In Africa, soil degradation accelerated after colonisation by European nations, but was exacerbated by several factors, including the mechanisation and commercialisation of agriculture, social systems such as overgrazing by cattle and the use of fertile land for non-agricultural purposes. The consequences are appalling: in South Africa, for instance, an estimated ten percent of the 1,2 million hectares under irrigation has already been lost as a result of salinisation alone.
Bush encroachment in South Africa, as elsewhere, is a good indicator of soil degradation. Research has shown that the rate of degradation has been less severe in developing areas than in white-owned commercial areas. In these rural areas the veld was regularly burned and stocked with a high population of goats, which kept the bush in check.
Commercialisation replaced wild ungulates with cattle and sheep. Their preference for grazing gave the bush a free hand and it encroached on grassland. Sheep in particular are responsible as they are short grazers, meaning that they crop the grass down very close to the ground.
After 1950 scientific agricultural planning caused bush encroachment in the rural areas. This was accomplished through injudicious cultivation of non-arable land, which later had to be abandoned, with resultant bush encroachment. Interestingly, however, it has subsequently been discovered that bush encroachment has improved the fertility status of some soils to near-pristine levels.
It is generally agreed that soil degradation has accelerated on a global scale since World War II, even though there are no acceptable hard facts available to quantify it. Even so, these guesstimates are telling: The UN's FAO (Food and Agriculture Organisation) estimates that between five and seven million hectares go out of production each year in Sub-Saharan Africa as a result of human-induced soil degradation.
The severity of this short summation of human-induced soil degradation is self-explanatory and obviously unsustainable and unacceptable. The large discrepancy between Burundi and the other countries is a result of the fact that this country's maize is produced on very steep slopes of 40 percent gradient, giving a clear indication of the effect slope has on accelerating the rate of topsoil loss from an area. The situation in South Africa is no different, with erosion rates twenty times as high as the world average. Estimates are that South Africa is losing 300 Mt or 2,5 t/ha/annum.
Consequences One of the first consequences of degraded soil is that the yield potential/carrying capacity of the soil is lowered, with a concomitant lowering in its capacity to produce aboveground biomass. Loss of aboveground biomass can have effects such as reduced harvests, diminished supplies of fuel wood and a reduced capacity to support stock.
Research indicates that a loss of 20-40 tons of topsoil per hectare can half crop yields on certain Alfisols that are a common soil type in much of Sub-Saharan Africa.
Consider, then, the extent of lost production against the loss of topsoil summarised in Table 1. This highlights the fact that yield losses are not only a symptom of nutrient loss in the soil. Nutrient losses, however, are astronomical.
In an FAO study conducted in Zimbabwe it was found that this country could be losing 1,6 million tons of nitrogen and 0,24 million tons of phosphorus per year through erosion. Replacement of nitrogen and phosphorus to these soils translates into US$1,5-billion at the 1985 price!
An inexpensive solution to this form of degradation is the application of agricultural fertilisers. This, however, should be weighed against the fact that these resources, too, are finite and that their incorrect application has caused, and is causing, environmental pollution on a global scale through the eutrification of fresh and saltwater resources.
Exposure of the surface soil to the elements has as its major effect a reduction in the capacity of the soil to store water. Four effects predominate with the removal of the vegetative cover:
Ø Exposure of the soil to the beating action of rain, causing the physical dispersion of the surface soil layer, resulting in crusting and splash erosion.
Ø Organic matter in the surface layer of topsoil is reduced to very low levels. This reduces cementing and stability of the soil structure. Reduced soil organic matter leaves the soil more prone to crusting and erosion.
Ø Crusting causes poor water infiltration, with the effect of reduced replenishment to groundwater and increased run-off. Crusting reduces the germination of seedlings and thus revegetation of bare patches.
Ø Degradation of vegetation on steep, rocky slopes leads to greatly increased run-off, flash floods and accelerated erosion on lower slopes.
Quite apparent from this is the fact that each of the above-mentioned results is mutually reinforcing in that the presence of one is an aggravating factor for the other.
Water erosion has the following effects on hydrology:
Ø Water erosion reduces the amount of soil that remains in situ and as such reduces its water storage capacity. Reduced storage of water within the soil profile dramatically reduces plants' capability to survive spells of drought between rain events.
Ø Erosion gullies act as canals that are efficient at draining water away from a catchment area, achieving the same end of reduced water storage. Watersheds are like large sponges that slowly release water and as such regulate stream flow. With gully erosion comes a change in the dynamics of the hydrology of a catchment area: once steadily-flowing perennial rivers become characterised by frequent periodic flooding and/or periodic abnormally low flow. In its worst form, once perennial rivers become seasonal.
Ø With the increased velocity of water flowing out of a watershed comes an increase in the sediment load carried by the river, which results in silting up of dams - to the detriment of all those dependent on water. The Welbedacht Dam is a good example. This dam was conducted in 1973 to serve as the main water reservoir for Bloemfontein. By 1993, however, it had silted up to such an extent that its storage capacity had shrunk from 114 million cubic metres to seventeen million cubic metres.
Alteration in the hydrology of an entire watershed such as this one has the effect of making the area prone to the effects of drought.
The building of the Aswan Dam in the Nile River valley is a particularly good example of chemical degradation that can be expected when a river's flow is interrupted by the construction of a dam such as this. The Nile valley is the only significant area of deep, fertile, level soil available in Africa. The construction of the Aswan Dam has drastically changed the hydrology of this river system, resulting in increased salinity and barren soil.
Here the changes in hydrology have reduced flooding that has interrupted the deposition of fertile soil and the flushing of salts from the system. The bulk of salts flushed from the Sudanese irrigation schemes now end up in the Aswan Dam, accelerating salinisation. The Nile delta is severely degraded, while the area below the dam is moderately so. Above the dam, in the Sudanese irrigation schemes such as Gezira, all is well.
In South Africa there are 1,2 million hectares under irrigation and an estimated ten percent of this area has been lost as a result of salinisation.