The Relation of Mechanical Analysis to Field Textural Classification

During the early work of the Soil Survey, mechanical analyses of representative samples were used to determine the quantities of sand, slit, and clay In the various textural grades defined by the field workers. Due to differences between the areas in which the field men worked, the definitions of the textural grades varied. As the work progressed, changes were made in the definitions until those in use today were evolved. Davis and Bennett (2) in a circular published in 1927 suggested some changes, and summarized the principal class definitions used at present. Textural class limits were shown on a triangular diagram (Fig. 1) which are difficult to disperse. In the case of some other soils, organic matter tends to granulate the soil and to give a coarser field texture. Only by removal of the organic matter can such granules be broken up completely (6). In the mechanical "analysis laboratory, practically complete dispersion is obtained by the pipette method (14.). The use of sodium hydroxide (1) instead of sodium oxalate as a dispersing agent for lateritic soils aids materially. It is essential, in connection with problems involving complete dispersion, to remember that all soil colloids do not disperse or remain dispersed to the same degree with any one treatment.