Brassica nigra defatted seed meal (DSM) and liquid formulation were added to three different soils in a pot experiment to investigate its effect on the growth of melon, on soil microbial size and on six enzyme activities. DSM was added as a pre sowing treatment; the liquid formulation, based on B. carinata oil and B. nigra DSM, was then repeatedly applied as fertilizer with allelopathic activity.