Vermicomposting with rock powder increases plant growth
The growth of earthworms in substrates enriched with rock (gneiss and steatite) powder, and the potential of vermicomposting in increasing solubilization of minerals present in rock powder and in promoting plant growth were evaluated. Cattle manure (400 g), was enriched with 0, 5 and 20% of gneiss or steatite powder. Each pot with this mixture received nine earthworms (Eisenia andrei), at a density of 1000 indiv. m−3. After 60 d, earthworms were collected, counted and weighed (fresh and dry). Maize was cultivated in a greenhouse in pots with an Oxisol that was fertilized with the vermicompost obtained above. Treatments with Oxisol fertilized with gneiss or steatite only and unfertilized soil were used as controls. Shoot length was measured weekly from the soil surface to the tips of the leaves. After 73 d, the plants were harvested, the roots washed from the soil and shoots and roots dried and weighed. Plants fertilized with vermicompost enriched with rock powder were taller and heavier than plants fertilized with non-enriched vermicompost. Plants grown on soil fertilized with rock powder but not with vermicompost were larger than plants grown on unfertilized soil. Vermicompost enriched with steatite powder resulted in a larger effect on plant growth than the mere sum of applying vermicompost of non-enriched manure and steatite alone to the soil. A similar, but non-significant effect was also observed for gneiss. The different effects between gneiss and steatite may be associated with the lower resistance to chemical weathering of steatite minerals compared to gneiss minerals, as well as the former being softer than the latter. The effect of vermicompost on the optimization of nutrient release from silicate rocks seems to depend on the rock type.