Agriculture is a very relevant sector for Brazilian economy and has contributed with about 30% of Gross National Product, in the last 10 years. In 2004, the fertilizer national production was 9,733,609 ton and 15,424,235 ton were imported. This situation is unfavorable for the Brazilian Balance Trade and instigate the search for alternatives sources to soil fertilization. One of the factors that is considering is the utilization of rock powder, but technical arrangements are needed to come viable the use. The aim of the present work was to test the potential potassium solubilization from minerals of silicatic rocks by Aspergillus niger, which occur commonly in tropical soils and is responsible for organic acid production. The results obtained with kimberlite powder are presented here. The kimberlite powder had granulometric size < 2 mm. The adapted medium used was sodium citrate (2.85g/L), NH4SO4 (1g/L), MgSO4.7H2O (0.5 g/L), CaCl2 (0.132 g/L), glucose (10g/L) and distilled water until 1 liter. The pH was adjusted to 7,0 with NaOH 0.05 N. The experiment was carried out in triplicates in 250 ml erlenmeyers flasks with 25 ml medium and 5g/L rock powder, inoculated with Aspergillus niger spores (1.6 x 107 spores/ml) at 30°C, 160 rpm for 20 days. Soluble K (ppm) and pH into the medium were analysed (Figure 1). Figure 1: Soluble potassium and pH into the medium treated with Aspergillus niger, Aspergillus niger + kimberlite powder and kimberlite powder. It can be concluded that Aspergillus niger fungus has high potential of potassium solubilization by biologic acidification, however the studies must continue aiming the optimization of methods that apply this potential in practical activities of use of rock powder as fertilizer. Key-words: Soil remineralization, rock powder, kimberlite.