The Case for Minimum Tillage and Alley Cropping in the Forest Zone of Ghana

With her rapidly increasing population, one of Ghana's most pressing concerns has been, and will, for the foreseeable future, be the need to feed her people. Ghana's climate and soils have great productive potential and the need for food must be met from within. The problem is most serious in towns where access to land is limited and where the number of mouths to feed is greatest. Such a situation results in the land around towns being rapidly overused and depleted, forcing food production into remoter areas.

By far the majority of farms in the forest zone of Ghana are smallholdings farmed with cutlass, hoe and a match. Because of the rate at which plants grow in Ghana, every farmer is fighting a constant battle against weeds which will otherwise choke the crop. This battle takes up great amounts of time during the growing season and, the larger the farm, the longer it will take. In practice the traditional cutlass and hoe farmer is severely limited in the area that can be cultivated by the need to control weed growth. The production capacity of a farmer is directly related to the area that can be cultivated. The traditional farmer operates a way of agriculture ideally suited to providing food for a small community well supplied with accessible land. It is an extensive system which grows crops for three or four years and then operates a fallow for considerably longer. It fails when the community becomes a town, as the scale of production is too great for both the agricultural system and the land. The fallow period is the first to suffer, and the soil rapidly becomes worn out through overcropping. The result is vast areas of exposed laterite and sparse scrub often seen surrounding towns. A temporary solution is for the farmer to farm more distant land. The distance travelled increases demands on the farmers time during the growing season, reducing productivity whilst spoiling more land. These then are the problems to which an appropriate solution is being sought.

It was thought that mechanization offered a solution, increasing productivity at a stroke. Such a solution takes no account of the majority of Ghana's farmers: very few can afford a tractor. The resulting machinery compaction and tillage takes no account of the notoriously fragile nature of the soil, nor the numerous obstacles such as tree roots, which damage expensive machinery in the forest zone. Finally it relies on a good infrastructure to service and maintain the machinery. This is lacking in many parts of Ghana. It has some success further north in the Ghana midlands, these being more suited to mechanized tillage. These areas are a long way from the demand, and transport is expensive. The northern areas enjoy a less favourable growing climate and have lower potential yields than the forest zone.

Minimum tillage is a widely used farming technique that can be adapted to provide a solution. It is based on the principle that the land should be disturbed as little as possible, neither ploughed nor hoed. Weeds are controlled by the use of chemicals and mulching. In the forest, chemical application is best done by one man with a relatively cheap knapsack sprayer, rather than a tractor-mounted unit. Dead weeds are left on the soil to provide a protective cover or mulch. This inhibits further weed emergence, and it also protects the soil against erosion caused by heavy rainfall. Eventually, the mulch will break down and add to the soil fertility. When planting, the seed has to be sown in the untilled soil. Planting implements vary in complexity from a cutlass to a tractor-drawn pneumatic seed drill. TCC advocates the use of the Rolling Injection Planter. This simple tool is made in Kumasi and Tamale, and can readily be manufactured by workshops in other localities. It is hand-pushed by one person. The area planted is much greater than can be achieved by the use of the cutlass alone, experience showing that two acres can be covered in four hours. Obstacles such as stumps can easily be avoided by this wheelbarrow-like machine. Laying mulch down between the crop rows will prevent much weed emergence. The few weeds that do emerge are easily dealt with.

Using minimum tillage techniques, the small-scale farmers are able to control the weed problem, and thus expand their farm size. Minimum tillage is a good way of conserving soil fertility. It is designed for continual cropping, therefore land is more productive. When minimum tillage techniques are used in conjunction with beneficial (leguminous) crops such as cowpeas, it is possible to reclaim previously barren land. (This feat has been demonstrated at Approtech Farms Ltd, Kumasi, where TCC has been involved with a long running minimum tillage programme.) All these features can improve the productivity of both the small-scale farmer and the land being farmed. The techniques are ideal for farms close to towns where the demand for food is greatest. So the food is being grown right where it is needed.

There are foreign exchange costs involved with minimum tillage farming. Such things as fertilizers, chemicals and sprayers are currently all imported. Any technological advance from traditional agriculture is difficult, if not impossible to envisage without some foreign exchange expenditure on agricultural inputs. In this case the money would be invested into a system that has been proven to work in the Kumasi region, rather than a mechanized system that has been repeatedly demonstrated not to be self-supporting in the forest zone. It would benefit large numbers of small-scale farmers rather than a few well-off farmers. It would also benefit the environs of towns as well as providing their food.

In the context of the present drive to revive the cash cropping of perennial crops such as cocoa and oil palm, minimum tillage does not involve burning. Every year bush fires destroy and set back huge areas of productive cocoa and palm plantations. Minimum tillage is compatible with farming perennial crops. Unlike methods relying on burning or soil disturbance, it poses no threat to the crop, and can be used confidently within vulnerable young plantations, before the trees attain maturity.

Alley cropping is the new form of a traditional technique in Ghana: agroforestry. Currently it is receiving attention from scientists throughout the humid tropics. It is a technique of managing a farm so that land fertility is protected and enhanced while need for the imported inputs necessary to grow crops on the same land is reduced year after year. TCC has been practising minimum tillage on an alley cropping plot, on a farm near Kumasi. Selected species of trees have been planted, which fix large amounts of nitrogen in the soil. These trees are planted in rows amongst the crops, and the canopy shade during fallow periods (dry season and between crops) is an effective weed suppressor. During cropping the trees are cut back to bare poles, to minimize competition between the crop and the trees. The trees provide an effective barrier to erosion throughout the year. The roots are permanently binding the soil, and this is coupled to the protective effects of either a canopy or a mulch. Nitrogen is provided to the soil, which will reduce, but not abolish, dependence on added fertilizer or manure. Shade provided by the canopy before planting the crop allows reduced use of weed killers compared to conventional minimum tillage. The technique is particularly suitable for small farms requiring a sustainable yield without a heavy dependence on chemicals. Yields will be relatively stable, reflecting the soil protecting, weed suppressing and nutrient recycling characteristics of the system.

Soil Fertility

Soil fertility varies enormously in the humid tropics, even within one field. Though a very few crops can grow in poor soils, it is common sense to keep agricultural land fertile, in order to enable a wide range of crops to be grown. Most of the fertility of a soil lies in the top few inches. It is vital to protect this layer.

Erosion

This is generally associated with bare soil. Land should never be burnt and left bare. The first rainstorm may result in the immediate loss of the topsoil. In the same way that a stream will dig itself a bed, so water running off a field will quickly dig down into the lower soil, washing away the fertile topsoil. Erosion problems become severe where the field slopes.

Overcropping

Every plant that grows on a field will take food from the soil. When the farmer plants high densities of crops such as maize, yams, and vegetables, these take a great deal of soil nutrient reserves in order to produce a harvest. Unless these food reserves are replaced, the soil will become gradually poorer until it is able only to sustain very low yields. Formerly this problem was solved by moving to another piece of land, leaving the denuded soil to recover, usually for a period of six or seven years. Where farmers own the land they work this rapidly becomes uneconomic. Any serious attempt at commercial food production must conserve soil fertility.

Overheating

The sun in Ghana is so powerful that it can bake the soil like a loaf of bread. If the soil surface is left bare, as after planting, the heat of the sun can kill off all but the most hardy weeds. Such temperatures will also cause the chemical breakdown of the soil's nutrient reserves.

Leaching

The topsoil contains plant food: nitrogen, phosphorus, potassium, and other nutrients, by binding them closely to soil particles. Some of these compounds, particularly the nitrates, are quickly dissolved in water. Rain water percolating down through the soil will tend to carry plant nutrients with it, out of the reach of crop roots.

Maintenance of soil fertility

Many of these problems can be solved by good management practices. The most important rule is that the soil should not be left bare. Also, burning or hoeing a soil only exposes it to the damaging effects of the sun and rain. Weeds, when killed, should be laid on the soil surface to protect it and prevent it from drying out. The same should be done with crop residues. This practice is called mulching, which is the most important feature of minimum tillage farming.

Soil erosion, which is caused by allowing water to run off bare soil, can be minimized by leaving the roots of dead weeds in the soil. The roots hold the soil together and prevent rain water from prising loose the soil particles. When the roots rot, they leave small holes through which surface water can percolate into the subsoil. The presence of a layer of mulch on the soil surface will also help to minimize erosion. A heavy mulch will prevent the hammer-hard raindrops from eroding the topsoil, and it will slow down the progress of the surface water as it runs down the field. As the water moves more slowly over the soil surface, so more will percolate down into the soil, adding to the water available to growing crops, and less remains to prise loose the top soil.

Increasing soil fertility

A balanced rotation is needed for farmers who continuously crop the same land, to put some of the nutrients back into the soil that the demanding crops take out. Nitrogen-fixing (leguminous) plants should form a regular part of this rotation. The use of a mulch will also help, as it will decompose and enrich the soil.

Mulching

The use of a mulch will buffer the soil against sudden temperature changes. It will also preserve soil moisture during short dry spells in the cropping season.

Improving soil fertility

Generally the two nutrients most limiting crop production in Ghana are Nitrogen and Phosphorus. Every season it will be necessary to provide crops with these compounds. This can be done by chemical fertilizer, or organic manure, or a mixture of both.

Fertilizers

These chemicals supply the main food requirements of crops in concentrated form. Currently three types are commonly available in Ghana. These are: 20:20:0 which contains nitrogen and phosphorus, and ammonium sulphate which is a concentrated form of nitrogen. Urea, another highly concentrated form of nitrogen, may also be available in some areas.

The use of a mulch will encourage crop roots to grow near the soil surface. This allows the farmer to place fertilizer on top of the soil. In conventional agriculture, fertilizer must be incorporated into the soil in order to be taken up by the roots. This represents a considerable saving of time and effort.

Manure

Many farms do not make good use of the natural resources available. Instead of spreading manure onto their soils they will pay money to buy chemical fertilizers which may not be as good. A rich manure provides all that a fertilizer can and more, as it is a soil conditioner. It will add nitrogen, phosphorus, and potassium, as well as trace elements needed by the crop plants. It also contains organic matter which enables the topsoil to bind these nutrients, and enhances root growth.

Manure from any animal can be used. Chicken manure is one of the best, but sheep, goat, pig and cow manure are also good. When using manure containing a lot of bedding sawdust or straw, this should be allowed to rot down before it is applied onto the field. Cow manure also should be allowed to rot well. This will guard against the germination of noxious weed seeds of plants such as Rottboelia exaltata (Twi-nkyenkyenma) which the cattle may have eaten. Compost is similar to manure in its benefits though it takes some effort to prepare. (See Appendix 1 for details on how to make compost.)

Land reclamation

Sandy soils often yield poorly because they are unable to bind nutrients tightly. Consequently, leaching is severe. Such soils require organic matter, and will benefit most from applications of manure and least from chemical fertilizer. A long-term method of improving sandy soils is to work clay or silt into the surface. The opposite is true of clay soils where sand spread over the soil surface will gradually improve the soil quality. If the topsoil has been removed from patches of the field, by erosion or by machinery, leaving exposed subsoil, these areas may be reclaimed by applying heavy doses of green mulch or manure, and then ploughing or digging this in. The ploughing will loosen up the subsoil and the organic matter will improve the structure. The field can then be farmed using minimum tillage, which will prevent recompaction, and large annual doses of manure or compost.

Drainage

Most crop plants cannot grow when the land is waterlogged for even part of the growing season. Swampy land can sometimes be salvaged by drainage, digging and regularly clearing ditches to remove standing water. These ditches should run directly to the lowest portion of the field. Where the field is very flat, the ditches can be gently sloped to prevent silting. Raised beds can be used to achieve the same effect as ditching.

Weed Control

The immediate practical problem which faces the farmer is the control of weeds. This problem has to be tackled before the farm can be established. The advantages and disadvantages of six common methods of weed control are discussed below.

Hoe and cutlass

This traditional method is often used in conjunction with burning. The bush is first cutlassed down and left to dry for a short period before being burnt. At planting, the soil is loosened by hoeing, when any emergent weeds are cleared up. To farm well and to provide the crop with a weed-free environment is a daunting undertaking. One farmer can maintain about 0.3-0.4ha (0.75 acre) of land weed-free using a hoe and cutlass. The limitations of this scale mean that the farmer relying on the hoe for weed control cannot produce substantial amounts of food without massive amounts of labour.

Burning

This is often the only practical way the farmer has of clearing sufficient land to plant. It is quick and easy. The burnt residue of the vegetation provides some phosphate and small amounts of minerals, but all the organic matter and various nutrients are lost. Burning is obviously a one-off technique, it cannot be used to weed the young crop. Usually a hoe is used for this. Regular burning encourages the growth of resistant weeds, particularly Imperata cylindrica (Spear grass), which is difficult to get rid of after it has infested cultivated land.

Mechanical and animal-drawn tillage

Ploughing, harrowing and rotovating are all weed control measures, and if done well can be very effective. Tillage demands that the field should have been thoroughly cleared and all roots removed. It enables a much larger area to be made available for planting than is farmed by traditional farmers. On many farms where the tractor is hired or where a plough is the only available equipment, subsequent weedings have to be done either with chemicals or using manual labour, which will be very costly.