Re: Upper boundary of argillic horizon and evidence of illuviation?

The lack of visible evidence of illuviation (clay films) does not preclude the upper part of an argillic horizon from being the ‘top of the argillic’. Competing pedogenic processes and the activities of soil flora and fauna often disrupt visible clay films. This is especially true for argillic horizons in ‘drying’ climates as translocation slows and the precipitation of soluble carbonates within the argillic disrupts clay films. Therefore, yes, the official stance is the top of and upper argillic horizon may not exhibit visible evidences of illuviation. Laboratory analyses, such as fine-clay/total-clay ratio should still point out the top of the argillic or boundary between the eluvial and illuvial horizons. Always note that soils exhibiting pronounced pedogenic development, such as a mature argillic, may have formed in multiple climate regimes and some pedogenic process erase or mask evidences of earlier pedogenesis.