agricultural production guidelines
m
Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 7.1 1999
SOIL EROSION IN VELD
P J K Zacharias
Department of Grassland Science, University of Natal
What is Soil Erosion?
The Erosion Process
The Solution
INTRODUCTION
The existence of mankind is dependent on a number of components. Of these, the soil plays a vital role in the maintenance of agricultural systems, as every form of agricultural endeavour requires an adequate soil mantle. Unfortunately, it is generally believed that only those farmers who plough the soil need concern themselves with soil conservation. This is not so, because the weathering of rock is a natural process, and will take place as part of the geological cycle. Similarly, relatively low soil losses are a natural part of this cycle, and it is therefore the task of every landowner to ensure that the erosion on his property is ‘natural’ and not ‘accelerated’. The aim of this Production Guideline is to consider the process of erosion and to suggest management strategies which will permit the control of erosion. This is vital to the long-term productivity of agriculture.
Erosion, when used in the popular sense, implies a nett soil loss, and is considered to be a bad thing. It is ironic that the process that gives us soil is also the one that removes it so dramatically. Because of this we need to make a clear distinction between ‘natural’ and ‘accelerated’ erosion.
The development of soil is part of the geological cycle, and results from the weathering of parent material. The nature of the parent material, therefore, has a major influence on the eventual character of the soil. Soil, of course, is not just crushed rock but a complex mixture of mineral rock particles, organic matter and micro-organisms. In order for soil to be formed from the weathered rock, these other components need to be mixed with it. In most cases this is achieved by the erosion of the parent material (chemical or physical), the transport of this, together with organic matter, by wind, water and gravity, and the eventual deposition of this mixture. The process is called soil genesis and results in the production of 200 to 3 700 kg of soil per hectare per year. The soil mantle is therefore added to at a rate of only 0.02 mm to 0.37 mm per year. This is an extremely slow process, and particularly so when compared with the rates of soil loss which have been recorded.Natural erosion
Accelerated erosion
The component of the soil erosion process that is of most concern to land users is accelerated erosion. It is true to say that, on average, erosion over the entire landscape must be less than genesis. If this were not the case, then eventually there would be no soil. Unfortunately, however, soil loss has become a major problem in Southern Africa.In KwaZulu-Natal, soil erosion has become severe, especially in certain catchments. Over ten years ago, it was estimated that 50 to 90% of the landscape of Bioresource Groups 13, 14, 16, 18, 21, 22 and 23 were severely eroded. KwaZulu-Natal now contributes a high proportion to South Africa's total annual soil loss of 300 million tons. This is an enormous amount of soil and represents enough soil to fill a line of 7 ton trucks seven and half times around the world! It has been estimated that the essential plant nutrients (nitrogen, phosphorus and potassium) lost each year as a result of soil erosion, are worth more than R1-billion.
One may ask the question, why is KwaZulu-Natal's soil loss so high? There are many reasons, but the combination of topography, soil types and the nature of our climate (with intense summer storms) largely account for this. KwaZulu-Natal produces about 25% of South Africa's runoff. This reflects the higher rainfall of the region but also indicates the poor state of its vegetal cover. A 25 mm/hour storm releases the same amount of energy that is required to plough to a depth of 250 mm. Such storms are not uncommon in parts of KwaZulu-Natal, and the following data illustrate the extent of KwaZulu-Natal's soil loss:
- the Mkomasi river has an average sediment load between November to March that is represented by one 5 ton truck of soil lost every 9 seconds;
- the Tugela river loses 7 million tons of soil each year; and
- the Mfolozi river discharges enough soil each year to topdress 600 rugby fields to a depth of 1 m.
There are no major rivers in KwaZulu-Natal that do not show major sediment deposits at their mouths. Casual observation of any river in KwaZulu-Natal during the rainy season shows very clearly that all catchments appear to have accelerated erosion. Together with urbanisation, it has been predicted that between 1975 and the year 2000 the world will lose 50% of its per capita cropping land. More than half of this will be as a direct result of erosion. Southern Africa currently accounts for more than 10% of the world's soil loss.
Soil erosion not only represents a physical loss of a stock-limited resource, but also causes major pollution of water, as water quality and soil loss are inextricably linked. The hydrological process is dependent on the soil mantle and its associated vegetation as a regulatory mechanism. Apart from this, sediment is a pollutant which reduces the longevity of dams, and increases the costs of water treatment. The potential use of dammed water in hydro-electric schemes is also reduced as a result of sediment loads. Flash-flooding increases, and breeding grounds in estuarine systems are destroyed by sedimentation.
Soil erosion is not a localised process, and consequently does not respect regional or farm boundaries. A poor management practice in one part of the catchment, or on one farm, will affect the lower catchment or one's neighbours. The soil erosion process is driven by energy and in simple terms, the greater the energy in the system the greater the potential for soil erosion.
The factors affecting soil erosion can best be summarised by use of the Universal Soil Loss Equation (USLE). The components of this equation show that soil erosion is a function of:
- rainfall erosivity;
- soil erodability;
- land slope;
- length of land slope;
- vegetation cover (canopy);
- management.
Most of these factors are inherent characters of the system, and are not easily controlled or changed by land managers. The two components that can be controlled by the land user, are management and plant cover. For a given situation, rainfall erosivity is the main factor that determines the potential for erosion because it supplies the energy required. The effectiveness of that energy is controlled by vegetation cover. This, and the level of management, are the only factors under the direct control of the manager.
Soil reclamation work has been conducted in South Africa for over 40 years. In KwaZulu-Natal, the most ‘successful’ effort has been on an expropriated labour farm which is now the Weenen Nature Reserve. Over the duration of that project, a great deal of money was used for the erection of physical structures. Today such structures are not economically justifiable because the production potential of the reclaimed land is very low. Indeed, at Weenen, the area was withdrawn from production entirely. With the increasing population pressures today, such total withdrawal is no longer feasible. In any event, the causes of the erosion are not in the dongas but in the catchment. Revegetation of the donga, or silting up of the donga does not reduce the erosion. Of course the silt that is used to fill the donga comes from the still-eroding catchment. Mechanical control of erosion therefore has limited application today. In a cropping situation, the correct establishment of a water management system is essential, but this is beyond the scope of this document. In order to control erosion, it is essential to treat the causes, and not merely the symptoms. An algorithm designed to assist in this process is presented in Figure 1.
Figure 1. The basic principles of remedying soil erosion.
One of the main causes of erosion is excess, or uncontrolled, runoff. The main responsibility of farmers and other land users, therefore, is to manage runoff, or ensure that this is kept at acceptable levels. This can only be done in the catchments which yield the runoff. For the crop farmer, this means that the appropriate crops must be planted, and that contour banks and waterways are adequately designed for their situation. For the grassland farmer, this means ensuring that veld and pasture management are such that the vegetal cover is not destroyed, in order that the vegetation can moderate the energy and erosive effects of the rainfall (Figure 2). For the grazier or plantation manager, attention must be given to the conservation of stream banks and vleis (wetlands). The vegetation of these areas plays an important role in the regulation of the discharge of water from their catchments. Injudicious burning and grazing of vleis, together with disturbance of the natural stream-bank vegetation, contribute to the accelerated erosion of soil in South Africa. Attention to these sensitive areas requires high priority.
Soil erosion continues to be a major problem in southern Africa. Because the effects of localised erosion influence the entire catchment, it is the responsibility of every land manager to control erosion to acceptable levels. If this is not done, and erosion continues at its current levels, important catchments will be irretrievably lost, and the productivity of southern Africa will be drastically reduced, because of lost arable land and reduced water production for industry.
Figure 2. A procedure for remedying common forms of soil erosion on farmland.