Gneiss and steatite vermicomposted with organic residues: release of nutrients and heavy metals

Purpose Use of silicate rock powders as fertilizer improves nutrient cycling, thus benefitting agriculture. The availabilityof nutrients to plants is often low, but can be increased when rock powder is vermicomposted and added. However, thesepowders can be rich in heavy metals, which may impair their use. We evaluated maize growth and heavy metals in plantsand soil after fertilization with vermicomposted gneiss or steatite powders.Method Vermicompost was prepared with cattle manure, with or without gneiss or steatite powders. The experimental unitswere kept in the dark at room temperature for 70 days. Subsequently, a greenhouse experiment was carried out with maizegrown in highly weathered oxisol soil and fertilized with vermicompost alone, or with gneiss or steatite powder.Results A small proportion of heavy metals was immobilized in the earthworm bodies, but did not hinder their growth.Maize growth was superior in the treatment with vermicomposted gneiss powder. The gneiss-enriched treatment contributedto increased Zn concentration in plants and may also be an alternative to Zn fertilization. High Ni and Cr concentrations insteatite powder apparently induced higher levels of these elements in plants. However, metal concentrations in the soil aftercultivation in all the treatments were below acceptable limits.Conclusion Increased plant growth in gneiss-enriched vermicompost suggests the possibility of using this material to enrichvermicompost with nutrients, thus improving the chemical quality of organic fertilizer. Use of steatite powder in agriculturedeserves further investigation.