Classification of volcanic soils in the Soil Classification System of Japan and verification based on forest soils

In 2017, the Japanese Society of Pedology (the Fifth Committee for Soil Classification and Nomenclature) established the Japanese soil classification system. In the classification system, all the soils derived from volcanic ejecta (volcanic ash, pumice, scoria, etc.) are classified into the Great Group of Andosols, excluding Man-made and Organic soils, although some volcanic soils were classified into other soil groups, such as the Podozols and Brown forest soils in the previous classification systems. The aim of the present study was to verify and propose revision for the new system based on physicochemical data collected from forest soils across the Japan archipelago. Approximately 20% of the forest soils have andic properties and were classified into the Andosols in the Soil Classification System of Japan, which is consistent with the Andisols in Soil taxonomy and Andosols in World Reference Base for Soil Resources, excluding a few soils with low andic horizon thickness. Many other forest soils that were not classified into Andosols possessed some of the regosolic andic properties, and it was challenging to classify them into Regosolic Andosols or Brown forest soils. Therefore, additional diagnostic activities on the volcanic ejecta and glass are required to classify such soils appropriately. In the new classification system, the Fulvic Andosols soil group was abolished and substituted with the Humic and Brown-humic subgroups under the Non-allophanic and Allophanic Andosols, respectively. The revision was consistent with the traditional forest soil classification system. In conclusion, the classification of volcanic soils in the Soil Classification System of Japan was verified using forest soil data in Japan.Abbreviations CFSJ: Classification of Forest Soils in Japan (1975); CSCSJ: Comprehensive Soil Classification System of Japan, 1st approximation (2011); SCSJ: Soil Classification System of Japan (2017); ST: Keys to Soil taxonomy, Twelfth Edition (2014); USCSJ: Unified Soil Classification System of Japan, 2nd Approximation (2002); WRB: World Reference Base for Soil Resources (2014, 2015 updated)