A paradox between Soil Taxonomy and World Reference Base in classification of the soils with clay-enriched horizons (a case study in Central Zagros, Iran)

Abstract

Both Soil Taxonomy (ST) and World Reference Base (WRB) classification systems include the plow layer as a criterion for diagnostic clay-enriched horizons under specific conditions. The present research aims to compare the results of the two classification systems when the plow layer does or does not exist directly above the illuvial layer. A representative toposequence is selected in the Chelgerd region, Central Zagros of Iran, and five pedons are classified along the southern slope according to the latest versions of ST and WRB. The results shows that for the soils with no plow layer directly above the illuvial layer, the argic horizon could be determined using WRB. The reason is that the clay content is higher in the illuvial horizon than the surface (eluvial) layer, whereas the argillic horizon can not be distinguished by ST due to the fact that clay illuviation’s evidence should exist in such conditions as another mandatory criterion. However, according to WRB, the same soil under agricultural land use has different soil classification as a result of changing A to Ap (plow layer) horizon. In this situation, clay illuviation’s evidence is necessary for both soil classification systems. We suggest that the plow layer should be removed from both systems as a criterion. In this case, the clay illuviation can be considered diagnostic characteristics/properties at the lower levels. Improving digital image analysis and morphometric techniques may help scientists to better quantify the soil characteristics such as particle size distribution and clay illuviation in the future.