Genesis and Relative Weathering Intensity Studies in Three Semiarid Soils

The role of clay illuviation and clay formation in situ in the development of argillic horizons in arid and semiarid regions has been the subject of some controversy among pedologists. This study included two profiles with argillic horizons and one profile without an argillic horizon.

Thin section examinations revealed very few illuviation cutans and some stress cutans. Fine clay (< 0.2µ) / coarse clay (2‐0.2µ) ratios increased in the argillic horizon of the two Haplargid profiles. Relative weathering intensities within the three profiles were estimated from CaO/ZrO₂ molecular ratios obtained by X‐ray spectrographic analysis of the silt fraction. These molecular ratios indicate that maximum weathering has occurred at or near the soil surface and that weathering has decreased with depth in all of the profiles studied. It is concluded that clay formation in situ is not solely responsible for the development of an argillic horizon in arid and semiarid soils. More likely, both clay formation in situ and enrichment by illuviation of fine clay (<0.2µ) are responsible for the argillic horizons in soils of arid and semiarid regions.