Rationalising research on hedgerow intercropping: an overview

The paper discusses some of the background issues to the plant-environment interactions that affect hedgerow intercropping in particular, and agroforestry in general. Putting forward various sets of conclusions that indicate where critical research problems lie. Hedgerow intercropping is one form of zonal agroforestry in which plant residues (from the hedge) are utilized to sustain crop production. Some comparative examples from tropical agriculture research are given of the effect on crop yields of applying organic matter to the soil. The need to main a balance of soil available nutrients is emphasised. In many systems this has involved using some fertilizers. Relatively large and consistently-applied amounts of plant residues are usually needed in order to improve the normally-measured soil chemical and physical parameters. A summary of three extensive reviews of tree planting in the tropics is given. These highlight the fact that continuous cropping on most tropical soils brings about "long term" soil deterioration. Tree clearing can cause major problems, but even in the "maximum production phase" nutrients are lost from the system. Any kind of cropping which removes a high proportion of the plant biomass can degrade soils. However, hedgerow intercropping in high rainfall areas (>1000 mm. p.a.) and in reasonably fertile soils (Alfisols) does, so far, appear to maintain crop yields. It is suggested that we need to know more about the "short-term" environmental effects of using plant residues that can help bring this about if we are to be able to extend the practice to other environments. In dry regions, hedgerow intercropping may have an important function in preventing soil erosion and rainfall run-off. The ability of individual tree species to enrich their microsite is discussed, but the rather slow rate at which this occurs should be noted. Factors involved are commented on. When trees are grown in some spatial arrangement to cover just a portion of the ground (as in hedgerow intercropping) their effects on the yield of adjacently-grown crops appears to be much greater than that resulting from the "equivalent" coverage in time when trees/bushes are used to improve soil fertility through a fallow phase, or by growing plot of trees in a rotation. A computer model available at ICRAF ("LANMODEL") helps to expose this paradox. Again, mixing trees and crops may offer a greater opportunity for the short-term environmental benefits, both aerial and adaphic.

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