Incorporation and maintenance of organic matter in the soil is beneficial for improving soil structure and moisture retention, and will encourage natural enemies of root-knot nematodes (biological control).  Biological control agents, primarily bacteria and fungi can also be purchased in formulations that can be added to the soil.  Soil fungi, like Purpureocillium lilacimun and Pochonia chlamydospora are known to reduce root-knot nematode populations by infecting eggs and egg masses.  The effectiveness of bacterial formulations, like those including Pasteuria spp. will vary among root-knot nematode species.