Laboratory data are critical to the understanding of the properties and genesis of a single pedon, as well as to the understanding of fundamental soil relationships based on many observations of a large number of soils. Development of both an analytical database and the soil relationships based on those data are the cumulative effort of generations of soil scientists at the Soil Survey Laboratory (SSL). The purpose of the Soil Survey Investigations Report (SSIR) No. 42, Soil Survey Laboratory Methods Manual is to document methodology and to serve as a reference for the laboratory analyst. It is expected that this document will continue to evolve and change over time as new methods are developed and old methods are modified or retired based on new knowledge or technologies. This manual is an historical document in that it describes both current and obsolete methods at the SSL. This manual also provides historical perspective, documenting the contributions of many soil scientists who have gone before us. Many of these scientists are noted in the section on contributors. Illustration are in full color and black and white.