Do Genetic Soil Horizons Exist?.

Genetic soil horizons are defined as layers of soil material which are oriented approximately parallel to the land surface that differ from adjacent layers with respect to physical, chemical, and/or biological properties; they result from pedogenic processes of additions, losses, translocations, and transformations acting on unconsolidated regolith. Technological advances in the realm of digital soil morphometrics that are increasingly able to characterize the continuous nature of the boundaries between these horizons coupled with the subjective nature by which horizons have been traditionally delineated in the field have given rise to questions as to whether genetic horizons are discrete bodies that extend in time and space. The objective of this work, then, is to examine whether these horizons actually exist discretely in nature or are, instead, simply conventions convenient for the interpretation, prediction, classification, and communication of soil processes and ecosystem functioning. If the latter proposition is true, then soil profiles may, perhaps, be best understood in terms of continuous depth functions rather than by discrete horizons although the latter may still be useful in organizing and understanding complex pedon information. In this paper, we propose that the validity of these two propositions can be tested through the examination of ecosystem functions. Three physical ecosystem functions—the capture or infiltration of meteoric water, the retention of plant-available soil water near the surface, and the reduction of water loss through evaporation—of optimally-expressed genetic horizons were investigated using the horizon properties of well-expressed soils found under disparate pedogenic environments. In each of these soils and settings, HYDRUS 1-D was used to simulate soil hydrology and evaluate the ecosystem functions provided by the relevant genetic horizons in comparison with unaltered parent material. The results from this thought experiment will be presented and discussed.