The invention relates to use of (a) compounds of phosphorus containing at least partly lipophilic organic radicals together with (b) urea and/or urea derivatives in selected quantity ratios of (a) to (b) as a soil additive for biologically controlling plant-parasitic soil nematodes by strengthening the correspondingly antagonistic soil potential, more particularly by strengthening the growth of antagonistic and/or nematicidal rhizosphere bacteria and/or corresponding mycorrhiza strains—and at the same time strengthening the growth of cultivated plants in soil. The multicomponent mixtures which strengthen bacterial growth are preferably introduced into the region of the plant roots in the form of an aqueous preparation using plant-compatible emulsifiers before and/or during plant growth. Components (a) and (b) are used in such quantity ratios that the ratio by weight of C to N is no higher than 6:1 and preferably no higher than 5:1.