1. Sector of Biology, Microbiology and Biological Processes, Dep. of Soil Science Univ. Federal de Lavras Lavras MG 37200‐900 Brazil
2. Dep. of Soil Science Univ. Federal de Santa Maria Santa Maria RS CEP 96501‐516 Brazil
3. Dep. of Agriculture Univ. Federal de Lavras Caixa Postal 3037 Lavras MG 37200‐000 Brazil
4. Dept. of Statistics Univ. Federal de Lavras Caixa Postal 3037 Lavras MG 37200‐000 Brazil