1. Departamento de Ciência do Solo; Escola Superior de Agricultura Luiz de Queiroz; Av. Pádua Dias 11 PO Box 9 Piracicaba SP 13408-900 Brazil
2. Natural Resource Ecology Laboratory; Colorado State University; Fort Collins CO 80523 USA
3. Department of Soil and Crop Sciences; Colorado State University; Fort Collins CO 80523 USA