Donald R. Nielsen, Professor of Soil and Water Science at the University of California, Davis, taught soil physics courses, integrating chemical and biological processes. His research and that of his students included miscible displacement, microbiological transformations, scaling soil properties and analyzing field soil variability. While at Davis, he collaborated with 90 soil scientists from 40 countries. Nielsen retired in 1994, but he continues to visit and encourage young colleagues worldwide. He has served as president of Soil Science Society of America, American Society of Agronomy, Hydrology Section of the American Geophysical Union and Soil Physics Commission of ISSS. He is chair of the US National Committee of Soil Science.