Soil erosion and conservation in the tropics : proceedings of a symposium / sponsored by Divisions A-2, A-6, C-3, S-4, and S-7 of the American Society of Agronomy and the Soil Science Society of America ; organizing committee, S. A. El-Swaify [and others] ; editorial committee, W. Kussow, S. A. El-Swaify, J. Mannering ; managing editor, David M. Kral ; assistant editor, Sherri Hawkins