Paul Richards (1983) Ecological Change and the Politics of African Land Use. African Studies Review 26:1-72.

• Exposure of the soil to erosive rainfall is minimized (especially where quickgrowing and slower-maturing varieties are planted together, or sequential planting is practiced);
  
• Spread of pests and diseases is minimized (neighboring plants are less likely to be of the same species);
  
• Use of available soil moisture and plant nutrients is maximized. Different plants, e.g. maize and cowpeas, have different complementary requirements, and root at different depths;
  
• Suppression of weeds at later stages in the cropping sequence is achieved through competition by established crops approaching harvest (weeding is a major constraint in tropical farming systems, more often determining farm size than labor constraints in clearing and cultivation);
  
• Plants with different growth characteristics and leafing patterns may be able to combine to maximize use of available sunlight, rather than compete, as would be the case where plants of a single species or variety were planted in denser stands;
  
• Risks of crop failure (due, for example, to drought) are minimized, because farmers mix varieties and species with different speeds of maturation and moisture requirements. Although subdividing the farm into several monocropped sections might achieve a similar result, intercropping offers significant operational advantages. It may be possible to sow insurance varieties in the same operation as the main crop. Gaps in the farm caused by the failure of some individual seeds to germinate might be plugged by adding in a hardier variety at a later stage. Combinations of plants can be varied to match variations in soil conditions (up and down slopes for example).

All in all, perhaps the greatest significance is the scope intercropping offers for a range of combinations to match individual needs and preferences, local conditions, and changing circumstances within each season and from season to season. In some cases, psuedo- rotation (response to declining fertility through intercropping adjustments) greatly extends the life of an individual farm plot in rotational fallow farming systems. The following may be cited as examples of the measured benefits of intercropping.

Michael Johnny (1979), on the basis of yield plot surveys on 31 upland (shifting cultivation) farms in southern Sierra Leone, found, on the average, 21 intercrop species per farm plot. Crops in addition to the main crop (rice) accounted for 53% of the market value and 73% of the food energy value of the total output of such farms. Yield figures for traditional rice farming systems in West Africa (c. 1000kg/ha.) frequently omit estimates of the value of intercrops. Irrigated rice farms may double rice yields, but produce no intercrops. In addition they require greater labor inputs. Several recent studies suggest that "improved" irrigation farming in Sierra Leone enjoys no clear cut advantage over intercropped upland shifting cultivation, although the latter might perhaps be suitably modified in areas of land shortage by introduction of high yielding rice varieties and chemical fertilizer (Karimu and Richards, 1981; Lappia, 1980; Spencer, 1975).

Maize yields in an experimental study of cowpea/maize intercropping in western Nigeria were more than double those from an equivalent number of stands of monocropped maize with improvement in cowpea yields as well (Table 2). Much of this improvement appears to have resulted from a reduction in pest damage in the intercropped plots (Ajibola Taylor, 1977; Norton, 1975).

In Norman's (1972; 1977) study of three villages in Zaria Province, northern Nigeria, farms where groundnuts were intercropped with grains gave returns to labor greater than 30% higher than monocropped groundnuts (Table 2). Monocropped grains gave slightly better returns to labor than grains in intercrops, but in this case the variability of the results appears to be smaller than in the case of the monocropped farms (coefficients of variation, 9% and 16% respectively). These data suggest that farmers' returns might tend to be more reliable with intercropping. Risk minimization is generally a more important criterion for small farmers than profit maximization (Low, 1975).

Intercropping, then, is one of the great glories of African science. It is to African agriculture as polyrhythmic drumming is to African music and carving to African art. It will be combined with mechanization only with difficulty, and is contrary to the ethos of agricultural commodity production under capitalism, which insists that agricultural ecologies operate only in their most attenuated form. But it is capable of producing remarkable results-labor productivities perhaps equal to if not better than plough agriculture in European peasant agriculture-and nowhere more so than in that intermediate zone where conditions are most dominated by intense, yet variable, rainfall and limited sunlight and the height of the growing season. It is here, where irrigation and the plough have least to offer, that intercropping techniques have been developed to their highest degree of complexity. Igbozurike (1977) insists therefore that intercropping should be seen to be, in effect, the heart of an African agricultural revolution, and that it must (and will) remain central to the furtherance of that revolution