RESOURCE CAPTURE AND UTILIZATION IN INTERCROPPING - NON-NITROGEN NUTRIENTS

The capture and utilization of P and K, two non-mobile soil resources, were examined by decomposing crop production/unit area into uptake/un it area (capture) and production/unit uptake (utilization efficiency). Resource capture and utilization efficiencies by intercrops were comp ared to those of sole crops by contrasting intercrop means against the weighted means of sole crops. Weightings were based on the proportion of each species in the intercrop. On average, intercrops took up 43% more P (-4 to 83%) and 35% more K (-10 to 87%) than the sole crops. Wh ere nutrients were not limiting, P and K uptake increased as dry-matte r yield increased, evidence that uptake was a function of crop growth rather than conversely. Even where soil P was deficient, uptake by a d ominated crop was decided more by factors that determined the outcome of competition than by P availability. The combined root systems, like ly to be larger and functional for a longer duration under intercrops than under either sole crop, were postulated to explain the greater ca pture of non-mobile nutrients like P and K. An enlarged root system pr ovides an expanded root surface area to which non-mobile nutrients can diffuse. For mobile nutrients which move to root surfaces largely by mass flow, shading of the dominated canopy may explain the positive as sociation observed between intercrop dry-matter accumulation and Ca ca pture. Shading reduces carbon assimilation and transpiration of the un derstory canopy and, therefore, would reduce mass flow of Ca as well. Competition for P and K by intercropped species, even when nutrient su pplies were abundant, was affected by degree of growth concurrency, ca nopy domination and planting geometry. When a dominating species was h arvested much earlier than the dominated species, the uptake rate of t he longer-duration crop usually recovered from competition after the f irst crop was harvested. Exceptions occurred when development of the d ominated crop was so badly impaired during concurrent growth that vege tative enlargement was inhibited after harvest of the early species. U ptake rates by pigeon pea and cassava, which are exceptionally long-ma turing species that have relatively low daily P and K uptake requireme nts, appeared capable of fully recovering even on soils in which nutri ent availabilities were marginal for other species.