The relationship of climatic and gelogical factors to the composition of soil clay and the distribution of soil types

Few soil problems have aroused more discussion and controversy than the assessment of the relative importance of climatic and geological factors in soil formation. At international Soil Congresses such as that held in 1927 in the United States it has been apparent that British and other workers familiar with small highly cultivated areas of irregular topography and varied geology attach much less importance to the climatic factors than do the Russians whose experience is largely of vast plains of fairly uniform loess material extending over well defined climatic zones. Purely practical considerations led under these extreme conditions to the development of geological and climatic systems of soil classification respectively, but both systems were found to require considerable modification when they were applied to other countries or when the scale of soil mapping was greatly changed. Even in the British Isles a generalised soil map would allow for considerable modifications of the climatic soil types by variations in local geology and topography. Both systems are open, however, to the more fundamental criticism that they are based not on the actual properties of the objects classified but on external factors which have influenced the formation of the soil to varying and unknown degrees. The Russian work has demonstrated that an essential preliminary a o all field and laboratory examination of soils should be the recognition and separation of the soil profile down to the unaltered parent material into a series of distinct horizons of approximately uniform composition and mode of formation.