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.
* 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.
* 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.
* Soil erosion is one of our worst environmental problems. South Africa loses about 300 to 400 million tonnes of soil each year!
* 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.
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.
Farmers Support Group. University of Natal, PO Box 375, Pietermaritzburg, 3200. Tel. 0331-68385/6/7.
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.