The Quantitative Evaluation of Soil Formation and Development by Heavy Mineral Studies: A Grundy Silt Loam Profile

TN ITS progress from the qualitative viewpoint to •*• the quantitative measure, the study of soil formation and development is beset with grave difficulties. In part, the problems have been solved by the application of climatic data to soil profile description on a geographical basis. However, we still lack two essential components of a genuinely quantitative pedology, namely, the means of measuring soil development and of determining its duration in time. As regards the former, it has long been recognized that any constituent unaffected by weathering processes might be used to provide the appropriate comparisons with the parent material. In practice, however, two difficulties have arisen. The proof that a given profile was indeed derived from the assumed parent material beneath it is not easy to furnish and has seldom been thoroughly attempted. There has, in addition, been no unanimity among pedologists as to the best choice for the immobile constituent. The comments of Milne (5) upon the lateritic profiles studied by Harrison (2) in British Guiana afford a vivid illustration of these difficulties. The measure of duration will not concern us here except to point out that it is usually based on evidence external to the profile itself. Soil profiles normally display clear internal evidence of their origin in the resistant-heavy minerals which they contain. We shall now show how the latter may also be used to measure profile development. Some preliminary consideration was given to this problem two years ago (4). Since then, several profiles have been examined using further refinements of the heavy mineral technic. The example here selected is by far the most favorable yet encountered. The selection of sufficiently resistant minerals which can be used as immobile indicators of the gains and losses of other constituents follows from the general experience of the sedimentary petrographers. Druif ( i ) , working in the Dutch East Indies, has examined this matter very closely in the case of tropical soils and has stated that even the most resistant species, such as zircon, tourmaline, and garnet, may show evidence of surface corrosion. However, it is possible, as he admits, that some of these examples may have arisen under abnormal conditions at some earlier stage of their geological history. From the available evidence, admittedly qualitative in nature, it seems certain that of all the commonly occurring minerals zircon is the most resistant. In choosing it as our ultimate indicator of profile losses or gains we shall be on the safest ground yet attained. As was pointed out in the earlier paper, it is first necessary to furnish convincing proof that stratigraphical and depositional differences are absent. The former are considered to be excluded when a semiquantitative study of the resistant heavy minerals under the petrographic microscope shows no change in the mineral suite throughout the profile. Depositional variation can frequently be detected by mechanical analysis alone, but the ultimate court of appeal is again the resistant heavy mineral suite. In a profile derived from uniform parent material the amounts of the resistant heavy minerals should bear a constant ratio to each other and the particle size distribution curve for each resistant mineral should remain constant. Assuming that these requirements are met, it remains to define the parent material, that is, to decide what horizon in the profile should be used as the basis for calculation. It should fulfill the condition that samples taken at reasonable distances above and below be practically identical with it, the permissible limits of variation being determined on samples at the same depth from nearby profiles. Whether the parent material defined in this practical sense actually represents the state of the soil system at zero time, Jenny's definition of parent material (3), will generally be a matter for debate. However, as evidence of this kind accumulates on soils of differing degrees of maturity, it should be possible, by measuring the processes which accompany the various stages of weathering and development, to set definite limits to this uncertainty. The evidence from a single profile will always give the minimum change compatible with the facts. OUTLINE OF METHOD The following procedure in calculation will be found to be simpler than that previously discussed.