An Historical Perspective on the Theory and Practice of Soil Mechanical Analysis.

In this paper the history of soil mechanical analysis is traced and evaluated to place our current concepts in perspective, from both a research and teaching viewpoint. The early period of research in mechanical analysis (from 1880 to 1920) is characterized by laborious separation techniques such as decantation and elutriation, which were replaced in the 1920s by the modern pipette and hydrometer methods. The pipette method is based on Stokes' Law and has become the standard method, whereas the Bouyoucos hydrometer method is empirical and its popularity is based on simplicity and speed of use at the expense of accuracy. Revised hydrometer methods based on Stokes' Law improve its accuracy, but continued use in research and instruction of the 2-h hydrometer reading tends to promulgate an approximation as a method of choice. Alternatives to the Bouyoucos method for use in soils teaching laboratories are discussed. Additional Index Words: Particle size analysis, Hydrometer, Pipette method, Bouyoucos, Stokes' Law, Sedimentation. T STUDY of the origins of scientific concepts is an interesting and ultimately useful endeavor from several points of view. For the research scientist, the historical background of a pertinent study area forms the basis of the current set of hypotheses and experimental approaches. For those involved in teaching, an historical perspective can be a valuable teaching tool to stimulate student interest by introducing the personalities and debates that molded our current ideas about a particular All authors, Dep. of Agronomy, Room 3111, Miller Plant Sci. Bldg., Univ. of Georgia, Athens, GA 30602. Contribution from the Dep. of Agronomy, Univ. of Georgia, Athens. Received 17 June 1987. •Corresponding author. Published in J. Agron. Educ. 17:24-28 (1988). topic. In both cases, the history of a concept often teaches (or reminds) us that the process of science, research, is often precisely that: re-searching for explanations, methods, and applications that have been studied many times before. An awareness of the process of science as the refinement of concepts over time, the replacement of one hypothesis by another, is a valuable lesson: "researching" will render out students' textbooks obsolete during their lifetimes, and hence the ability to continually evaluate and apply new hypotheses may become as important to our students as familiarity with a "static" body of current concepts. The study of soil is relatively young as a science, yet our roughly 100-yr history offers many examples of this scientific process, perspective to our current knowledge and illustrating the development of modern concepts. Most introductory texts describe at least briefly the search for the "principle of vegetation" during the 18th and 19th centuries (Russell, 1973 gives a more complete account), and the advances in soil genesis and classification made by the Russians in the 1880s. Use of these historical scenarios in either an anecdotal or problem-solving approach can be effective in adding interest and diversity during lectures. An important aspect of soil science commonly discussed in many soil courses is particle-size distribution and its determination by mechanical analysis, another topic with a long but less well-known history. We mention Stokes and Bouyoucos in our lectures, and describe the pipette and hydrometer techniques, perhaps with little feeling for how these people and others arrive at their conclusions and what these techniques mean to us today. Our interest in this topic was spurred by an inconsistency discovered in class between two methods of mechanical analysis, which resulted in a thorough review of the historical literature on the theory and methodology of particle-size analysis. Our goal began as a simple clarifica24 J. Agron. Educ., Vol. 17, no. 1, 1988 tion of methods and ended as a search for the origins of those methods. The results of our "re-search" revealed a number of interesting, little-known facts on mechanical analysis and provided a good example of science as the "sifter" of sound, workable ideas from those not as sound or workable. DERIVATION OF STOKES’ LAW Stokes’ Law is typically used as a starting point in discussing soil mechanical analysis. G.G. Stokes (1850) initially formulated the equation that bears his name to describe the motions of pendulums in viscous media such as water. His major contribution was to describe the frictional resistance of a body moving through a viscous medium as a function of the radius (r) and velocity (v) of the body and of the viscosity of the medium (7):