Tuesday, 11 July 2006

Proposed Modification of the Definition of Mollic Epipedon Based on Experience from Soils Developed in Cold-Temperate Climates.

Jan Eriksson and Holger Kirchmann. Swedish University of Agricultural Sciences, Department of Soil Sciences, Box 7014, Uppsala, 750 07, Sweden

Swedish soils formed in calcareous parent material are often classified as Mollisols/Phaeozems according to the present criteria in Soil Taxonomy/WRB. In most cases the depth requirement for the mollic epipedon is satisfied only because the soils have been plowed to 25-30 cm. This is un¬satis¬factory since these soils in many cases do not systematically differ from Inceptisols/Cambisols normally formed in medium to fine textured materials. Weaknesses of present criteria for mollic epipedons: Since the classifications systems are not genetic, soils outside steppe areas can and should be classified as Mollisols/Phaeozems as long as they fulfill the criteria in the classification keys. A mollic epipedon produced by plowing is accepted since normal farm practice should not change the classi¬fica¬tion. However, as we understand it the concept Mollisols/Phaeozems refers to soils with favorable physical and chemical properties and thus being more fertile than average. Many of the Swedish “Mollisols” have been proved to be relatively poor farmland. A mollic epipedon containing less than 15% the calcium carbonate need only to contain 0.6% organic C or more. Due to cold and humid climate, which favor slow decomposition, there are few if any agricultural soils in Sweden with intact plough-layer that have an organic C content as low as 0.6 %. Thus the present limit value for organic C content is not a useful criterion to distinguish soils characterized by a humus rich A-horizon in cold temperate areas. Another property that should be typical for soils classified as Mollisols (mollis=soft) is a very good structure due to high faunal activity and accumula¬tion of well humified organic matter. The Swedish “Mollisols” do not consequently have a better structure than comparable Inceptisols. We suggest, based on documented descriptions of Swedish soils, that the criteria for Mollisols/Phaeozems should be improved. In Soil Taxonomy the limit for carbon content in the mollic epipedon should be 2.5 % (as in calcium carbonate rich soils) for soils with udic or aquic moisture regime and cryic or frigid temperature regime. This limit is close to the median organic C content of the A-horizon in Swedish agricultural soils. A higher limit for organic C would also in most cases imply a better than average structure of the soils that would still classify as Mollisols. In WRB the carbon content criterion is bit more difficult to handle since moisture and temperature regimes are not used. In soils with a solum less than 75 cm, a weak soil development that may be due to cold climate and/or short time for soil development, the thickness requirements for a mollic horizon is waived. Introducing the proposed elevated limit for organic carbon in the mollic horizon in thin soils could probably work in WRB.

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