SOIL IMPROVEMENT BY TREES IN SUB-SAHARAN AFRICA

Trees can influence both the supply and availability of nutrients in t he soil. Trees increase the supply of nutrients within the rooting zon e of crops through (1) input of N by biological N-2 fixation, (2) retr ieval of nutrients from below the rooting zone of crops and (3) reduct ion in nutrient losses from processes such as leaching and erosion. Tr ees can increase the availability of nutrients through increased relea se of nutrients from soil organic matter (SOM) and recycled organic re sidues. Roots of trees frequently extend beyond the rooting depth of c rops. Research on a Kandiudalfic Eutrudox in western Kenya showed that fast-growing trees with high N demand (Calliandra calothyrsus, Sesban ia sesban and Eucalyptus grandis) took up subsoil nitrate that had acc umulated below the rooting depth of annual crops. Sesbania sesban was also more effective than a natural grass fallow in extracting subsoil water, suggesting less leaching loss of nutrients under S. sesban than under natural uncultivated fallows. Nutrient release from SOM is norm ally more dependent on the portion of the SOM in biologically active f ractions than on total quantity of SOM. Trees can increase inorganic s oil N, N mineralization and amount of N in light fraction SOM. Among s ix tree fallows of 2- and 3-year duration on an Ustic Rhodustalf in Za mbia, inorganic N and N mineralization were higher for the two tree sp ecies with lowest (lignin + polyphenol)-to-N ratio (mean = 11) in leaf litter than for the two tree species with highest ratio (mean = 20) i n leaf litter. Trees can also restore soil fauna, which are important for SOM and plant residue decomposition. Some agroforestry trees have potential to provide N in quantities sufficient to support moderate cr op yields through (i) N inputs from biological N-2 fixation and retrie val of nitrate from deep soil layers and (ii) cycling of N from plant residues and manures. The cycling of P from organic materials is norma lly insufficient to meet the P requirements of crops. Sustained crop p roduction with agroforestry on P-deficient soils will typically requir e external P inputs.