It is of course possible to have soils without designated A horizons at the soil surface if they lack qualities associated with such a designation. I believe one of the key concepts for designation of soil material as an A horizon is the need for it to be present at the land surface (or under O horizon) for sufficient time that 1) evidence of pedogenic processes are apparent in the composition or structural development, or 2) any soil material at the surface that has been plowed or otherwise modified by cultural practices.
If the soil material at land surface is undisturbed by cultural practices (not plowed), then it remains designated as C horizon until enriched by the organic material of decomposing plants, soluble constituents have been reduced or removed by percolating waters, or development of soil structural units is discernable. This definition still leaves a lot of room for interpretation by the describer. The definitions of and conditions for horizon symbol assignment are kept intentionally vague so that they do not imply a diagnostic level expression exists. Horizon symbology is used by the describer to convey observed evidences of soil alteration, whether that process has occurred sufficiently to result in a defined diagnostic feature (ochric or mollic epipedon, argillic horizon, calcic horizon, etc.), or meeting other requirements to meet a specific taxonomic classification. That is, the horizon symbology assigned is often left to the discretion of the describer thus enabling them to tell the pedogenic story they are observing regardless of whether diagnostic criteria are met.
So, what’s the deal with a soil that has been plowed or otherwise modified by cultural practices getting a ‘bye’ and always receiving an A horizon designation, whether it be old, stable, and well-developed soil or recently deposited alluvium or aeolian material? Primarily, it was to retain the A horizon concept and designation for soils formed in place and subsequently cultivated, or otherwise modified. Once disturbed by cultivation, it might be impossible to ascertain a pedogenic history. Agricultural practices often result in destruction of natural soil aggregates and structure and alters the expected distribution of organic and chemical constituents compared to those in corresponding undisturbed soils. One of the early key concepts in development of Soil Taxonomy was the proviso that normal agricultural tillage practices should not alter taxonomic class as compared to the same soil in an untilled state. While the nomenclature and symbology used in soil profile description and conditions that are diagnostic in Soil Taxonomy are not codependent, they should, however, provide for consistent, clear communication of the qualities and properties important to the use and management of the soil.
