Radiation use efficiency of sole (cereal) crops, for instance, is found to be equivalent to radiation use efficiency of that crop in intercropping, while the radiation use efficiency of legumes in intercropping is often higher than in sole cropping (Tsubo and Walker, 2002). Marshall and Willey (1983) found intercropping gave higher biomass (28 percent) per unit of land than the same species grown separately. Millet in intercropping intercepted 2.1 times more radiation and used it with the same efficiency, hence produced twice as much biomass. Groundnut intercepted 27 percent less radiation but used it with 47 percent higher efficiency to give the same yield. Some of these gains in total yield per unit area can be explained by the plant's flexibility in maintaining leaf area development, photosynthetic capacity and yield when subject to strong competition for light (Keating and Carberry, 1993).
Many agronomic studies have improved soil water storage by groundcover and tillage practices and enhanced water use by combining crops. Inter-cropping is found to strongly affect the use of water by the crops. Water capture by intercrops hardly differs from water capture by sole crops, while water utilization efficiency of intercrops exceeds that by sole crops by 18-99 percent. These gains are due to various reasons: a larger portion of evapotranspiration may be captured for transpiration, due to the faster canopy closure; the dominant species with an inherently high water utilization efficiency, such as use of C4 plants, may occupy a larger portion of both above- and below-ground resources, resulting in higher overall water utilization efficiency; finally, a favourable micro-climate of shading and vapour pressure, created for the shorter crop, may reduce transpiration (Morris and Garrity, 1993).
Overall, intercrops take up some 40 percent more phosphorus (-4 to 83 percent) and potassium (-10 to 87 percent) than sole crops. The increased uptake is more likely from the increased growth rate of the crops rather than increased availability. The larger and better functioning root systems resulting from improved growth probably explain the greater capture of non-mobile nutrients like phosphorus and molybdenum. On the other hand, species which are dominated may have reduced uptake of mobile nutrients, such as calcium, due to reduced mass flows as a result of reducedtranspiration (Morris and Garrity, 1993).
Finally, intercrops are often less damaged by pest and diseases than those grown as sole crops, but effectiveness is unpredictable. Three mechanisms may prevent an attack, and all reduce the population growth rate of the attacker: plant associations make the intercrop combination a poor host; the combination interferes directly with the activities of the attacker; the altered environment favours natural enemies. The large variability in the impact of intercropping on pest and disease incidence and yield allows only a few generalizations (Trenbath, 1993). Certain crop combinations can for instance exacerbate a disease situation rather than reducing it. At the same time, the ability of diversified systems to suppress pest and diseaseincidences deserves specific attention.
