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4. Aims and techniques of soil management
Countries
share experience
Covering
the land helps
Zero
tillage gains ground
Keep
tillage to the minimum
Farmers
practice contouring
Rotation
reduces erosion
Terracing
requires labour
Controlling
windblown soil
Overgrazing challenges conservationists
Stern reclamation measures needed
Forests
aid conservation
Watershed
approach required
The aims of soil conservation in a world that needs more food can be stated briefly:
Translating these deceptively simple aims into action will be a demanding task. At a minimum, developing workable soil conservation programmes will require a number of concurrent activities, calling for a determined and continuing commitment to conservation by each country's leadership. Other requirements include:
Countries embarking on such a course of action will not be starting from scratch. The lessons learned by pioneers in modern soil conservation, like the USA and Australia, are available to other countries to modify and apply to their own problems, as are the experiences of older societies, like the terrace-builders of Asia, the Near East and Latin America. There are also a number of research stations throughout the world that are spending a great deal of time and money trying to improve resource management and farming systems. Like the basic aims of soil conservation, the principles they have learned can be put very simply.
Keep the soil covered with living plants - or the residue of dead ones. Slow the movement of water downhill, and help rain to soak into the soil where it falls. Get more organic matter into the soil, and avoid stripping trees and other protective vegetation from hillsides. Where erosion from wind is a problem, break its force with trees or other plants and do a better job of holding down the soil. Most important as a guiding principle, people should ask no more of each parcel of land than it is capable of giving.
Soil conservation consists of safeguarding, not just cropland, but all kinds of useful land. Nearly all land is useful for some purpose. Some kinds of land - flat, fertile, and well drained - are ideal for growing crops. Other kinds can be used for crops only if protected with conservation measures. Still others should not be used for annual crops at all, but kept under grass and never grazed or should not be grazed past their limits. Other land is practically erosion-proof if left in trees but will deteriorate rapidly if the trees are cleared. And much steep land with shallow soils should not be used by man at all, but should remain in its natural state, a source for groundwater recharge and a home for wildlife. Good land use is probably the best conservation practice of them all.
The most inexpensive way to prevent soil from washing or blowing is to keep it covered. In most places, vegetation should cover at least 70 percent of the ground surface to provide adequate protection against erosion. Unfortunately, the most critical erosion on farmland occurs at those times of year when the soil is bare or when row crops are in the seedling stage. The farmer's preoccupation during this time of year is to eradicate the unwanted vegetation, which appears in the form of weeds.
But if a cover of living plants is impractical at this stage, a cover of dead plants is not. Many crops produce substantial crop residues in addition to the parts that are harvested or grazed. These residues may be ploughed under, left on the soil surface as mulch, or partially mixed with the surface soil when preparing the seedbed for the next crop. Used in sufficient quantity, mulches provide a measure of protection to the soil - much more than if the land were clean-tilled.
Even better ground cover is obtained through a system developed during the last two decades. Called "no-till" or zero tillage, it involves planting an economic crop which keeps the soil covered, such as a cereal; harvesting it but leaving the stubble standing in the field; killing it with a single application of a chemical herbicide, and drilling the seed and fertilizer for the next crop into the soil through the stubble of the old crop. The soil is neither ploughed nor disked, but left alone as much as possible, reducing fuel and labour costs and lowering the risk of compaction.
In normal years, the crop germinates on time and yields are generally equal to or larger than yields from crops grown in clean-tilled fields. The roots of the old crop discourage weeds and improve soil structure and, as a result, the soil retains more moisture. In an unusually wet year, this may prove a disadvantage, but in an abnormally dry year, crops grown with no-till farming suffer much less stress than crops grown with traditional tillage. Most important, soil erosion, even on steep land, is usually negligible.
Variations of no-till farming, as well as other forms of mulch tillage and minimum tillage, are the fastest growing set of conservation and energy saving practices in the United States today. They were used on some 30 million hectares of cropland during the 1980-81 season with generally good results.
While the zero tillage idea is new to the West, certain forms of it have been practiced for a long time in parts of Asia and Africa, primarily to save labour. In the savanna zone of Africa, for example, farmers dig holes with a hoe one pace apart and plant maize seed in each hole, without any other form of seedbed preparation. If grass is growing between the holes, it is lightly slashed or left untouched. This system is well suited to many tropical soils, in which intensive tillage leads to rapid breakdown of soil structure and loss of moisture. More sophisticated forms of zero tillage, with herbicides used for weed control, hold even more promise for improvement of agriculture in the tropics.
Mulches of crop residues also help reduce erosion in tropical countries. Mulches can be straw, maize stalks, banana leaves or palm fronds, depending on what is available locally. Laid on the soil surface, or mixed into it, they reduce the surface area exposed to the impact of raindrops and decrease the velocity of runoff water by imparting roughness to the soil surface. When the mulches decompose, they add organic matter to the soil. Rain also infiltrates the soil more easily under mulch, since there is no sealing and no crust on the soil's surface.
Living mulches of grass and legumes are being tried in some places between rows of vegetables, with weeds regulated with herbicides. The living mulch helps to prevent crusting, controls erosion, and improves soil filth.
The use of vegetation can be combined with various other conservation techniques, like contour farming, to protect soil from washing. Instead of planting rows of crops straight up and down the hill, a farmer practicing contour cultivation plants rows at right angles to the direction of the slope, following the contour of the land. The practice works best on fields that slope uniformly in one or two directions, and it is used for orchards and vineyards as well as field crops.
To make contouring succeed, natural drainageways have to be identified and planted with grass to allow stormwater to move safely off fields without creating gullies. Grassed waterways have to be graded accurately and made wide enough so that runoff water is spread evenly over a wide channel. Waterways are permanent measures and require a dense, vigorous growth of vegetation to be successful .
If runoff in the field is still too rapid, it may be necessary to control it further and promote absorption by cutting contour furrows at intervals on the slope. More severe problems may be handled with diversions, which are channels with a supporting ridge on the lower side to intercept runoff. Diversions move water on a gentle gradient to a grassed waterway or other outlet.
Contouring is often combined with stripcropping, in which strips of a crop like maize alternate on the side of a hill with strips of denser vegetation, like small grain, grass, or a legume. Much of the soil washed downhill from each strip of row crops is trapped by the strip of denser vegetation growing below it. The cover crop can be grazed or cut for hay or silage.
Farming systems that rotate crops with pastures of grass, clover and legumes reduce soil erosion in two ways.
Firstly, the period of time that the soil is exposed to water and wind is reduced sharply. In a maize-wheat-grass rotation in USA, for example, the grass pasture is not ploughed until spring, just before the maize is planted. Wheat follows the maize in the autumn or fall, immediately after harvest, and grass and clover growing in the wheat stubble also reduce erosion.
Secondly, crop rotation with legumes usually improves soil fertility, particularly nitrogen content and increases the ability of the soil to absorb and retain moisture. Land planted with maize following grass loses considerably less soil than maize following maize.
In the Corn Belt of central USA, three-year rotations of maize, small grain, and pasture were used with success for many years. On sloping land, two consecutive years of pasture have proved even more effective. In New England, a common rotation was potatoes, followed by oats and then clover. In recent years, however, more and more American farmers have abandoned crop rotation in favour of continuous crops of maize or maize and soybeans, to the detriment of the soil. Today, however, there is renewed interest in crop rotation among many growers.
In tropical countries, a grass fallow rotation system, with suitable cooperation between farmers and graziers, has occasionally proved effective in replacing shifting cultivation, as has mixed cropping of maize with legumes.
Mechanical measures for controlling erosion are usually more costly than those that depend primarily on vegetation, and they generally require more labour, materials, and technical skill to install. They include many different kinds of terraces, diversions and grade stabilization structures.
Terraces, one of the most effective means of reducing soil erosion, are ancient inventions. They vary widely over the world - even within a single country - depending on tradition, type of soil, degree of slope, rainfall, and many other factors. In India, as in many countries, terracing is by far the most effective and widely practiced field measure for controlling runoff and preventing erosion. Terracing also helps conserve soil moisture and improves groundwater storage and crop yields.
In general, terraces break up a long slope into a series of short ones. Each terrace collects and controls the excess water from a definite area of the slope above it. Water collected in a terrace channel may be conducted to protected outlets where it will cause no damage, such as well sodded pastures or natural waterways. If the soil in a field is permeable enough, terraces may be built level and water allowed to stand and soak into the ground.
A few types of terraces may be familiar to the layman. Bench terraces, one of the oldest methods of erosion control, transform steep land into a series of level shelves or steps running across the slope. Bench terraces make cropping of steep slopes possible and safe; the steps are separated by almost vertical risers of rock or earth protected by a heavy growth of vegetation.
On slopes of 10 percent or less, terraces can be built with wide, gently graded banks, with a large shallow channel above each one. On gently sloping land, such terraces are usually constructed so that crops may be grown on them as well as in the intervals between them. These broad base terraces can be farmed with modern machinery. On slopes greater than 10 percent, terraces with grassed back slopes, or contour bunds, often prove effective.
In the USA, an increasing number of terraces today are laid out so that they are parallel to each other, permitting greater ease in using and turning large tillage machinery. Any excess water is removed with perforated underground drains. Where installed, parallel terraces are popular with farmers. The chief barrier to building more of them is the expense.
Even when properly constructed, terraces will not provide erosion protection for very long unless they are kept in good repair. Failure of terraces in some countries is the result of poor maintenance, not faulty terrace design. A crucial time for terrace maintenance is after each heavy rain during the first year after construction, when soil may settle. Breaks or low places should be mended promptly before damage becomes serious. Unless kept in good condition, terraces may cause more erosion than if they were never built.
The techniques for reducing erosion from wind are considerably different from those for slowing down water erosion. One approach is to plant rows of trees at right angles to the prevailing wind to break its force and provide shelter for the field just beyond. Shelterbelts diminish the length of land open to wind blowing and perform a function similar to that of terraces in controlling erosion from water.
Starting in the Dust Bowl days of the mid-1930s, USDA's Forest Service planted 220 million trees as "nets to catch the wind" in parts of the USA subject to wind erosion. Also, plants other than trees, like tall wheatgrass, have also proved effective in blocking wind and drifting snow.
An obvious solution to erosion from blowing is to maintain a surface on the field that cannot be moved by the wind. One answer is to keep the field covered with plants or mulch. Stalks of maize, for example, when disked into the topsoil, roughen the surface and interfere with the mechanics of wind erosion.
Another method is to plough the land into a rough cloddy condition and leave it without disking until planting time, so that the clods resist the force of the wind for as long a period as possible. The cumulative effect of blowing soil, or avalanching, can also be effectively reduced by growing crops in strips placed at right angles to the prevailing wind. Intervening strips are frequently fallow, but erosion on these exposed soils is cut sharply by the stripcropping.
Overgrazing challenges conservationists
Answers to overgrazing are far from simple and range conservationists will need to approach the problem differently in different regions and different societies. Some of the more obvious solutions are usually unacceptable to local people, like reducing numbers of livestock or migrating to different locations. Another approach is to introduce hardier and more nutritious species of forage to overgrazed areas and to harvest and retain at least a part of the forage to feed livestock during the dry season. Reseeding of degraded rangeland may also prove necessary.
Still another approach is to improve management of indigenous pasture and rangeland through rotational grazing, and by avoiding concentrations of animals that trample down soil and vegetation on stock routes and near watering points. Sources of water should also be widely distributed.
Stern reclamation measures needed
Where overgrazing has resulted in desertification, stern procedures are called for to halt the spread of desert and to begin reclamation. They include:
These activities, singly or in combination, should lead to eventual reclamation of many desert lands, provided of course there is sufficient rainfall or irrigation water available. Large sand dunes will remain, but they can be prevented from moving and covering productive land by planting of suitable vegetation.
It has been said that Japan would have been washed into the sea long ago had it not been for its forest-covered hills and mountains, maintained through the centuries for the specific purpose of controlling erosion by water. As trees are cut and used, Japanese farmers constantly replant from tree nurseries kept supplied with planting stock.
Forests provide excellent cover on hillsides, but when first established, young seedlings do not provide enough cover to prevent erosion. Like other crops, trees need to be planted on the contour or on terraces if the land is steep .
Some forests can be harvested from time to time without causing great damage to the land, but in other places, where the forest soil is exceptionally light, trees are the best protection for the land and must be cut selectively, if at all.
On wastelands, a plant cover of grasses, legumes, brush, and trees should be established and maintained. If cutting and grazing are prohibited, the land may slowly recover and someday become productive.
A watershed is an area of land that drains into a single waterway or body of water. A small watershed of a few hectares that drains into a small stream forms part of a larger watershed, which in turn forms part of a larger watershed, until the combined watersheds may become a major river basin draining millions of square kilometres of land.
Of all the units possible for containing a conservation project - villages, states, provinces, countries - the only "natural" project area is that of a watershed. It is natural because it allows planners to focus on all the effects of downhill runoff in a given area and to plan accordingly to control or contain it. In a degraded watershed, which lacks forests and cropland conservation measures, water running downhill too fast erodes soil and washes out crops. It pollutes streams or fills lakes with sediment. It causes frequent flash floods and contributes to bigger floods downstream. In a well managed watershed, on the other hand, most of the stormwater soaks into the soil, increasing groundwater supplies and providing crops, pastures, and trees with needed moisture. Floods are controlled.
A typical small watershed project concentrates first on land treatment. Development starts at the top of the drainage basin and proceeds gradually down the slopes to the lowlands. In humid areas, the highest elevations are usually forest, and forest is often the resource most in need of repair. The benefits of forest cover on mountainsides are numerous, and in many steep watersheds where forests have been cut, the only answer to local problems is to replant them.
Farther down the slope, eroding pasture and orchards may have to be improved to reduce soil loss and eroding cropland protected with contour stripcropping or terraces. Areas unsuited for crops may have to be returned to grass or trees. All these land treatments are directed toward minimizing the amount of runoff water - and soil - that reaches the lowlands.
Even with adequate land treatment, excess runoff from unusually intense storms may have to be contained in various parts of the watershed behind small dams. The stored water may also be used for irrigation or local water supply or released slowly and safely at a later time to proceed downstream.
One of the virtues of the watershed approach to soil and water management is that while a large and badly eroded watershed can be selected for development, the work can be carried out gradually, one sub-watershed at a time. The work can be spread over a number of years, depending on the availability of funds and trained manpower. If and when all the sub-watersheds have been treated, then the larger watershed will also have been improved in a sound and systematic manner.
As a rule, watershed development requires the contributions of many disciplines and aims at improving the quality of life of local residents of the area as well as management of resources. New crops and cropping systems may form part of development, as well as better marketing facilities, new roads, new facilities for health and education and a general improvement of the environment.
In recent years, many countries, in such widely separated locations as Korea, Jamaica, Indonesia, and the USA, have conducted resource development projects on a watershed basis. Multilateral and national assistance agencies and banks have also supported the watershed approach with grants, loans, and technical assistance. Today, watershed planning and development is the preferred approach to soil and water conservation and management.