tBrassica nigra defatted seed meal (DSM) and liquid formulation were added to three different soils in apot experiment to investigate its effect on the growth of melon, on soil microbial size and on six enzymeactivities. DSM was added as a pre-sowing treatment; the liquid formulation, based on B. carinata oil andB. nigra DSM, was then repeatedly applied as fertilizer with allelopathic activity. Brassica materials werealso evaluated in combination with the melon seed treatment with an isolate of Trichoderma harzianum.This beneficial fungus was previously selected for tolerance to toxic compounds released by brassicaDSM. At the flowering stage, brassica treatments determined a doubling in shoot dry weight in all soils;this was paralleled by an increase of both soil microbial biomass (+46%) and enzyme activities, up to+90%. The combination of the applications of brassica materials and seed treatment with T. harzianumalso increased root length by 20%. These effects were most probably due to enhanced microbial activityand enhanced biogeochemical cycles of nutrients caused by the treatments with brassica materials andthe stimulation of root activity by T. harzianum.