Abstract: Barshad's method of calculating clay formation from the chemical analyses of the whole soil, the clay fraction, the nonclay fraction, and the mechanical analysis was applied to a large number of soils to determine the effect of climate, topography, parent material, vegetation and time on amount of clay formation. The amount of clay formed from 100 g of the nonclay fraction of the parent material was used as the yardstick for comparing clay formation in the different soils.
It was found that for a true evaluation of the effect of these factors on clay formation, it is necessary to determine clay formation in every horizon of a profile and that all soil profiles should be sectioned into horizons which are as nearly alike in thickness as possible. Maximum clay formation in a soil profile occurs in the horizon which is about 2–10 in. below the surface rather than at the immediate surface; as a rule, the amount of clay forming decreases below this maximum horizon. An increase in moisture and temperature enhances clay formation. The temperature factor mainly affects clay formation in the Great Soil Groups in the following order: Podzols < Gray Brown Podzols < Brown Earths < Chernozems < Prairies < Red and Yellow Earths (Latosols) < Laterites. Poor drainage enhances clay formation. Grass-type vegetation is more effective in clay formation than tree-type. Clay formation is greater from the finer grained and the more basic rocks. The more easily rocks fragment, the more clay is formed. The increase in age of a soil is believed to be expressed in the same manner as an increase in the intensity of clay formation, namely, as an increase in clay formation with depth and as an increase in the thickness of a soil profile. It was concluded that clay formation is relatively slow : clay forms alb a rate ranging from 0.00001 g to 0.002 g of clay per year per 100 g parent material or from 0.6 to 120 lb of clay per acre foot of parent material per year.