ILEIA Newsletter • 1.2 • March 1985
Integration of farming systems in the world economy puts a stress on possibilities for maintenance of soil fertility under 'low external input' circumstances, as increased losses by export of farm products have to be compensated. Some alternatives to fertiliser use may, however, be present, depending on the local situation. Several non-agricultural "developments" interfere with traditional techniques for maintaining soil fertility. If sufficient labour is available, losses from the system can be reduced and recirculation increased. Contrary to the high-input fertiliser option, no general "green-revolution" recipes are possible. A solution for each situation will have to start with an analysis of strong and weak points of the present situation.
Agriculture can be ‘mining’, using land for reaping a temporary profit without bothering about the future. Another extreme is to use the land as a ‘waste-dump’, enriching the soil with potentially useful elements, but in such an excessive way, that eventually they can become toxic. On a world scale the two types of agriculture are coupled at the moment, import of large amounts of animal feed into Western countries leading not only to soil exhaustion in exporting areas, but also to pollution of soil and groundwater in importing areas. Obviously, a system in between these extremes, maintaining -and possibly improving -soil fertility in a form of 'sustainable' agriculture, offer us the only way to survive on this 'only one earth'. Two options for maintaining soil fertility seem to be open: a ‘high external input’ option, which uses large amounts of (fossil) energy and other limited mineral resources for the manufacture and transport of fertilisers to maintain the above-mentioned nutrient flow) and a ‘low external input’ option, which often involves a high input of local labour to use a variety of practical solutions to reduce the losses from the nutrient cycle and to 'harvest' additional nutrients from the environment. Of course, mixed strategies using parts of bath options are possible, but for the sake of clarity we will concentrate on these two types here.
This figure shows a nutrient flow scheme, relating agricultural production to consumption in towns, without recycling. It is also a nutrient game! It can beplayed by 2-10 players with 1 die. Every player acts as a mineral and starts in the soil. Each turn, the number on the die shows your path through the cycle: you can be taken up into plants, washed into the subsoil, etc. The game ends when nobody can continue playing.
'High external input' systems can only be economical when a high financial output is guaranteed. Therefore, it is no wonder that fertiliser use in developing countries is often restricted to cash crops for export (coffee, cotton, tobacco, etc.), as both the farmer and the government of the country need a cash income (or hard currency) to buy fertilisers from the (world) market (see 2). Fertiliser use is linked economically to integration with cash economies, just as it is linked ecologically to irreplaceable losses from the local nutrient cycle by export of agricultural products. In subsistence farming (local consumption in the village) or for 'self reliance' (consumption in the towns) often no money is available for expensive inputs such as fertilisers. 'Low external input' solutions may be the only choice for farmers in such circumstances, provided there is no shortage of labour. In traditional farming systems a variety of techniques exists for managing the soil. In many cases the soil is temporarily and locally depleted in a form of 'shifting cultivation', but as no farm products leave the area, a nutrient cycle is still functioning. Nutrients are being concentrated around the homesteads, until the village is moved to a new place and natural vegetation can recover the space and use the nutrients stored. Within this period of 'village shifting' it is customary to have a 'field shifting', leaving the land fallow after a few years' cropping.
Problems in shifting cultivation systems have arisen recently, because for many reasons the decision to shift the village has become more difficult: the vicinity of roads and/or the presence of permanent services in the form of wells, school s, medical posts can all lead to local overexploitation of the land, even if sufficient land is still available at some distance. Increasing population densities generally lead to a reduction in time available for the restorative action of a fallow (forest) vegetation. In this intensification process a critical point exists, beyond which the decrease of soil fertility becomes self-stimulating, as lower yields will lead to a higher demand and even shorter duration of the fallow, etc. In practice, however, some possibilities exist to counter such effects. By employing more labour, soil fertility of the cropping fields can be increased, e.g. by improving the systems of collecting organic debris from the village or by collecting mulch material from neighbouring land. Historically, such techniques have mainly been developed in places where arable land was short in supply. An example is found in the flood plain along the river Nile in the Sudan, where much low-lying grazing land can be used for keeping cattle, concentrating manure on the limited higher grounds around the homesteads for growing crops (see (1) and (3) for further references). The constant supply of sediments to the grazing lands by the rivers allows such a system to be stable for thousands of years. The sediment load of rivers has been used for permanent agriculture in a more direct way, for example in the famous Nile-delta agriculture, which persisted for thousands of years at reasonable yield levels, and in the traditional sawah-system of Southeast Asia. The latter system is labour-intensive to the extent that almost any amount of extra labour spend on improving the water management and dyke system may result in sufficient increase in yield to maintain the extra labourers, thus accommodating the large increase in population, be it on a low level of material wealth. New and modernised techniques for 'low external input' farming will have to draw upon the traditional techniques for each specific environment and may try to avoid weak points and better exploit strong points. We will now briefly discuss three aspects of maintaining soil fertility:
A. Maintaining fertile soil layers.
B. Reducing nutrient losses.
C. Obtaining extra inputs.
A. Maintaining fertile soil layers
Erosion of the topsoil can be so much faster than depletion of nutrients, that any discussion on maintenance of soil fertility has to start with prevention of erosion. Erosion requires human activities at the field level (terracing, contour planting, windbreaks planting, etc.), but, on a more detailed level, it is also related to soil structure. Possibilities for infiltration of rainwater determine the amount of surface run-off, which is the main cause of erosion in intensive-rainfall areas. A good surface structure of the soil can be maintained by having a permanent soil cover through mixed cropping, the use of leafy green manures and/or ample mulching. Topsoil structure can be spoiled by heavy machinery on the land, by soil tillage and by some types of fertiliser, e.g. the classical Chilesalpeter (sodium nitrate) on clay soils. Termite activity in the soil can be important for keeping the soil porous and hence less susceptible to erosion. The use of pesticides which kill termites can in this respect be harmful (although some termite species can damage crops to such an extent that some control is needed).
B. Reducing nutrient losses
Losses of nutrients from the topsoil in arable land are of various kinds. Leaching of nutrients to lower layers of the soil in (temporary) periods of excess of rain-fall over evapotranspiration can be an important loss, depending on the climate (rainfall distribution), soil type and the nutrients concerned (degree of buffering soil nutrients) and on the crop's root system. Intense root branching in the topsoil may effect the uptake before nutrients are leached; deep root development enhances the recovery of leached elements. As in tropical climates the start of the rainy season often induces a 'flush' in the mineralisation of soil organic matter, early planting to make use of flush is an important way to reduce losses due to leaching. Usually, labour is in short supply in this period. Raving deep-rooted crops in the mixed cropping field is equally important. Deep rooting trees can take up (leached) nutrients from deeper layers and via leaf fall (mulch) concentrate them in the topsoil. In the case of nitrogen losses, soil pH is important as a low soil pH may inhibit deep root development, but at the same time causes much of available soil nitrogen to be in the ammonium form, which is less subject to leaching than the nitrate form occurring at higher soil pH. Applying lime may increase the risk of leaching of nitrogen, as well as the chances of recovery due to a deeper root development (the cost-benefit balance cannot be predicted in general terms).
Nutrient losses to the air are potentially important as well. Nitrogen can be lost by the process of denitrification in wet soils (in the presence of nitrate and organic matter and absence of oxygen), especially in rice fields larger losses occur in this way. Burning the remains of forest vegetation after land clearing in shifting cultivation can be useful for the farmer as it increases the short-term availability of most nutrients in the ash, but it can result in losses up to 100% of the nitrogen content of the vegetation (and even some of the soil nitrogen if the burn is to hot). Incomplete burns, which may be better from a point of view of nutrient losses, may inhibit subsequent farm work and thus require a higher labour input.
A third type of nutrient losses from the soil is in fact desired by the farmer: the removal of harvestable products. Whether or not this is a definite loss depends on the destination of the products and on the amount of recirculation. If the products are used for local consumption the return to the land of the nutrients stored in kitchen and human waste is possible (e.g. old pit latrines can be subsequently used for planting exacting crops such as bananas homestead sweepings can be deposited on the nearby vegetable garden, etc.) If the products are used as animal fodder, the manure produced can be used to concentrate nutrients on parts of the arable land used most intensively. The use of manure and crop residues for fuel for cooking conflicts with this type of use. If the farm products are sold to the nearby town, a return flow in the form of town refuse compost and night soil is feasible, although usually such products are used mainly in intensive horticulture near the towns, depleting the land further away. 'Modern' sewage and waste treatment systems, following the Western fashion generally preclude the possibilities for such recirculation. If farm products are exported, no return of the nutrients is possible, and especially for 'bulky' animal feeds such exports are in fact devastating for the exporting countries (tapioca from Thailand, soybeans from South America and cottonseed-cake from Africa being well known examples).
C. Obtaining extra inputs.
Inputs of nitrogen occur in the form of low amounts of nitrate in rainfall (the result of lightening, and recently increased to considerably higher amounts due to air pollution in industrialised countries). Certain groups of micro-organisms are able to fix nitrogen from the air in plant available forms, especially when in symbiosis with green plants (which supply energy to the micro-organisms). Leguminous crops with root nodules, the floating Azolla-ferns in marshes and sawahs and free-living micro-organisms in the vicinity of plant roots can all fix substantial amounts of nitrogen, under favourable conditions. Nitrogen fixation is often limited by external factors such as a low availability of phosphate (of which bath leguminous crops and Azolla have a rather high requirement). Mineral nutrients can be obtained from geological processes such as weathering and erosion. In young volcanic soils, weathering may yield significant amounts of nutrients, provided a deep-rooted vegetation makes efficient use of them. In many landscapes the sediment load of rivers resulting from erosion elsewhere in the catchment area is an important input to agricultural land, the sawahs being the best example. Continued production in the sawahs therefore depends on the continuous, slow erosion of hilltops. An increased rate of erosion might give temporary gains (although flooding problems are more likely to occur), but certainly spoils the system in the long run. The construction of dams for irrigation is a major obstruction to soil fertility maintenance as siltation of the reservoir replaces fertilisation of temporarily flooded riverbanks.
Several favourable aspects are combined in certain tree crops which can be used for intercropping in agroforestry: a deep root system, nitrogen fixation (e.g. Prosopis in dry lands, Leucaena in humid tropics and Sesbania in weylands) and the ample production of mulch material to keep the soil covered. In other cases, the phosphate requirement of leguminous crops can be the bottleneck. If locally available, raw rock phosphate -with low direct availability to most crops- can sometimes be used by leguminous crops. The leguminous crops create an acid root environment, comparable to the industrial process of making "super-phosphate". Formation of "mycorrhiza" (root-fungus symbiosis) may help in this respect.
Meine van Noordwijk
Almastraat 1, Groningen, the Netherlands
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