Application of Clay Crystallite-Size in Soil Genesis Studies

Many soils have clay-enriched horizons that are thought to result from pedogenic processes. Various hypotheses are proposed for expected clay size-distributions in soil profiles. The average size of clay crystallites determined by the amount of broadening of x-ray diffraction peaks ranged from 160 to >2000A 0 , but the lack of trends in four loess-de• rived soils did not permit the hypotheses to be evaluated. An unexpected relationship in a well expressed illuvial horizon was noted: as the percent of accumulated clay inol'eased to the obse1ved maximum the average crystallite size also increased rather than decreased as proposed. This suggests that smallsized illuvial clay particles undergo reorganization to form larger stable cry~tallites. Knowledge of clay crystallite size is expected' to supplement other data in interpreting soil genesis. Soil genesis is the study of the development of horizons in a soil. It involves, in part, estimating the 01iginal properties of the unconsolidated material .and attempting to reconstruct the possible conditions and events which have given rise to the presentday observable features. The ideal situation exists where initially a soil material is isotropic in all measurable properties and the soil horizons result from a rearrangement or removal of various components, such as carbonate and clay. Concepts to explain changes have classically assumed the initial materials to be homog.enous, thereby focusing attention on the kinds of processes that would differentiate the parent material into recognizable soil horizons. Loess approaches the concept of an ideal soil parent material in that it (a) has a mixed mineralogy capable of responding to a weathering environment, ( b) has a medium texture which slows up percolating water but does not unduly restrict the removal of weathering end products, ( c) has been exposed long enough on stable landscapes to reflect the influence of the climate and vegetation characteristic of a region, and ( d) has less variability of other physical and chemical attributes than many materials in which soils have developed. Starting with such a material, a disparity of clay content between upper and lower horizons in a profile is commonly thought to result from a translocation of the inherited clay from the upper zone to the underlying zone. Processes of clay formation and clay destruction may also account for some of the observed differences in clay content throughout a profile. The weight of several 1 Contribution of the Department of Agronomy, Cornell University, Ithaca, New York, as Agronomy Paper No. 758.