The invention relates to the combined use of (a) phosphorus compounds with at least partially lipophilic organic radicals and (b) urea and/or urea derivatives in selected proportions of (a):(b) as soil additives for biological control of phytoparasitic earth nematodes by increasing the correspondingly antagonistic potential of the soil, especially by promoting the growth of antagonistic and/or netamaticidal rhizospheric bacteria and/or corresponding mycorrhiza strains, while at the same time strengthening the growth of autochtonal plants. Preferably, the substance mixtures promoting bacterial growth are introduced into the root area of the plants in the form of an aqueous preparation in combination with phytocompatible emulsifiers before and/or during the raising of said plants. The components of subclasses (a) and (b) are used in such proportions that the weight ratio of C to N does not exceed 6 and is preferably 5.