PRESSURES OF SOIL   |
Land is the source of most of our food and raw materials and also provides us with living space. It is also the final resting-place of 90% of the waste we produce. As population and economic activity increase there is growing competition for the limited land available. South Africa has limited high potential agricultural land, most of it already cultivated. Future increases in production must come from greater output on existing land rather than bringing new areas under cultivation. In many parts of the country soil is being eroded faster than it can be replaced, undermining long term fertility (DEAT, 1997). Below are the definitions of terms used in the above
linked table of the underlying geology of the Greater Johannesburg area: SOIL TYPES, PROPERTIES AND AGRICULTURAL POTENTIAL Soils can be classified according to the grade sizes of sediment particles that they contain (Strahler & Strahler, 1992):
The depth (thickness) of the soil is a fundamental criterion in the evaluation of the agricultural potential of the area (Buttrick & Stapelberg, 1992).
Texture of the soil refers to the relative ratio of the grain size fractions – thus its fineness or coarseness. There are basically sand, silt and clay, according to the main constituent, but also mixtures, called loams. The soil structure refers to the presence of aggregation (lumps or clusters) of soil particles. The soil texture and structure helps determine the soil porosity and agricultural potential. The porosity is a measure of the volume of pores per volume of soil and the average distances between those spaces. A soil with more pores can hold more water and air, which also increases the agricultural potential.
The average size of the spaces or pores in the soil determines soil permeability, the rate at which water and air moves from upper to lower soil layers (Miller, 1993). In the study done by the Council for Geoscience (Buttrick & Stapelberg, 1992), the soil depth (thickness), texture and drainage (effective passage of water through the soil) was assessed to determine the agricultural potential of the soil. If a soil is classified as falling in the moderate class with respect to its depth and texture, but is characterised by a layer of soil that inhibits drainage, the profile as a whole will classify as having a poor agricultural potential. If a soil would have a high potential in terms of depth, but moderate potential in terms of texture, the soil as a whole would have a moderate agricultural potential. The lowest category thus always determines the classification of the soil (Buttrick & Stapelberg, 1994). Lastly the consistence of the soil refers to the quality of stickiness of wet soil, the plasticity of moist soil and the degree of coherence or hardness of the soil when it holds small amounts of water or is in a dry state. CONSTRUCTION MATERIAL RESOURCES Various quarries are being exploited for construction materials, and were assessed on the basis of various criteria:
Building material is classified on the basis of various properties such as mode of deposition, grading and source material. Natural sand is a fine aggregate (0,5-2,0 mm) resulting from the natural weathering of rock. The mechanical crushing of rock creates manufactured sand, while stone aggregate is rock that is crushed to specified sizes after quarrying or mining. LANDFORMS
(Buttrick & Stapelberg, 1994) SLOPE CATEGORIES Slope categories are commonly seen as: DOLOMITIC LAND Dolomitic land refers to land underlain directly or at shallow depth (less than 100m) by dolomite rocks. Dolomite is a sedimentary rock (formed by precipitation of shallow water in lakes and oceans) having the composition calcium magnesium carbonate. Carbonate minerals are highly susceptible to chemical weathering. The main problems associated with development on dolomitic land, are the generation of ground movement events, such as sinkholes and dolines and also groundwater contamination. GEOTECHNICAL PROPERTIES Various geotechnical land classes were identified during the geotechnical study of the area (Buttrick & Stapelberg, 1994).
AGRICULTURE The city relies on the input of food from the agricultural regions in the outskirts. With urbanisation and population growth, more food is required from smaller agricultural areas. This encourages over-cultivation, with large amounts of pesticides, water and fertilizers being used to increase crop yield. This in turn means that the soil quickly becomes nutrient deficient and overexploited land erodes easier, and the land will eventually be unable to support life. Lots of nutrients are also washed into rivers and cause problems such as eutrophication (excessive growth of algae in a stream or lake, as a result of the input of large amounts of nutrient ions, especially nitrate and phosphate). MINING The prevailing wind direction on the Johannesburg mining reef is north, northwest. This will determine the slopes most likely to be eroded by wind, as well as the areas of dust pollution. Other mining pressures include ungrassed old dumps and slimes dams, as well as incorrect mining and rehabilitation methods, which increase erosion. INFORMAL SETTLEMENTS AND POVERTY Pressures due to poverty and informal settlements include a lack of services and housing space, but is detailed in the poverty and land pollution sections. URBANISATION, POPULATION GROWTH AND DEVELOPMENT Pressures include higher densities of housing, increased removal of vegetation and disturbance of steep slopes, destabilisation of slopes due to roads, an increase in impermeable surfaces and an increase in sewerage and waste. REFERENCES Buttrick, D. & F. Stapelberg 1994: A report to the Central Witwatersrand Regional Services Council on an Engineering Geological Study of the Central Witwatersrand Regional Services Council area. The Council for Geoscience. Department of Environmental Affairs and Tourism 1997: White Paper on Environmental Management Policy. DEAT: Pretoria. Miller, G.T. 1993: Environmental Science – Sustaining the Earth. (4th ed) Wadsworth: California. Strahler, A.H. & A.N. Strahler 1992: Modern Physical Geography. (4th ed.) John Wiley: New York. |
