EVALUATION AND PROPOSED REVISIONS OF CRITERIA FOR ANDOSOLS IN THE WORLD REFERENCE BASE FOR SOIL RESOURCES

The proposed criteria for Andosol classification in the World Referenc e Base (WRB) of Soil Resources were evaluated using the Tohoku Univers ity World Andosol Database (TUWAD). The WRB proposal defines a diagnos tic andic horizon comprised of vitr-andic, alu-andic, and sil-andic su btypes, and eight second level soil units, Several notable refinements to the WRB criteria were apparent from evaluation of soil properties from TUWAD and are proposed as revised criteria. Our analysis indicate d that the most effective criteria for differentiating the andic horiz on were glass content, phosphate retention, oxalate extractable Al and Fe, pyrophosphate-extractable Al, and bulk density. The vitr-andic ho rizon is distinguished by its low degree of weathering, evident from i ts low oxalate extractable Al and Fe, low P retention, and high glass content, The ratio of pyrophosphate to oxalate extractable Al is used to separate andic horizons dominated by Al-humus complexes (alu-andic, Al-p/Al-o greater than or equal to 0.5), from those dominated by allo phanic materials (sil-andic, Al-p/Al-o < 0.5). When Al, data are not a vailable, the Si-o value of 0.6 could be used as an approximate criter ia. We also propose revisions to the nine second level soil units base d on the dominant pedogenic processes occurring in Andosols: noncrysta lline material formation and organic matter accumulation. These soil u nits are based on the degree of weathering (oxalate extractable Al, Fe , glass content), the dominant weathering agent (organic acids vs, car bonic acid), type (melanic car fulvic) and quantity of organic matter, and high 1.5 MPa water-holding capacity. The revised criteria determi ned by this study effectively differentiate Andosols based on their do minant pedogenic processes, and the relationship of soil properties to productivity and utilization.