Primary soil forming processes as influenced by the composition of parent material and environmental conditions

Abstract

Two rock types, a quartz diorite and andesite, differing physically and mineralogically, but being similar chemically and occurring under different climatic conditions were studied chemically and petrographically and the results compared with the chemical characteristics of their residual clays and soils. It was found that under different climatic conditions the feldspar of the rock weathered in about the same manner, but in one case the products of weathering were carried away due to excess in moisture; in the other case accumulations occurred. The products of weathering in the case of the quartz diorite, under extreme rainfall conditions, were practically all removed with the exception of the silica and alumina which combined to form a residual clay. Even in this case under extreme conditions the clay minerals forming the residue might be eventually broken down. In the case of the andesite, occurring under low rainfall conditions, the only constituents removed were calcium and magnesium; the other constituents remained about the same. The ferromagnesium minerals in the quartz diorite were also broken down by hydration, and iron was lost. In the case of the andesite, however, the iron being chiefly as magnetite was attacked very little in the process of weathering and very little change was observed. In all cases the study of the silicon dioxide-aluminum oxide ratio is of material assistance in tracing the progress of these various weathering processes.