SOIL PRODUCTION ≠ SOIL FORMATION

Gilbert (1877) proposed a key relationship between soil production (SP), i.e. the rate of bedrock conversion to “soil”, and soil depth. This model has re-emerged in the Earth sciences, and the SP function has quickly gained acceptance among geoscientists as providing a powerful basis for understanding soil-landscape evolution. SP is emphasized in new geomorphology textbooks and is highlighted as essential to understanding the collapse of past civilizations (Montgomery, 2007). Likely one important reason for the rapid acceptance of SP is that the SP rate can be explicitly numerically defined and thus readily integrated into numerical models for landscape evolution. Increasingly, the terms soil formation and SP are used virtually interchangeably, but SP differs in many fundamental ways from soil formation as formally defined by most soil scientists and in the groundbreaking textbook Soils and Geomorphology (Birkeland 1999). Consistent with Gilbert’s original concept, SP should be applied to soil-mantled, vegetated uplands slopes dominated by diffusive transport of slope materials. On slopes with positive curvature, curvature controls the SP rate and soil depth increases with slope gradient. SP cannot be used, however, as a basis for understanding soil-geomorphic relationships in many landscapes; e.g., SP is not obviously applicable to slopes with no or negative curvature (e.g., many mid- to toeslopes), where the conservation of mass equation for slopes would produce zero or negative SP rates. It does not apply to (1) many dryland slopes subject to dust inputs and highly susceptible to minor climate changes that strongly influence vegetation, bedrock weathering, pedogenic, and slope-forming processes that favor non-steady state conditions, or (2) where diffusive processes are relatively unimportant. On relatively stable landforms, soil formation in surficial materials progressively produces horizons unaccounted for by the SP model. In many landscapes (e.g., loess plains, alluvial fans, fluvial terraces and flood plains, eolian sand sheets, and pediments), an appropriate conceptual model for understanding soil formation may be the “Soil Factors” model, (“CLORPT”) emphasized by Birkeland. According to this model, the soils formed through SP would be regarded as a special type of soil topofunction.