Natural factors that affect soils include the geology, soil types, their properties, landforms, dolomitic land, geotechnical properties and slope categories, while human factors include mining, agriculture, informal settlements and poverty, urbanisation, population growth and development.

UNDERLYING GEOLOGY

The Greater Johannesburg area has a firm base of granitic rock, intruded some 3 200 million years ago, when the continent was forming. Interspersed with this are intrusions of greenstones (lava resulting from volcanic activity). On top of the base is the more recent accumulation of sedimentary rocks (mostly shale and quartsite), deposited between 2 800 and 2 600 million years ago. The great weight of the sedimentary layer caused the land mass to depress and tilt, forcing the palaeo-ocean to retreat.Subsequent, much more recent continental upliftment resulted in the Witwatersrand Basin being raised some 1 800m above sea level. The more recent, and slowly ongoing, process of erosion - caused by the weathering of rocks by water, wind and ice – has created the gently undulating landscape of today. Increasingly the Witwatersrand ridge acts as a watershed between the two oceans. Rain falling in Parktown drains into the Braamfontein Spruit, Jukskei, Crocodile, then Limpopo Rivers to spill out into the Indian Ocean, while rain falling in Mondeor drains into the Bloubos Spruit, Klip, Vaal, then Orange Rivers to enter the Atlantic Ocean in the Northern Cape (Butchart, 1995). Overlying the Witwatersrand sediments in the south, are the Ventersdorp lavas of 2 700 million year age. An important Graben (depression in a normal fault) section of these lavas is also present through Bezuidenhout Valley and the Johannesburg CBD. Limestone rocks (dolomite) overlie the rocks through Lenasia, while shales, quartzites and lavas, of the Transvaal Sequence, occupy the areas of Grasmere, Ennerdale and Orange Farm. These latter are approximately 2 500 million year age. All the GJMC rocks have been invaded by younger volcanic dyke intrusives, which influence structural stability and groundwater flows (Williams, 2000).

SOIL TYPES, PROPERTIES AND AGRICULTURAL POTENTIAL

According to the Strakhov-model of 1967, South Africa, and thus the Greater Johannesburg area, lies in a semidesert/desert latitude zone of almost no weathering and soil formation, due to the drop in precipitation and vegetation cover, and higher temperatures and evaporation rates (Selby, 1985). This leads to the lack of soil development in the areas. A variety of soil properties reveal the classification of soil, like the thickness, texture, structure and drainage, which in turn shows the agricultural potential.

CONSTRUCTION MATERIAL RESOURCES

Prior to development these resource areas should be carefully evaluated in terms of their potential future and alternative sources of construction material.

LANDFORMS

Various landform groups, including crests, slopes, plains, drainage features, solution features, surface water and cultural landforms are found in the area, and determine the relief of the area.

SLOPE CATEGORIES

The slope categories depict the general topographic character of the area.

DOLOMITIC LAND

These areas are underlain by the rock type dolomite, which poses various environmental problems associated with development.

GEOTECHNICAL PROPERTIES

These properties indicate anticipated potential geotechnical constraints on development.

AGRICULTURE

The principles of energy use and flow in natural ecosystems also apply to agricultural ecosystems. Agriculture however relies on inputs on energy that are ultimately derived from fossil fuels, whereas natural ecosystems only rely upon solar energy. The most obvious is the fuel that runs the machinery. The application of fertilizers and pesticides, tillage and the transportation of the crops all need a cultural input of fossil fuels. The fact that man maintains this cultural system, places pressures on the soil and other natural components.

MINING

Gold mining is the reason for the existence of Johannesburg, the city of gold. The extraction of minerals from the mining belt leads to toxic groundwater pollution, while incorrect methods and ungrassed dumps leave soils susceptible to erosion by wind and water.

INFORMAL SETTLEMENTS AND POVERTY

Increased population growth leads to an increase in poverty, informal settlements and environmental degradation. Various pressures impact on the land, including the lack of services and a lack of space and housing, forcing more people to live within the 1:50 year flood line. This also means that more vegetation is removed for housing and cooking.

URBANISATION, POPULATION GROWTH AND DEVELOPMENT

The increase in population growth and people flocking to the city for job opportunities increases the pressures on Greater Johannesburg.

REFERENCES

Butchart, D. 1995: Wild About Johannesburg. Southern: Halfway House.