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

Use of silicate rock powders as fertilizer improves nutrient cycling, thus benefitting agriculture. The availability of nutrients to plants is often low, but can be increased when rock powder is vermicomposted and added. However, these powders can be rich in heavy metals, which may impair their use. We evaluated maize growth and heavy metals in plants and soil after fertilization with vermicomposted gneiss or steatite powders. Vermicompost was prepared with cattle manure, with or without gneiss or steatite powders. The experimental units were kept in the dark at room temperature for 70 days. Subsequently, a greenhouse experiment was carried out with maize grown in highly weathered oxisol soil and fertilized with vermicompost alone, or with gneiss or steatite powder. 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 contributed to increased Zn concentration in plants and may also be an alternative to Zn fertilization. High Ni and Cr concentrations in steatite powder apparently induced higher levels of these elements in plants. However, metal concentrations in the soil after cultivation in all the treatments were below acceptable limits. Increased plant growth in gneiss-enriched vermicompost suggests the possibility of using this material to enrich vermicompost with nutrients, thus improving the chemical quality of organic fertilizer. Use of steatite powder in agriculture deserves further investigation.