The inorganic components of soil are weathered rock, air, water and minerals. The organic matter is the decomposing fragments of plants and animals. The spaces between the small particles that make up the soil are filled with air or water. Living plants and animals 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. Important plant foods include nitrogen (helps leaves and stems grow), phosphate (helps roots and fruits develop) and potassium (stimulates overall plant health). When plants die, they return the nutrients they initially absorbed from the soil, back to the soil, and enrich the soil. In this way soil plays a very important role in the recycling of nutrients (Enviro Facts, 1999i).

Soil takes thousands of years to develop from the parent rock – 10mm of soil takes between 100 and 1000 years to form. In South Africa 1mm of soil takes about 40 years to form. The time depends on the speed of weathering (parent rock being broken down into small particles). Weathering can be physical (frost, temperature changes, salt chrystallisation), chemical (chemical action of water, oxygen, carbon dioxide and organic acids) or biological (tree roots that widen crevices and cracks).

The soil profile generally consists out of three main layers (horizons): the topsoil (100 –200 mm deep) or darker layer, where air, water and humus allow plants to grow in; the sub-soil, a more clay-like layer which acts as a reservoir (water store) for the plants, and the bedrock or parent material, which is the underlying layer from which the first two horizons are formed. In South Africa sub-soil can be transported or residual, or both. Transported soil originates from wind, water or gravitational processes, while residual soils are the in situ (undisturbed) decomposed product of the underlying rocks. Soil horizons are set apart from other soil layers by differences in physical and chemical composition, organic structure, or a combination of those properties. Soil horizons are developed by the interactions, through time, of climate, living organisms, and the configuration of the land surface (relief) (Strahler & Strahler, 1992).

A city, like Greater Johannesburg, is an artificial man-made environment, dependent on technology and imported energy. The system is kept at an artificial equilibrium, and depends on the surrounding soil, agriculture and natural resources to ensure its wellbeing and sustain economic development. Soils are unfortunately deteriorating at an alarming pace due to poor management practices.

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