.

KwaZulu Dept. Nature Conservation. Head Office, P/Bag X98, Ulundi, Tel. 0358-700552.