Argillic horizons must meet three basic qualities. These are:
1) They have appropriate thickness based upon texture and nature of the illuviated zone, and
2) have evidence of illuviation (e.g.- clay films, bridging, oriented clay) unless disrupted by shrink-swell in soils significant content of expansive clay, and
3) have a specified increase in clay content from overlying eluvial horizons.
The third requirement is waved if the eluvial (upper) horizon has been removed, or the soil has been plowed to the depth of the illuvial horizon, or if there exists a lithologic discontinuity (by alluvial deposition).
In the situation you describe, if the illuvial zone meets the thickness requirement and has evidences of illuviation such as clay films, bridging, etc., then the plowed soil is considered to have an argillic horizon even though the pedon does not exhibit the required clay increase. However, some evidence of plowing (mixing) should be evident to the depth of the illuvial zone.
