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 Reference Base (WRB) of Soil Resources were evaluated using the Tohoku University World Andosol Database (TUWAD). The WRB proposal defines a diagnostic andic horizon comprised of vitr-andic, alu-andic, and sil-andic subtypes, 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 indicated that the most effective criteria for differentiating the andic horizon were glass content, phosphate retention, oxalate extractable Al and Fe, pyrophosphate-extractable Al, and bulk density. The vitr-andic horizon is distinguished by its low degree of weathering, evident from its 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 or equal than 0.5), from those dominated by allophanic materials (sil-andic, Al(p)/Al(o) < 0.5). When Al(p) data are not availabe, the Si(o) value of 0.6 could be used as an approximate criteria. We also propose revisions to the nine second level soil units based on the dominant pedogenic processes occurring in andosols : non-crystalline material formation and organic matter accumulation. These soil units are based on the degree of weathering (oxalate extractable Al, Fe, glass content), the dominant weathering agent (organic acids vs. carbonic acid), type (melanic or fulvic) and quantity of organic matter, and high 1.5 MPa water-holding capacity. The revised criteria determined by this study effectively differentiate andosols based on their dominant pedogenic processes, and the relationship or soil properties to productivity and utilization. (Résumé d'auteur

Partial Rejuvenation of the Soil in Intavas Techno Demo Farm, Bukidnon, Mindanao, Philippines by Volcanic Ash Material

Intavas Techno Demo Farm is situated in high elevation of 1, 220 meters above sea level, with cool temperature and without pronounced dry season. The soil has low pH (5.24, average) and medium total carbon (4.3%) in the surface. Phosphorus retention is relatively high (77.5%, ave.), low cation exchange capacity (12 cmol(+) kg^ ave.) and low base saturation (13%, ave.). Clay content is 45% (ave.) and bulk density of 1.1 Mg m^ in average. Micrograph of the thin section of Bw_1 horizon reveals that void content is very small (6%). Voids are vughy and are not interconnected. In crossed nicol there were no clay illuviation seen. Total element analysis revealed that Intavas soils have about 60% Al_2O_3 but very low in SiO_2, 16.05% only. This indicates an advanced weathering that Si has been leached out and Al precipitated. As to the primary minerals, opaque minerals predominate with the presence of hornblende in heavy minerals. For light minerals, weathered particles predominate with the presence of mica, quartz and volcanic glass. Selective dissolution analysis reveals that Al_o is relatively high as compared to Fe_o and Si_o. However, these elements have the same pattern that irregularly decrease with depth and increase in the buried A horizon. Fe_p is relatively high followed by Al_p that significantly decreases with depth. The data show that amorphous materials are higher in surface and buried A horizons. Electron micrograph of BA shows no defined form that can be considered as amorphous materials. Bw_2 shows angular shapes that correspond to gibbsites and halloysite as indicated by spheroid configurations. X-ray diffraction pattern exhibits small and weak peak at 7 A in BA horizon. This peak corresponds to the presence of halloysite. The sharp and strong peaks at 4.8 A and 4.3 A in BA and Bw_2 horizons correspond to gibbsite. According to the selective dissolution analysis, electron microscopy and X-ray diffraction analysis, the dominant clay mineral in BA horizon is gibbsite and followed by amorphous material and small amount of halloysite. On the other hand, gibbsite dominate Bw_2 horizon followed by halloysite. Based on the physical, chemical, mineralogical and thin section analyses, the TDF soil can be classified as fine clayey, acid, isothermic, gibbsittic, Oxic Dystrudepts. It can be considered as soil with partial rejuvenation by volcanic ash material. As compared with soils without rejuvenation, this soil has higher total carbon, phosphorus retention, lower bulk density relatively, high amorphous materials in the surface and halloysite in the subsoils