Abstract
In the past few decades there has been growing concern about the exhaustibility of non-renewable soil resources in developing countries in the tropics to meet the needs of present and future generations. Land degradation is a major problem in many parts of the tropics, including subSaharan Africa, mainly owing to overexploitation of vegetation and soil resources and adoption of inappropiate farming methods. The challenge is to increase the sustainable agricultural productivity of the land with acceptable inputs to meet increasing human needs, while maintaining the soil resource base and minimising environmental degradation. One technology that can meet this need for rainfed upland farming with low external chemical inputs is the alley cropping system. This technology integrates trees and shrubs in spatial zonal arrangements with food crops in the production system. The presence of woody species in the alley cropping production system has been shown to contribute to (1) nutrient recycling, (2) reduction in soil nutrient leaching losses, (3) stimulation of higher soil faunal activities, (4) soil erosion control, (5) soil fertility improvement and (6) sustained levels of crop production. These experiences can be utilised in developing sustainable and environmentally friendly agroforestry systems.
Ical inputs, to meet the ever increasing need for food animal feed fibre, and fuelwood has already ready resulted in accelerated land degradation and loss of soil productivity (stoorvogel et al 1993 tolba1993) despite many decades of research on the intensification of agriculture on rainfed uplands in the humid and subhumid tropics efforts to develop altemative sustainable and more productive food crop production systems have far had only limited success (Kang et al 1991)
Traditional rainfed upland farming in the tropics is mainly practsed in areas dominated by either high base status alfisolf or acid and low base status oisols and ultisols all are fragile soils with moderate to very low fertility status results of many years of research have shown that the following
Management components are needed to maintain the productivity of these soils
1.maintenance of adequate soil cover and organic matter in the surface soil
2.minimal surface soil disturbance
3.judicious fertilizer and lime use
4.intercropping
5.fallowing and crop rotation.
Although it is known that external chemical in puts are needed to achieve sustained high crop yields their availability and cost (without subsidies) may be prohibitive for their use in food crop production in many developing countries.
In addition continuous use of only high external chemical inputs can result in increased land degradation on poorly buffered upland soil (Pichot et al 1981 Kang and Balasub ramanian 1990) It is therefore imperative that alternative integrated soil and nutrient management systems are developed that can maximize incorporation of biological nutrient sources and organic material.
This will also allow lower use of external nutrient inputs.
Thus there is increasing interest in developing alternative production systems that involve the incorporation of improved herbaceous and woody fallows.
The main aim has been to develop sustainable and more productive food crop production systems using low external chemical inputs as alternatives to the traditional farming systems one such system that incorporates the use of woody species is the alley cropping system.
This paper will elaborate on the merits of alley cropping in soil productivity maintenance and nutrient recycling on high base status soil.
2 Alley cropping technique
Alley cropping also known as hedgerow intercropping and contour farming is a zonal agroforestry subsystem that involves the intercropping of multipurpose trees and shrubs (MPTS) and food crops.
In alley cropping the MPTS are grown in managed hedgerows and the crops are grown in all (Fig. 1)
This zonal agroforestry arrangement allows the integration of a wider range of woody species than mixed agroforestry systems because the spatial arrangement of plant components limits intimacy.
The arrangement also facilitates more flexible management options to reduce competition and increase complementarity between trees and crops (Kang and Wilson 1987).
During the cropping period the hedgerows are periodically pruned to minimize shading of the companion crops.
When there are no crops in the field the hedgerows are allowed to grow freely to cover the land.
The technique is scale neutral.
Although originally developed for smallholder family farms it can also be mechanized for large scale farming.
The concept of alley cropping which is sometimes also called improved bush fallow system is based on the following principles.