Microbial Dynamics and Nutrient Mineralization in Soil Amended with Cacao Pod and Water Hyacinth Composts: Implication for Nitrogen Fixed by Soybean

Composting organic wastes can be relatively cheap and environmentallyfriendly sources of mineralizable nutrients for legumes. We amended 5 kg topsoil (0-15 cm) samples of an Ultisol (Lixisol) with 0, 2.5 and 5 t ha(-1) each of cacao pod (CC) and water hyacinth (WC) composts with and without starter fertilizer (F) (25 kg N ha(-1) as urea and 26 kg P ha(-1) as single super phosphate). The amended soil samples were used for 16-week laboratory incubation experiment and two-consecutive 8-week plantings of soybean (Glycine max. L. Merril) in the screenhouse. Measurements were carried out on some indices of microbial activity, namely, microbial respiration, N and P mineralization as well as on nodulation and N-2 fixed by soybean. The microbial respiration, and N and P mineralization increased with application rates of the composts, but generally declined with incubation duration. Applications of 5 t ha(-1) each of CC and WC with or without F increased (p <.05) the microbial respiration (24-93%), N mineralized (21-35%), and P mineralized (32-180%) during the incubation period. The highest of these increments was, however, obtained with WC. Increases of 1162 and 981% (CC2F and WC2F) and 28% (CC2F) were also recorded in nodulation and the amount of N-2-fixed, respectively. It was concluded that 5 t ha(-1) water hyacinth or cacao pod composts with starter fertilizer is capable of enhancing soybean N-2 fixation by serving as mineralizable nutrient sources prior to the onset of nodulation and N-2 fixation.