CONTRIBUTION OF AGROFORESTRY TREES TO NUTRIENT-REQUIREMENTS OF INTERCROPPED PLANTS

A major tenet of agroforestry, that trees maintain soil fertility, is based primarily on observations of higher crop yields near trees or wh ere trees were previously grown. Recently objective analyses and contr olled experiments have addressed this topic. This paper examines the i ssues of tree prunings containing sufficient nutrients to meet crop de mands, the timing of nutrient transfer from decomposition to intercrop s, the percent of nutrients released that are taken up by the crop, an d the fate of nutrients not taken up by the crop. The amount of nutrie nts provided by prunings are determined by the production rate and nut rient concentrations, both depending on climate, soil type, tree speci es, plant part, tree density and tree pruning regime. A large number o f screening and alley cropping trials in different climate-soil enviro nments indicate that prunings of several tree species contain sufficie nt nutrients to meet crop demand, with the notable exception of phosph orus. Specific recommendations for the appropriate trees in a given en vironment await synthesis of existing data, currently only general gui delines can be provided. Tree biomass containing sufficient nutrients to meet crop demand is not enough, the nutrients must be supplied in s ynchrony to crop needs. Nutrient release patterns from organic materia ls are, in part, determined by their chemical composition, or quality. Leguminous materials release nitrogen immediately, unless they contai n high levels of lignin or polyphenols. Nonlegumes and litter of both legumes and nonlegumes generally immobilize N initially. There is litt le data on release patterns of other nutrients. Indices that predict n utrient release patterns will assist in the selection of species for s ynchronizing with crop demand and improve nutrient use-efficiency. Fie ld trials with agroforestry species ranging in quality show that as mu ch as 80% of the nutrients are released during the course of annual cr op growth but less than 20% is captured by the crop, a low nutrient-us e efficiency. There are insufficient data to determine how much of the N not captured by the crop is captured by the trees or is in the soil organic matter, the availability of that N to subsequent crops, or ho w much of that N is lost through leaching, volatilization or denitrifi cation. Longer term trials are needed.