- CAUSES AND AGENTS OF SOIL EROSION
Erosion is defined as a levelling process with the soil particles being carried, washed or rolled downwards by the force of Gravity. Natural erosion has always occurred, but accelerated erosion takes place when the process is influenced and speeded up by man. If the natural conditions of Climate and Topography in an area are such as to encourage erosion, man will often speed up the process by bad farming practices.
Erodibility is the susceptibility of a soil towards natural erosion affected by both the Soil Texture and the Soil Structure.
Erosivity is the ability of rainfall to cause erosion. It depends on the total rainfall of an area and the force and frequency of storms.
Agents of erosion are the factors which loosen or break down the soil crumbs and particles. The main agents of erosion are wind, water, rain, temperature, biological factors and poor farming practices.
- Suitable land use such as not trying to grow crops in areas where the climate and the soils are unsuitable. Leaving the natural vegetation undisturbed.
- Suitable farming practices such as strip cropping, stubble mulching, and partial cropping to conserve water, minimum tillage using a chisel plough instead of a disc or mould board plough, maintaining good grazing systems and not overstocking.
- By planting trees and bushes to act as windbreaks to reduce the force of winds. These are effective over a distance equal to 25 times their height. Young trees which are 6m high will cut down the force of wind for 150m from where they are growing. The disadvantages of windbreaks are that they take water and nutrients away from the crop and also shade the crop. They hamper aerial spraying and in low rainfall areas the crop may not grow very well.
Figure 1: the effect of a windbreak on crop protection
This is the main agent of erosion in Central and Southern Africa. Rain effects soils in two ways, firstly by the Raindrop action and secondly by the Runoff action. Energy is used in all types of erosion, in breaking down the soil particles and crumbs, in causing turbulence in surface run‐off, in the scouring action and in carrying away particles and stones.
Because of the speed and force with which raindrops strike the soil surface, Raindrop Action has 250 times more energy than the surface runoff action. The erosive power of the raindrop is in the energy which is dissipated when the falling drop hits the soil. This energy helps to destroy the soil structure breaks up the soil crumbs and causes “Capping” on the soil surface.
In practice, the effects of raindrop action which are important are:
- The flattening effect on the soil.
- The movement of soil downhill.
- The breakdown of the soil structure which, in turn, can reduce the nutrient holding capacity of the soil by as much as one half.
- The reduction in the infiltration rate of the rain water. Fast falling raindrops tend to bounce off the soil surface rather than penetrate it.
The chief factor affecting the erosive power of raindrops is the intensity of the rainfall. As intensity increases, so the actual size of the rain drop increases, causing more energy to be released when the drop hits the soil. Rainfall of an intensity of less than 25 mm per hour is virtually non‐erosive. The average rainfall intensity is greatest in the tropics and sub‐tropics but varies greatly with the locality, season, and time of year. High intensity storms cause more damage early in the season, when there is little cover on the soil. In the areas around London, England, 2% of the rainfall is erosive. This figure is 5% for most temperate areas. Around Pretoria 40% of the rainfall is erosive. It is therefore obvious why, in this part of the world, mechanical protection works are essential on arable land, and in other countries they may not be required.
Erosion due to raindrops splash can be prevented or reduced by providing a ground cover such as a growing crop or mulch.
The principal effect of raindrop splash is to loosen and detach the soil, while the principal effect of run‐off action is to wash the soil away. The most important part of the erosion process is the detaching action of the raindrops. Without this, water running over the surface has a very minor effect on erosion because there are no loose particles to transport. Some detachment of particles can occur due to run‐off alone, depending on the speed and volume of the running water.
The cracking and flaking of rocks and soil crumbs due to temperature changes has a slow effect on soil erosion. However, where ice is involved, this action can be very much faster.
Biological factors which cause erosion are mainly the action of plant roots or lichens which crack and flake rocks and stones. This is a very slow process.
POOR FARMING PRACTICES
This is one of the most significant factors in causing soil erosion. The simplest, most effective and cheapest weapon in the fight against erosion is a good crop on arable land and good grazing management on veld. Leaving bare soil exposed to the effects of heavy rain is the worst crime that a farmer can commit. Animals treading on rock or wearing a path through grazing land can greatly increase the rate of erosion.
- TYPES OF SOIL EROSION
The following types of erosion are caused by surface run‐off:
- Sheet Erosion: Here the velocity (speed) of the run‐off water is too low to detach particles from the soil, although particles which have been detached by raindrop splash are carried away. Thus humus and soil nutrients are lost.
- Rill Erosion: Uneven ground causes the collecting of water from sheet flow into small channels or rills. The increase in the speed of the water due to the deepening of the rills increases the extent of the erosion.
- Gulley Erosion: This is the result of unchecked rills erosion, where the rills have widened and deepened and the flow of water has sped up. With gulley erosion, massive detachment and transportation of all particle sizes can take place.
- Streams: Gulleys lead eventually to streams, where the erosion effect is extremely great, particularly at bends and on steep gradients. The speed of flow detaches and transports stones and soil with tremendous force. The best prevention of stream erosion is to allow the natural vegetation to grow and avoid cultivating near the stream bank.
On arable land, sheet erosion is the commonest, with the main damage being done by rain drop action. Farmers usually prevent rills from forming on arable land. On the veld, sheet erosion can lead to the formation of rills and gulleys particularly where there are cattle tracks.
Figure 2: The following diagram shows the relationship between the amount of erosion occurring
and the mean annual rainfall.
The higher rainfall areas have enough vegetative cover to protect the soil from erosion. In low rainfall areas, although the ground is not as well covered, there is generally less total run‐off because more of the rain is absorbed into the soil. In areas with a long, dry season and fairly high rainfall is one of the areas most susceptible to erosion.
SPECIALISED TYPES OF EROSION
- Pedestal Erosion: This takes place where an easily eroded soil is protected by a root or stone. The surrounding soil is washed away, leaving a column or pedestal of uneroded soil.
- Pinnacle Erosion: Usually associated with highly erodible soils. Gulley’s show deep, vertical rills in their sides and these cut back until they join and leave pinnacles. Soils subject to this type of erosion are recognized by slow water intake when dry and no cohesion when wet.
- Piping: This is the forming of underground channels often in soils subject to pinnacle erosion. Water infiltrates down to an impermeable layer, runs along the layer until it finds an outlet, usually into a gulley.
- Slumping: Occurs mainly on river banks, the sides of gulley’s or coastal collapse.
- Puddle Erosion: This is a type of soil degradation with no actual loss of soil taking place. This type of erosion results in the loss of soil structure and the washing into depressions of the finer particles. Therefore results in a structure loss of soil and greatly reduced fertility.
MEASUREMENT OF EROSION
Many formulae have been developed to determine the soil loss occurring in a particular area. One example is the ‘Universal Soil Loss Equation’:
- A = R x K x LS x P x C, where
- A = the soil loss
- R = rainfall erosivity
- K = soil erodibility factor
- L = length of lands
- S = slope of the lands
- P = conservation practice factor
- C = crop management factor
This formula looks, and indeed is rather complicated, but all you need to remember is that this type of formula is used mainly to compare the erosion taking, place in different areas. On this basis, erosion is a far more serious problem in Africa than in North America, and it is as well to remember that in America by 1934, of the total of 160 million hectares of farmland,
- 20 million were completely ruined
- 20 million were almost ruined
- 40 million had lost half their top soil
- 40 million had lost from a quarter to half their top soil