In a pot experiment using a strongly P-fixing Andosol from Nicaragua, the effects of sugarcanefilter cake application on the growth of white mustard (Sinapis alba L.) were compared with those of 13 C-labeled pea residues. The application of pea residues led to a 50% increase and the application of filter cake to a 30% decrease in soil organic matter-derived microbial biomass C compared with the control. In contrast, the application of filter cake resulted in a four times higher content of substrate-derived microbial biomass C than that of pea residues. The application of organic substrates generally increased microbial biomass N. Mustard growth led to significant increases in microbial biomass P in the control, but also in the organic-amendment treatments, which always resulted in decreased microbial biomass C : P ratios. Mustard growth also led to increased contents of Bray-1-extractable P, but this increase was only significant in the filter cake treatment. The application of pea residues had no effect on the yield of shoot C, but a positive effect on the yield of root C in comparison with the nonamended control. In contrast, the application of filter cake significantly depressed yields of shoot C and root C, due to N immobilization, presumably due to the high concentration of lignin.