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UNESCO - EOLSS SAMPLE CHAPTER LAND USE, LAND COVER AND SOIL SCIENCES – Soil Conservation - David Sanders SOIL CONSERVATION David Sanders World Association of Soil and Water Conservation, Bristol, England Keywords: soil conservation, soil erosion, wind erosion, water erosion, erosion control. Contents 1. Introduction 2. The Past Problems of Land Degradation 3. Modern Soil Conservation 4. Erosion Processes and Soil-Conservation Technology 5. Soil-Conservation Policies and Approaches 6. Changes in Approaches and Policies 7. Soil-Conservation Research 8. Future Trends in Soil Conservation Glossary Bibliography Biographical Sketch To cite this chapter Summary Soil conservation is about solving the problems of land degradation, particularly soil erosion. Land degradation has been a problem ever since humans settled the land and started to cultivate the soil and grazed domesticated animals. At times, land degradation has become so severe that it has contributed to the decline of civilizations. Over the years, farmers devised ingenious practices and systems of land use to protect and rehabilitate their lands but most have been abandoned as population growth has placed greater pressure on the land. Modern soil conservation is largely based on research work started in the Unites States early in the twentieth century. Soil erosion takes place when particles are detached and then transported by wind or water. Until recent years, soil erosion was seen as a physical problem and treated mainly with engineering works. Most of these were based on the use of the “contour principle”—the construction of barriers built on or near the contour. Alternatively vegetation or “biological” measures can be used. Basically, this is done by ensuring that there is sufficient vegetation, either living or dead, left on the surface to protect the soil from the effects of wind and water. Recently, it has become appreciated that erosion is only the symptom of a deeper problem—incorrect land use and bad management. The emphasis has therefore moved from engineering solutions to biological measures and better land management. It has also been realized that conservation projects can only succeed if the land users are fully ©Encyclopedia of Life Support Systems (EOLSS)
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SOIL CONSERVATION David Sanders World Association of Soil and Water Conservation, Bristol, England Keywords: soil conservation, soil erosion, wind erosion, water erosion, erosion control. Contents 1. Introduction 2. The Past Problems of Land Degradation 3. Modern Soil Conservation 4. Erosion Processes and Soil-Conservation Technology 5. Soil-Conservation Policies and Approaches 6. Changes in Approaches and Policies 7. Soil-Conservation Research 8. Future Trends in Soil Conservation Glossary Bibliography Biographical Sketch To cite this chapter Summary Soil conservation is about solving the problems of land degradation, particularly soil erosion. Land degradation has been a problem ever since humans settled the land and started to cultivate the soil and grazed domesticated animals. At times, land degradation has become so severe that it has contributed to the decline of civilizations. Over the years, farmers devised ingenious practices and systems of land use to protect and rehabilitate their lands but most have been abandoned as population growth has placed greater pressure on the land. Modern soil conservation is largely based on research work started in the Unites States early in the twentieth century. Soil erosion takes place when particles are detached and then transported by wind or water. Until recent years, soil erosion was seen as a physical problem and treated mainly with engineering works. Most of these were based on the use of the “contour principle”—the construction of barriers built on or near the contour. Alternatively vegetation or “biological” measures can be used. Basically, this is done by ensuring that there is sufficient vegetation, either living or dead, left on the surface to protect the soil from the effects of wind and water. Recently, it has become appreciated that erosion is only the symptom of a deeper problem—incorrect land use and bad management. The emphasis has therefore moved from engineering solutions to biological measures and better land management. It has also been realized that conservation projects can only succeed if the land users are fully

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involved in the planning and implementation of schemes. Meanwhile, the emphasis in research has moved from studies that just looked at how much soil was being lost to investigations into the effects of soil loss on soil productivity. 1. Introduction Under natural conditions, different environments evolve which vary according to the available variety of plants and animals and the existing soil, climate and topographic conditions. These environments exist in a state of dynamic equilibrium, a state where they are usually able to adjust and recover after some unusual event such as the outbreak of a disease, a fire, or a drought. Wherever humans have settled and cultivated and grazed the land with their domesticated animals, these conditions have been upset and changed. In some places, and over time, new forms of land use have evolved which are in harmony with the soil, climate, and topography and a new, productive, and stable environment has been established. Unfortunately, this has not always been the case and over large areas of the world’s surface land use systems are to be found which are not in balance. Where this is the case, various forms of land degradation are taking place.

Figure 1: Badly eroded hills in northeastern Turkey, cleared of their natural forests and heavily overgrazed

Soil conservation is about solving the problems of land degradation, particularly accelerated soil erosion. Accelerated soil erosion is a result of the operation of the physical forces of wind and water on soil, which has become vulnerable, usually because of human interference with the natural environment. For this reason, soil erosion can be viewed as a symptom of bad land use and management.

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Soil conservation is fundamentally a matter of determining a correct form of land use and management. A correct form of land use and management is one that provides a higher level, or a different form of productivity from that available in the natural state. This new form of productivity must, however, be one that must be capable of being sustained indefinitely. Soil conservation can be defined as the combination of the appropriate land use and management practices that promotes the productive and sustainable use of soils and, in the process, minimizes soil erosion and other forms of land degradation. 2. The Past Problems of Land Degradation The problem of land degradation is not new. In fact, soil erosion, soil salinity, and related forms of land degradation have been with us from the time when humans first domesticated animals, settled, and started to raise crops, at least 7000 years ago. At times, the problem of land degradation has become so severe that it has contributed to, if not caused, the decline of great civilizations in such places as China, Mesopotamia, Egypt, North Africa, and Greece. Confronted with the problem of land degradation, over the centuries farmers have developed ingenious strategies and systems of land use and management to protect and rehabilitate their lands. Many of these have been very effective and the remains of some of them can still be seen in old terracing systems in several countries, including Yemen, China, and Peru, as well as in farming systems such as shifting cultivation which is still practiced in parts of the tropics.

Figure 2: Beautifully built terraces in Yemen carefully maintained over the centuries

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Great interest has been shown in these traditional soil-conservation systems in recent years and a number of studies have been conducted to find out more about them and if they can be adapted to present day conditions. One of the more thorough of these studies was undertaken by the soil scientist Hallsworth. He concluded that modern research shows that the most effective way to control erosion is through the maintenance of soil cover and by reducing the gradient of channels and surfaces over which water flows. His study indicates that these principles have been used by farmers for centuries and have been incorporated into traditional conservation systems such as terracing, mixed cropping, mulching, and shifting cultivation. Modern research, Hallsworth claims, completely confirms the value of traditional conservation practices, some of which have been used for at least 1000 years. But, conditions have changed considerably over the last hundred years. Population numbers have increased substantially and with this increase has come much more pressure on the land to produce more food, fiber, and fuel. At the same time, mechanization has been introduced to large areas of the world’s agricultural land and the economics of farming have changed dramatically. As a result of these changes, many of the effective, traditional, conservation measures have been abandoned or neglected. Nevertheless, many fine examples of traditional conservation works can still be seen such as the carefully terraced vineyards and olive groves in Mediterranean countries and the beautifully terraced rice fields in The Philippines and China.

Figure 3: Steep slopes terraced and irrigated for rice growing over large areas (Java, Indonesia)

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3. Modern Soil Conservation The history of modern soil conservation as we know it today is short. Probably the first scientific investigations of erosion were those carried out by the German soil scientist Wollny between 1877 and 1895. Wollny used small plots to measure the effects of such factors as vegetation and surface mulches on the interception of rainfall and soil structure. He also looked at the effects of slope and soil type on runoff and erosion. However, the lead in erosion research has been provided by the United States of America where farmers were experimenting with mechanical conservation works as far back as the 1850s. 1907 was an important year in the history of soil conservation. In that year, the United States Department of Agriculture declared an official policy of land protection. In the same year, Iceland established what was to become the world’s first soil conservation service. In the United States, the first quantitative experiments were laid down by the Forest Service in 1915 in Utah. These were soon followed by trials in Missouri in 1917, which led in 1923 to the first published results of field plots. Other research work soon followed, stimulated by Congress allocating special funds for the purpose in 1923. In spite of this early research work, it was some years before modern soil-conservation measures were applied in the field on a large scale. Eventually a large program was started in the United States in the 1930s. The fact that this program was started owes much to the work of an outstanding soil conservationist, Hugh Hammond Bennett, a man now regarded by many as the “father” of modern soil conservation. Early field trials and a number of surveys showed that much of the country’s rich agricultural land was being badly degraded by erosion. Armed with this information, Bennett was able to convince Congress of the need to devote considerable funds to soil conservation. But there was one single event that greatly affected government and public opinion. At this time, huge areas of the Great Plains were suffering badly from wind erosion, to the extent that the area became known as the Dust Bowl. On 12 May 1934, a spectacular dust cloud swept across the country from the Great Plains to beyond the Atlantic Coast. It blotted out the sun over a large part of the nation and sifted through the windows of New York skyscrapers, bringing home to everyone the seriousness of the problem. Following this event, not only were funds allocated for soil conservation but also a soil conservation service was established to carry out a program which grew and has continued until today. Meanwhile, soil erosion was becoming recognized as a serious problem in many countries and large-scale soil-conservation programs were launched in Africa and, a few years later, in Australia. Soil-conservation services were subsequently set up in a number of countries in Africa and in Australia, New Zealand, and India. 4. Erosion Processes and Soil-Conservation Technology Soil erosion takes place when particles of soil are detached and then transported to a different place. The agents for this detachment and transportation are wind and water. The study of soil erosion is therefore normally divided into water erosion and wind erosion and dealt with as separate subjects. While this division may be convenient, it must be

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remembered that whether the erosion is by wind or water, the causes are frequently the same, or similar, as are many of the principles of control.

Figure 4: Much of Lesotho’s agricultural land has been lost through gully erosion in spite of the use of contour works.

4.1. Water Erosion Raindrops falling on bare soil break down the structure of the surface soil and detach particles. If the land is sloping and the water cannot be absorbed by the soil, or detained by the microtopography, the water moves off down the slope in the form of runoff, carrying dislodged particles with it. The basic factors affecting water erosion are how prone the soil is to erode (the soil’s erodibility), the intensity of the rainfall (the rainfall’s erosivity), the slope of the land, and the way in which the land is used and managed. 4.1.1. Mechanical or Engineering Soil-Conservation Measures Until relatively recently, soil erosion has been looked upon largely as a physical problem—the effects of the erosive forces of wind and water on exposed soil. Consequently, there has been a trend to treat erosion as an engineering problem. Control measures have largely consisted of what are known as “mechanical” or “engineering” soil-conservation measures—physical measures to prevent and control erosion. In the case of water erosion, mechanical soil-conservation measures include a wide variety of conservation practices and structures but, in essence, most of them aim to either reduce

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the length or the slope of the land. By doing this, the movement of water over the surface is stopped or slowed down to a velocity that will not cause erosion. Water is encouraged to be absorbed by the soil or is led off to an area where it can be safely dispersed. A simple but important principle in the use of mechanical soil-conservation measures is the “contour principle.” This involves the construction of barriers of various types, horizontally across the slope, either on or very close to the true contour of the land. These structures vary from a plow furrow to structures made of earthen banks, grass strips, shrubs and trees, or “trash lines” formed from the residues of past crops to lines of stones, stone walls, or even lines of logs. Probably the simplest form of mechanical erosion control is contour plowing. This consists of cultivating the land on or close to the contour instead of up and down the slope or round and round the field. When this is done, each furrow acts as a small dam, catching water as it runs down the hill and encouraging it to soak into the soil. This simple conservation measure may be enough by itself to prevent the runoff of water and erosion where slopes are gentle and the rainfall intensities are low. Contour banks have been used successfully to control erosion over large areas of cultivated land, particularly in the USA, Australia, and parts of South America. These often involve complex systems with earthen banks carefully built on grades close to the contour, but with a sufficient slope to allow excess water to gently run away. The excess water is usually diverted to artificial waterways down which the water can safely run until it reaches a natural stream or watercourse. These systems have proved to be very effective on cultivated land where the farms and the fields are large. However, it is a system that has failed to be accepted by farmers in Africa and Asia where the fields are generally small and fragmented. Under these conditions, the complex contour systems are too hard to manage as they must cross numerous field boundaries. Also, the banks and waterways take up land and are relatively unproductive—something that is unacceptable to small farmers who are, understandably, unwilling to lose the use of any part of their small plots. Where the land is fragmented and split up into small plots, the contour principle is often applied in other ways. This includes crop residues being stacked in lines along the contour. Every line acts as a barrier to water as it moves down the slope, slowing down its movement, helping it to be absorbed by the soil and catching soil that has been detached and that is moving down the slope. Barriers of living vegetation are also commonly used under a wide variety of conditions. The barriers are usually of grass, which has been planted or left to grow naturally in narrow strips along the contour at intervals across the slope of a field. The grass strips act in the same ways as crop residue barriers, trapping moving soil, slowing down moving water, and encouraging it to sink into the soil. In recent years, a similar practice has been promoted with trees and shrubs rather than grass being grown in contour strips. The idea is that the trees and shrubs will act as barriers in the same way as grass strips, with the additional advantage that the trees and shrubs can also provide a valuable asset to farmers, particularly in developing countries where the timber is badly needed for fuel and building material. In addition, the trees and shrubs used are

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usually legumes, plants that have the ability to “trap” nitrogen from the air, return it to the soil and make it available to other plants. This is very important as a shortage of nitrogen is one of most common causes of crops not growing well.

Figure 5: Grassed strips have been used to control water erosion in parts of Lesotho. One of the oldest soil-conservation practices is the construction of stone lines or stone walls built across the slope of fields on, or close to the contour. Examples of this technology can be found over large areas of the Mediterranean and the mountainous parts of Asia. Like barriers of vegetation, stone barriers slow down the movement of water and trap moving soil. In time, the trapped soil builds up behind the stones and forms benches or terraces. Many of the terraced olive fields of the Mediterranean have been formed in this way, with the terraces usually taking many years to form. Mechanical engineering techniques are also used in other ways besides the many applications of the “contour principle.” Structures of various types are frequently used to stop the growth of and to heal gullies that have been formed by water erosion. The structures used include dams, silt traps, and structures to protect eroding gully and stream banks. Dams are popular with farmers as they serve two purposes: dams catch and hold water that is used for domestic purposes, livestock, and sometimes irrigation, but they also slow down the movement of water and catch silt that would otherwise cause problems in streams or on fields further down the slope.

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Concrete and stone structures are commonly used at the top of gullies to prevent them from getting any bigger. Structures of various designs are also used along the course of gullies as silt traps. Here they act like steps—each one effectively raising the level of the gully floor and, in the process, slowing down the movement of the water. As it slows down, the water loses its capacity to carry particles of soil and the material, known as silt, drops to the gully floor and is trapped. If well-designed and maintained, silt traps can stabilize eroding gullies and, if there is enough silt, eventually result in the gully filling up and providing productive land again. Silt traps are used very effectively in some of the badly eroded parts of China where large areas of land have been reclaimed through their use. 4.1.2. Vegetative or Biological Soil-Conservation Measures Besides the “mechanical” or “engineering” techniques outlined above, another way of treating soil conservation has been extensively used. This is usually referred to as “vegetative” or “biological” conservation. The underlying principle here is that soil only becomes subject to erosion if it is bare and exposed to the erosive forces of wind and water. It follows from this that if the soil can be kept under a permanent or near-permanent cover of vegetation, then little or no erosion will occur To understand this concept fully, it is important to realize the force that both wind and rain can exert on bare soil. For instance, the energy dissipated by a 50 mm rain storm is theoretically capable of lifting 18 cm of soil 1 m into the air. If the raindrops are large, they fragment soil clods and disperse them in all directions. If there is a cover of vegetation on the surface, either living or dead, the soil is protected as the energy of the falling raindrops is dissipated when they hit the vegetation. Research into this subject shows that the vegetation does not even have to provide a complete cover to be effective; if only about 40% of the soil’s surface is protected by low-level vegetation (not more than 1 m above the surface) and evenly distributed cover, erosion can be reduced by as much as 90%. Not only this, but a cover of vegetation on the ground slows down the movement of water across the surface and allows it to sink into the soil, becoming available to the roots of plants or percolating down to the water table. Also, if vegetation can be retained, and allowed to break down and become part of the soil, the physical and chemical properties of the soil are improved. This, in turn, makes the soil less susceptible to erosion and more conducive to plant growth. A great range of biological conservation measures have been developed and used. In the case of grazing land, this can simply amount to ensuring that the land is never overgrazed and that sufficient cover is always retained to protect the soil. For land that is cropped, the problem is more complicated as it is difficult to cultivate without exposing the land to the wind and rain for at least part of the year. One practice that has become very popular in recent years is to use mulches. New types of plows and cultivators have been designed which can break up the soil without burying all the residues or becoming blocked by the straw and stalks in the process. One of the primary reasons for cultivation is to kill weeds, but this is now often done by spraying chemicals rather than cultivating. A wide range of agrochemicals are now available for this purpose, some of them selective so that they will kill the weeds but not effect the crop.

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A practice called “relay cropping” is often used in tropical countries. It is very commonly used in China. With this system, different crops are planted in a rotation but the farmer does not wait until one crop is harvested before the next crop is planted. So, for example, seedlings of maize may be planted in narrow strips running through a wheat crop. The maize is planted out a few weeks before the wheat is harvested so that, when the wheat crop is harvested, the maize plants are already big enough to provide a partial cover to the soil. A system called “agroforestry” has been widely promoted in the tropics in recent years. Here tree and field crops are grown together in the same field. The trees are often grown in narrow strips, often on the contour, and are usually cut at different times so that they do not provide shade that would affect the field crops. The trees may be either fruit trees or trees which have the ability to trap nitrogen from the atmosphere and return it to the soil where it can be used by other plants.

Figure 6: Trees that have the ability to return nitrogen to the soil are grown in strips between annual crops in this system of agroforestry in Indonesia.

Trees are used in many ways to protect the soil. They are particularly effective as wind breaks and are frequently used to control erosion and reclaim badly degraded land. However, trees are seldom very effective on their own as soil cover needs to be no more than about 1 m above the surface of the soil to prevent water erosion. A good ground cover of grasses, shrubs, and/or leaf litter is needed if the trees are to effectively control water erosion.

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4.2. Wind Erosion Both engineering and biological measures are also used to control wind erosion. To understand how these work it is necessary to understand something of the mechanics of wind erosion. This is complex but, broadly, the movement of soil particles is caused by wind forces exerted against the surface of the ground. As the wind moves over the surface a gradient develops in its velocity; the velocity being lowest near the ground and increasing with height. At some point, near the surface of the ground, usually in the range of 0.03 to 2.5 mm, for a bare, relatively smooth surface, the wind velocity is zero. For a very short distance above this level, the flow of air is smooth and laminar, and above this is turbulent air flow. It is this turbulent air which produces the forces which cause soil movement. Soil particles, or other projections on the surface protruding into this layer of turbulent air, absorb most of the force exerted on the surface. If these projections are sufficiently large, or firmly attached to other particles, they may resist the force of the wind. If, however, they are not attached, or light, the wind may lift them from the surface and initiate soil movement. It, therefore, follows that if we are able to increase the roughness of the surface, and the projecting material is either too heavy or too firmly attached to be removed by the wind, then the soil can be protected.

Figure 7: In Inner Mongolia, China, 20 cm of soil have been blown away exposing the roots of the remaining plants.

In view of this, the control of wind erosion is based on four basic concepts:

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• to produce, or to bring to the surface, aggregates or clods which are large enough to resist the wind force;

• to roughen the land surface to reduce wind velocity and trap any moving soil; • to establish barriers, at intervals, to reduce surface wind velocity; and • to establish and maintain vegetation, or vegetative residues, to protect the soil.

As with water erosion, a great variety of different soil-conservation practices and techniques are used to control and prevent wind erosion. Also, like water erosion, they are commonly classified as “mechanical” or “engineering” and “vegetative” or “biological”. The mechanical or engineering measures consist mainly of wind breaks of different types. These are most commonly belts of trees and shrubs, grown in strips across the direction of the prevailing winds. Care has to be taken to use the right type of vegetation as badly designed wind breaks can have the effect of increasing, instead of decreasing, erosion. Some wind must pass through the barrier, otherwise eddies form and these can cause the soil to erode. Well-formed wind breaks can provide protection for a distance of about 20 times their height.

Figure 8: This photo from China demonstrates how vegetation can protect the soil.

Other mechanical measures include simple practices such as cultivating the soil so as to produce a rough, cloddy surface and a practice called surface ridging, where the soil is formed into ridges in lines across the direction of the prevailing wind. Obviously, the best way to prevent wind erosion, as with water erosion, is to have the soil densely covered with vegetation at all times, but when it is necessary to grow crops and

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carry out a number of other agricultural practices, this is not always possible. However, vegetation can be used in a number of ways, including the use of strips of vegetation left at intervals across the direction of the wind. But more common is the practice of “no-till” or “ minimum tillage” cropping systems. Under these systems, the crop residues are left on the surface or partly worked into the soil, so that the soil is always covered, or partly covered, with a rough layer of living or dead vegetation. Well-managed grazing lands are seldom over grazed and every effort is made to always leave enough grass and other vegetation on the surface to protect the soil. Some remaining grass has protected this “pedestal” but nearly 2 m of soil have been blown away from the unprotected surrounding areas. 5. Soil-Conservation Policies and Approaches Soil erosion, and the other related forms of land degradation, now constitute an extensive and serious problem in many parts of the world, particularly in the developing countries. In fact, soil erosion is now one of the most widespread environmental problems facing us globally. In spite of the importance that the subject has been given by national and international organizations, the problem has been increasing in recent years. For example, despite the US government having spent an estimated US$18 billion on soil conservation between the mid 1930s and early 1990s, and presently spending over US$2 billion per year on soil conservation, that country is losing an estimated 6000 million tons of topsoil annually. The picture is similar in other parts of the world. There have been successful soil-conservation programs but, overall, not enough has been achieved and generally farmers and other land users have been slow or reluctant to take up practices that would protect and rehabilitate their land. There are a number of reasons for this but they stem from a lack of understanding of the real causes of the problem by those who have been responsible for the programs and projects. Far too often, soil erosion has been seen as a problem in itself, rather than the symptom of bad land use and management. Usually plans have been prepared with a heavy emphasis on physical conservation measures such as contour banks, silt traps, and gully control measures. The problem has usually been seen from an engineering perspective with emphasis given to physically keeping the soil in place and with planners seldom giving much attention to incorrect land use or bad land management. Often, contacts between the planners and the land users have been minimal and seldom have the real reasons for the misuse of the land been properly investigated. Plans developed in this way have usually paid little attention to the effects that they would have on future production or the immediate needs of the land user. This approach has almost inevitably led to problems. Soil erosion is usually a fairly gradual process and its effects are frequently masked by the use of fertilizers, better crop varieties, or even year-to-year rainfall fluctuations. Farmers, particularly in developing countries, are daily faced with a range of pressing problems, including earning enough to clothe, feed, and house their families, paying for the educational expenses of their children, paying off debts, and meeting their social obligations.

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Seldom is erosion seen as an urgent problem by farmers, nor are farmers impressed with statistics about tons of soil being lost from their land. Farmers, therefore, have generally not been very interested in soil-conservation programs which appear to offer them little if anything in the way of obvious benefits and which do little to solve their more immediate problems. Soil-conservation programs have therefore tended to proceed very slowly unless farmers have been either forced or paid to participate and the maintenance of conservation works has always presented a problem. In some African countries, strong penalties were imposed at one stage to force farmers to carry out conservation work. This made soil conservation so unpopular that conservation laws were taken up as a cause leading to independence—local politicians encouraging farmers to break conservation structures to oppose the colonial administration. In an attempt to encourage soil conservation, farmers in many countries have been paid to install conservation works. This has seldom been successful as the farmers have usually neglected, or even deliberately destroyed, the works once the payments have ceased. With such little incentive for farmers and other land users to involve themselves, in retrospect it is not surprising that conservation programs have often been disappointing. Faced with failure, too frequently the planners have looked upon the farmers, the pastoralists and those who rely upon the woodlands and forests for their livelihoods as being part of the problem, rather than the potential for solution. Until recent years, few attempts had been made to analyze the real causes of land misuse, such as population pressure, land tenure systems, labor shortages, poor supply systems, lack of markets, and the need for credit. 6. Changes in Approaches and Policies The approaches to soil conservation began to change in many countries in the 1980s. At last, it became widely recognized that, ultimately, the way a country’s land is managed and used depends upon the perceptions and the actions of its many thousands of individual land users. With the right incentives, these people have the ability to quickly bring about fundamental changes in land use and management for the better. It was seen that the challenge was to create the conditions that would provide motivation for this to happen. Importantly, it became recognized that land users will readily adopt conservation practices if those practices provide them with some tangible and obvious benefit in the short term. Thus there has been a general move away from many of the old mechanical erosion control measures, which may be effective in controlling erosion but do little to increase production or increase incomes, and a move towards biological measures, many of which are highly productive. The most striking example of this has been the promotion of what is generally called conservation tillage, no-tillage, or minimum tillage. With this system, a permanent cover is maintained on the surface of the land. This is done by retaining crop residues, instead of removing, plowing, or burning them. Where necessary, cover crops are also grown between the cash crops to ensure an adequate cover of vegetation at all times. The benefits of conservation tillage are: (1) the soil is covered at all times providing protection against the erosive forces of wind and water, (2) the vegetation breaks down and in the process improves the chemical and physical properties of the soil, (3) more rain water is

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absorbed by the soil instead of running off, (4) yields are maintained or increased, and (5) production costs are usually decreased. With such obvious benefits, farmers have been quick to embrace this new technology. The spread of conservation tillage has been particularly dramatic in Latin America where the area under this system of cropping increased 20-fold between 1987 and 1997 in Brazil, Argentina, Paraguay, and Uruguay. Conservation tillage systems are also being increasingly used in other parts of the world. While this has been happening, increased emphasis has been placed on the involvement of the land users in the whole process of identifying the problems of land degradation, developing solutions and then implementing the agreed programs. To facilitate this process, a rather bewildering range of methodologies has been developed. These go by such names as Participatory Rural Appraisal and Planning, Participatory Technology Development, Farming Systems Research and Extension, and Diagnosis and Design. While they may vary in detail, they all have the common aim of involving the land users in the whole process, using their local knowledge, raising their enthusiasm and obtaining their support and cooperation. Of the many programs now attempting to achieve greater participation of the land users, the largest, and apparently the most effective to date, is the Landcare Program in Australia. Probably the most important feature of the Landcare Program is that the land users have taken over the managerial responsibilities of all its projects. So successful has Landcare been since its launch in the mid 1980s that a quarter of the country’s farming population had joined the program by 1994 and membership has continued to grow since. The Landcare approach is now being copied, adapted, and applied in a number of different countries as far apart as South Africa and Iceland. With Landcare, not only the farming communities are involved, but also urban groups, schools, nongovernment organizations, and institutions. Another feature of Landcare is the way in which it has brought together investment from the government, private enterprise, and land users themselves. Recently, interest has also been shown in traditional soil-conservation measures in developing countries. It is known that many traditional land-use systems were very effective in preventing soil degradation and conserving the soil. Unfortunately, with changing conditions, particularly with more intensive land use being forced upon communities because of greater population numbers, many of the conservation effective land use practices of the past have been abandoned. Scientists are now looking at these traditional practices and attempting to modify them to present conditions so that farmers will practice them again. Although good progress is being made, much more remains to be done and progress is still slow. Among the reasons for this is the fact that even where land users are fully involved in conservation schemes and are prepared to adopt more conservation-effective systems of land use, conditions may not allow them to do so. These conditions may include financial constraints, social and political pressure, land tenure systems, the lack of markets, or of the availability of the necessary farming inputs. To overcome this, a wide variety of incentives and disincentives have been developed for soil-conservation programs. These are now widely used and can be found in any soil-conservation program in one form or another.

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The incentives used may be direct and in the form of cash payments for labor and cash for grants, subsidies, and loans. They may be in the form of food payments in developing countries and the provision of agricultural implements, or they may come in a combination. Incentives may also come in an indirect form, such as tax concessions, guaranteed prices, the regulated prices of inputs, and more secure land tenure systems. Disincentives are also used and come in an equally wide range of ways, varying from fines and taxes to exclusion from beneficial government programs if the individual does not comply with some regulation or requirement laid down by the government or local administration. It is now acknowledged that, if incentives and disincentives are used carefully and in the right way, their use can radically alter the way in which land users use and manage their land. Unfortunately, a serious problem still exists in that many government policies are frequently designed to overcome other problems, such as inadequate food production and low farm incomes Many of these unintentionally encourage land users to adopt practices and land use systems which lead to soil erosion. For example, subsidies paid in the European Union countries have encouraged farmers to cultivate steep slopes which should have been left under pasture and to dig up old hedgerows to make cultivation easier. 7. Soil-Conservation Research Soils are complex systems. They usually consist of weathered and partly decomposed rock, water, air, organic material formed from animal and plant decay, and thousands of different life forms, mainly microscopic. Soils are normally in a form of dynamic equilibrium with the balance of their various components changing with variations in rainfall, temperatures, the surface cover and, of course, the way they are managed by humans. Soils vary enormously both chemically and physically and they vary greatly from place to place. Because of this complexity and variation, research on soils is difficult and some of the effects of erosion on soils are still not well understood. Soil-conservation research in the twentieth century was dominated by the work done in the United States. Work in other parts of the world has been largely based on what has already been done there and the research methodologies developed there. Because of the complexity of soils, and also because soil erosion has until recently been seen largely as a physical problem, most research work until recently was concentrated on measuring soil erosion in terms of soil loss—usually expressed as tons of soil lost per hectare or per acre. The data provided from this type of research are very useful, for example, to engineers who are looking at the rate at which dams and streams are likely to fill with silt. The data are also useful for some soils—soils which are deep and which do not vary much with depth, the sort of soils found over large areas of the US’s cropping lands. Early research, therefore, resulted in the gathering of vast amounts of information about soil loss in the USA. Armed with this information, it became possible to develop mathematical models through which soil loss could be predicted with reasonable accuracy under different management practices. The best known of these models is one developed by the research worker Walter H. Wischmeier and his colleagues, the Universal Soil Loss Equation or USLE as it is

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commonly referred to. (The equation is expressed as A = R × K × L × S × C × P, where A is soil loss in tons per acre, R is a rainfall erosivitiy index, K is the erodibility of the soil, L is a length factor, S is a slope factor, C is a crop management factor, and P is a conservation practice factor).

Figure 9: Different crops are grown on these trial plots in Kenya and the water and soil that run off them are trapped in the tanks and measured.

This equation has two applications. First, it can be used to predict how much soil will be lost from a field under certain conditions by erosion. Second, by deciding what an “acceptable” amount of soil erosion could be (this is usually taken as five tons per acre per year), a suitable crop management factor (C) and conservation practice (P) can be worked out and applied on the farm. With a huge data bank, built up over the years on hundreds of individual plots over a large area, the US Soil Conservation Service was able to use the USLE as the basis for its calculations and recommendations to farmers for many years. In recent years an improved version of the equation has come into use—the Revised Universal Soil Loss Equation (RUSLE). This model is able to make better use of new data and it can also take into consideration the effects of some conservation practices with which the USLE was not designed to cope. Owing to the prominence of the research work in the United States, many in other parts of the world based their research on the USLE and attempted to modify it to the conditions in their own countries. This has often proved unsuccessful as few countries outside the USA have sufficient data for the USLE to be applied with any degree of accuracy. In some cases, estimates have been used instead of sound data and this has led to results that have been very misleading.

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Soil-conservation research workers are now attempting to establish the effects of soil erosion in terms of soil productivity rather than just as total soil loss. This is very complex and involves establishing the effects of erosion on such facets of a soil as its physical properties, chemical composition, ability to absorb and hold water, and organic matter content. Although much of this work is still in its infancy, it is already becoming clear that the productivity of soil is affected differently by erosion, depending on the type of soil and the climatic conditions. Thus, in a shallow tropical soil, the loss of the organic matter in the top few millimeters is of the greatest importance as it is here that most nutrients vital for plant growth are stored. With this interest in soil productivity, rather than just soil loss, a number of new models have been developed. An example is the Erosion Productivity Impact Calculator (EPIC). Most of these models are very complex and require a huge input of data—data that are not available in most parts of the world at present. Not all research is on the physical aspects of soil conservation. With a greater participation by the land users in soil-conservation programs, and the realization that soil-conservation practices will not be adopted unless they are perceived to be worthwhile by these people, a considerable amount of research is now going into the socio-economics aspects of soil conservation. While some of this research is along traditional lines, for example cost: benefit studies, some more innovative research is now being undertaken into such subjects as how land user attitudes and behavior towards soil conservation are influenced by subsidies and other incentives, including improved land tenure systems, markets and pricing, and new technology. Farmers are always innovative and some research workers are looking at new conservation farming practices that are being developed all the time in different parts of the world by enterprising land users. In addition, more research workers are carrying out some of their trials on farmers’ fields to ensure that any new technologies they produce will work under real farm conditions and be readily accepted by farmers. Recently, considerable interest has also been taken in traditional systems of soil conservation, particularly in Africa and Asia. Many effective traditional conservation measures have been abandoned and research workers are looking into why this has happened and, where appropriate, if traditional practices can be modified and adapted to fit in better with present day conditions. One particularly interesting research program was launched by the World Association of Soil and Water Conservation in 1992—a World Overview of Conservation Approaches and Technology (WOCAT). As the name indicates, this program is making a global study to identify and document the more successful approaches and technologies for soil conservation. As the results become available, they are passed on to planners to use in national and international projects and programs. 8. Future Trends in Soil Conservation As the principles of ecology become more widely understood by planners, it is probable that land degradation—and soil erosion in particular—will be seen more as an

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environmental problem than a physical problem. This will result in solutions being sought in the way that land is used and managed rather than in engineering measures. With this, we can expect to see more emphasis placed on cropping systems, such as conservation tillage systems which will not only serve to reduce erosion but which will also lead to increased production and reduced costs. Similarly, there will be an emphasis placed on productive, but sustainable, land use systems for grazing, orchard, and forest lands. The lesson has now been learned that the way land is managed depends on the thousands of different people who use it and that little progress can be made unless the land users themselves are involved in the whole process of planning and implementing soil-conservation programs. We can therefore expect to see more programs of the Australian Landcare type being started. Planners are also now becoming aware of the importance of incentives and disincentives to the way land users react and use their land. It is probable that not only subsidies, but also a wide range of incentives, such as tax concessions and improved land tenure systems, will be more frequently used to encourage people to manage their land in a more sustainable way. Similarly, we can expect to see more disincentives used to discourage bad land use. In some countries, these are likely to include the greater use of fines and the exclusion from government programs of those who will not implement basic conservation measures on their land. Glossary Accelerated soil erosion:

The erosion of soil at a much more rapid rate than normal, primarily caused by the activities of humans.

Agroforestry: Land-use systems in which trees or shrubs are grown in association with agricultural crops, pasture, or livestock and in which there is ecological and economic interaction between the trees and the other components.

Conservation tillage: Noninversion tillage that retains protective amounts of plant residues on the surface as a mulch.

Contour banks (or contour bunds):

Small banks (usually 20–50 cm high) built on or close to the contour to catch and/or slow down the movement of runoff water and soil.

Land degradation: The deterioration of land through such processes as soil erosion, salinization, acidification, pollution, or sediment deposition.

Minimum tillage: The minimum tillage necessary for crop production under the existing soil and climatic conditions, normally infers noninversion of the soil and retaining some or all crop residues on the surface. A form of conservation tillage.

Mulch: Any material, but usually straw or other plant residues, left on the surface to protect the soil.

No-till: Similar to minimum tillage with minimum or no mechanical soil manipulation. A form of conservation tillage.

Rainfall erosivity: The ability of the rain to cause erosion, relative to rainfall intensity.

Relay cropping: The growing of crops in a sequence but with the new crops

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being planted before the previous crop is yet harvested. In this way the soil is never left entirely bare.

Runoff: The portion of the total precipitation on an area that flows away.Shifting cultivation: The production of food crops, usually for subsistence,

alternating with fallow when the land is allowed to revert to forest or grass.

Soil conservation: The combination of appropriate land use and management practices that promote the productive and sustainable use of soils and in the process minimize soil erosion and other forms of land degradation.

Soil erodibility: The susceptibility of a soil to erode. Soil erosion: The wearing away of the soil by water, wind, ice, or other

geological agents. Soil structure: The combination or arrangement of primary soil particles into

secondary particles, units, or peds. Water erosion: The erosion or removal of soil primarily through the forces of

water. Wind erosion: The erosion or removal of soil primarily through the forces of

wind. Bibliography

Bennett H. H. (1939). Soil Conservation. 993 pp. New York: McGraw-Hill Book Company, Inc. [This is a classic early text by the “father” of modern soil conservation.]

Campbell A. (1994). Landcare—Communities Shaping the Land and the Future. 344 pp. Sydney, Australia: Southwood Press. [This book gives the background to the Landcare movement in Australia and describes how it works.]

FAO (Food and Agriculture Organization of the United Nations) (1992). Protect and Produce—Putting the Pieces Together. 35 pp. Rome, Italy: FAO. [This is a booklet that outlines the problems of soil erosion and what can be done about it.]

Hallsworthy E. G. (1987). Anatomy, Physiology and Psychology of Erosion. 176 pp. New York: John Wiley & Sons. [This book presents a good background to the effects of erosion and farmers’ fight against it over the centuries.]

Hudson N. (1995). Soil Conservation. 391 pp. London: B. T. Batsford Limited. [This is a classical textbook on the technology of soil conservation.]

Sambatpanit S., Zobisch M., Sanders D. W., and Cook M. (eds.) (1997). Soil Conservation Extension: From Concepts to Adoption. 487 pp. New Delhi, India: Oxford & IBH Publishing Co. [This is a book based on the proceedings of a conference held in Chiang Mai, Thailand, which explored the concepts, strategies, implementation and adoption of soil conservation.]

Shaxson T. F., Hudson N. W., Sanders D. W., Roose E., and Moldenhauer W. C. (1989). Land Husbandry—A Framework for Soil and Water Conservation. 64 pp. Published in cooperation with the World Association of Soil and Water Conservation by the Soil and Water Conservation Society. Ankeney, IA: Soil and Water Conservation Society. [This is a booklet describing many of the approaches now used in soil-conservation programs.]

Tato K. and Hurni H. (eds.) (1992). Soil Conservation for Survival. 419 pp. Ankeney, IA: Soil and Water Conservation Society. [This book consists of a number of papers which were presented at the 6th International Soil Conservation Conference, Addis Ababa, Ethiopia, in 1989. They provide an insight into a large range of soil-conservation issues in different parts of the world.]

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Biographical Sketch David Sanders started his professional career with the Soil Conservation Authority, Victoria, Australia, before joining the Food and Agriculture Organization of the UN in 1965. While with FAO he headed the soil-conservation section for many years providing advice to governments on their soil-conservation policies and programs as well as supervising soil-conservation projects in many countries. Since retiring from FAO in 1995, David has remained actively involved in soil conservation and has served as President of the World Association of Soil and Water Conservation and as a member of the Board of the International Soil Conservation Organization. He has contributed to a number of books, written numerous papers, and has appeared regularly as a keynote speaker at international soil-conservation conferences over the years. To cite this chapter David Sanders, (2004), SOIL CONSERVATION, in Land Use ,Land Cover and Soil Sciences, [Ed. Willy H. Verheye], in Encyclopedia of Life Support Systems (EOLSS), Developed under the Auspices of the UNESCO, Eolss Publishers, Oxford ,UK, [http://www.eolss.net]

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