MILESTONES IN SOIL CHEMISTRYSparks, Donald L.Soil Science: doi: 10.1097/01.ss.0000228050.62345.96 Technical Articles Abstract Author Information An array of pioneering research, dealing with various aspects of soil chemistry, has appeared in Soil Science for the past 90 years. In this review, two papers that have shaped the field forever are featured and discussed. The paper by Mattson (1931) and the many others that he published in Soil Science established the importance of variable or pH-dependent surface charge on colloids and soils and the pronounced impacts on various soil chemical processes and reactions. He proposed the idea that soil colloids have an isoelectric point, that ion selectivity exists on soil surfaces, and that ligand exchange was an important process affecting ion sorption. In the second paper, using X-ray diffraction, Hendricks and Fry (1930) definitively showed that the major portion of the inorganic fraction of soils was crystalline. They presented results of total chemical analyses, optical studies, and X-ray diffraction on geological specimens of montmorillonite, bentonite, halloysite, kaolinite, and dickite; the results were related to data on 23 samples of colloids from soils. Their landmark paper paved the way for numerous studies on clay mineralogy throughout the world. Department of Plant and Soil Sciences, University of Delaware, Newark, DE. Dr. Sparks is corresponding author. E-mail: email@example.com Received May 4, 2006; accepted May 8, 2006. © 2006 Lippincott Williams & Wilkins, Inc.