Humic substances: natures̕ most versatile materials : Proceedings of the 20th Anniversary Conference of the International Humic Substances Society ; July 21-26, 2002, Northeastern University, Boston, Massachusetts, United States of America. Boston: IHSS Humic substances: natures̕ most versatile materials : Proceedings of the 20th Anniversary Conference of the International Humic Substances Society ; July 21-26, 2002, Northeastern University, Boston, Massachusetts, United States of America