Smectites in iron-rich calcareous soil and black soils of Taiwan

The iron-rich calcareous soil (Typic Rhodustalf) from the Penghu island gro up represents a volcanic area. The black soils (Typic Haplustert, Vertic En doaquoll, Typic Hapludolls) are typical of eastern Taiwan. Four A horizons and a pedon from the iron-rich calcareous soil and four pedons from the bla ck soils were studied to analyze soil properties and clay compositions. The objective was to compare the properties of smectites developed from differ ent parent materials. The materials were studied by using conventional X-ra y diffraction (XRD) of K- and Mg-saturated clays and involved the alkylammo nium (C = 12) method and the Greene-Kelly test. The mean-layer charge of sm ectites (0.48-0.52 cmol(c)/O-10(OH)(2)) in the iron-rich calcareous soil wa s found to be higher than the black soils (0.43-0.48 cmol(c)/O-10(OH)(2)). A smectite of higher charge developed from the basalts. This smectite is en riched in Fe and Mg, and lacks Si, thereby forming beidellite and/or nontro nite. In contrast, under high precipitation, elevated temperature, base sat uration (e.g., Na, K, Ca, Mg), and about equal wet and dry cycles per year in the black soil environments, smectites developed from the complicated ge ologic site of eastern Taiwan. These smectites transformed to smectite-kaol inite mixed-layer clay and thus, resulted in lower-charge smectites. The K fixation capacity of the iron-rich calcareous soil was higher than the blac k soils.