Musokotwane Environment Resource Centre for Southern Africa
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Soil erosionis one of southern Africa's major environmental problems. It ruins agricultural land and reduces crop yields.
More than a thousand million tonnes of southern Africa's soil are eroded every year. Experts predict that crop yields will be halved within thirty to fifty years if erosion continues at present rates.
Though the exact extent of erosion and land degradation is not known, patches of land throughout southern Africa have been abandoned because they no longer support crops or grazing. Usually older people in these areas can remember when these were fertile lands.
SOIL EROSION MEANS SOIL LOSS
Soil erosion is measured as tonnes of soil lost from a hectare in a year. An area losing 50 tonnes of soil from each hectare is shown as 50 tonnes/ha/yr.
There is no single average figure for soil erosion in southern Africa. Soil loss rates vary from place to place, from less than 1 tonne/ha/yr to over 200 tonnes/ha/yr.
Ten tonnes of soil is the same as 1 mm of soil spread over a hectare. This amount of soil fills more than a hundred wheelbarrows. An area losing 100 tonnes/ha/yr is losing a thousand wheelbarrow-loads of soil.
Compared to Asia and South America, southern Africa's rates of soil erosion are not very high. However, the amount of soil lost through erosion is not a good measure of the land's ability to produce crops. Losing 10 tonnes/ha from poor thin soil is much more of a problem for farmers than losing 50 tonnes/ha from deep fertile soils.
RAINDROPS ARE THE MAIN CULPRIT
Raindrops come in a variety of sizes. In southern Africa the average drop size is such that when they hit the ground they are travelling at about 20 km/hr.
Obviously, one raindrop does very little damage. Damage begins during a typical heavy storm when there are millions of raindrops bombarding each square metre of soil.
Rain causes erosion when it falls at more then 25 mm per hour. Rainfall intensities in Europe and North America seldom exceed this rate. Compare this with southern Africa where rainfalls of 30 or even 40 mm per hour are common, and bigger storms a fact of life.
One mm of rainfall per hectare weighs 10 tonnes. Imagine the power of an average rainfall of 40 mm (400 tonnes) striking bare soil at 20 km/hr.
Raindrops dislodge individual particles of soil. These loose soil particles are easily washed away over the surface of the ground.
After heavy rains, the fine mud left on the ground dries out. As it dries the mud forms a hard crust that seals the soil surface, making it difficult for rainwater to penetrate. Once a crust has formed, the rain, instead of seeping into the soil, washes over it.
Rainfall that is not absorbed by soil is called 'runoff' because it runs off over the soil surface. It is runoff that carries soil particles away.
HILLS AND VALLEYS
Erosion is worse on slopes and hillsides. Even small slopes make a difference.
Think of runoff as a round stone. A stone won't roll by itself on flat ground. But on a hill, the steeper the slope, the faster the stone will roll. Once it is moving fast, it knocks into other objects and causes them to move too.
Runoff washing downhill picks up a load of soil particles, sand and small stones. This gives the water added erosive power, like grit on sandpaper, tearing up more soil particles from the ground and scouring out small channels that turn into gullies.
Wind erosion is mainly a problem in dry flat areas where winds blow strongly such as Botswana and Namibia and parts of South Africa, Zimbabwe and Tanzania.
Erosion by raindrops makes soil more susceptible to erosion by wind. Fine soil particles dislodged by rain are easily picked up by wind.
Dust storms pick up the finest and most fertile parts of the soil. Dust from Africa blows many thousands of kilometres. It has been found as far away as South America and is said to contribute to the fertility of soils in the Amazon basin.
THE KIND OF SOIL MAKES A DIFFERENCE
Soils that contain both clay and organic matter (the remains of decayed dead leaves, roots, insects and so on) are more resistant to erosion. Organic matter and clay are the glue that stick soil particles together in clumps which do not easily wash or blow away.
Soils with plenty of organic matter behave like a sponge absorbing rainfall.
Soils in places with cooler climates and long rainy seasons tend to build up organic matter. Because of our long dry season most southern African soils contain low amounts of clay and organic matter.
Ploughing, a standard agricultural practice, can increase erosion in several ways. This is especially true if ploughing is done every year.
Ploughing damages soil structure by crushing the soil, producing large amounts of
powdery material. The dust mixes with rainwater and forms mud. The mud seals the soil
surface so that rainwater can't soak in. This leads to increased runoff and soil loss.
As organic matter is used up, soil particles and nutrients have nothing to cling to and are easily washed away by rain. The soil loses its fertility and its ability to withstand erosion.
Most soils in southern Africa that have been cultivated continuously for ten or more years are in poor condition, degraded and erodible.
GRASS, TREES AND SHRUBS
Very high erosion rates are always associated with bare soil. A study in Shinyanga in Tanzania compared erosion in nearby areas and found that vegetation made all the difference. Areas with plenty of grass and trees had rates of erosion around 5 tonnes/ha/yr. Areas with poor vegetation cover had erosion exceeding 100 tonnes/ha/yr.
Plants protect soil from erosion in several ways.
* Trees, shrubs and grasses break the force of raindrops. Raindrops hit the plants first, breaking into smaller droplets that fall more gently to the soil surface.
* Plant roots physically hold the soil in place.
* Plants and their roots provide an environment for earthworms, ants and termites, and moles. These creatures construct underground tunnels which help rainfall soak into the ground and reduce runoff.
* Plants that have many stems growing close together (like grass) help the soil absorb
water by providing thousands of little holes through the soil surface.
* Dead plants lying on the surface protect the soil from the impact of raindrops and shade it from the sun. In time the dead vegetation rots and becomes part of the soil, adding organic matter which helps it absorb water.
Burning grass or crop residues affects soil as well as plants. By removing the vegetation, fires take away organic matter which would have improved the soil, and remove the soil's protection from rain and sun.
Burning also tends to seal the soil surface. This increases resistance to water penetration which means there is less rainwater seeping into the soil and more runoff to cause erosion.
RUNOFF DAMAGES LAND
Increased runoff reduces the productivity of agricultural land in three ways:
When rainfall can't penetrate the soil, less rainwater is absorbed and available for use by plants. In some areas the amount of water that soil is able to absorb has dropped by a third. This has the same effect on crops that a 33 percent drop in rainfall would have.
Nutrient losses are the hidden cost of soil erosion. You don't see nutrients disappearing, but this is the most serious impact of erosion.
Erosion selectively removes nutrients such as nitrogen and phosphorus by removing the organic matter and clay particles. In one study, scientists compared the amount of nutrients in soil with the amount of nutrients in sediment eroded from that soil. They found the sediment contained more than twice the concentration of nutrients.
Erosion removes more than naturally occurring soil nutrients. It also removes fertilisers. A Zimbabwean study of soil nutrients lost through erosion found that an erosion rate of 30 tonnes/ha/yr, typical of many commercial and subsistence farms, removed half the applied fertiliser.
In South Africa, if plant nutrients such as nitrogen and phosphorus which are washed away by erosion were to be replaced by commercial fertilizer the cost would exceed R 1000 million each year.
Sheet erosion (rainfall washing over the land) is the most serious type of erosion. It makes topsoil shallower. In Zimbabwe, sheet erosion is estimated to remove 50 tonnes/ha/yr from 'average communal land.'
Sheet erosion is the starting point for gully erosion. Rainfall runoff starts out as a thin film of water on the ground, but as it flows downhill the runoff tends to concentrate and gouge out its own drainage channel. The steeper the land and the longer the slope, the greater the damage.
Gullies are not a cause of erosion. They are a result of sheet erosion, a consequence of removing vegetation on a hillside through cultivation, burning or grazing. Natural drainage channels can't handle the extra runoff caused by the reduction in vegetation. Runoff pours into drainage channels like a waterfall. The channels are undercut and 'scoured', dug deeper and wider, by the stormwater.
Gullies expand in years of above average rainfall, especially after a period of drought when the vegetation cover has died.
Sometimes cattlepaths or roadways provide starting points for gullies. Roadways that cross a slope can act as a collection ditch for rain. When stormwater overflows and escapes, it pours down the hill gouging out a gully. An undrained roadway running down a hill can also start a gully by providing a route for stormwater to pour downhill.
Though gullies look terrible, sheet erosion actually causes much more soil damage in southern Africa than gullies do.
A survey of a section of Luangwa Valley on Zambia's eastern border found that where severe gullying occurred, the amount of soil removed by sheet wash was more than double that of gullying. On average 510 tonnes/ha/yr were removed by sheet wash, compared to 127 tonnes/ha/yr removed by gullies.
Soil takes a very long time to renew itself. It is a slow process involving the breakdown of rock into small particles and the accumulation of organic matter.
The rate of soil formation varies from place to place depending on rainfall, vegetation cover and other factors. In most cases the amount of soil formed every year is less than 1 tonne/ha/yr. At this rate, it takes between 100 and 1000 years to build 1 cm of topsoil.
In undisturbed nature, there is almost no loss of soil through erosion, and the rate of erosion is balanced by the rate of soil formation.
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