Mineral characteristics of volcanic ash-derived soils affecting boron adsorption

Volcanic ash-derived soils are naturally deficient in the essential micronutrient B, but they also have maximum adsorption capacities (b_B) that are as much as 40 times greater than in nonvolcanic soils, further exacerbating B availability. Because short-range-order (SRO) aluminosilicates (allophane and imogolite) and iron (Fe) oxyhydroxides are the main clay-sized minerals in these soils, we hypothesized that their relative abundances would explain variations in boron sorption capacity (b_B). We characterized 23 volcanic ash-derived soils (Andisols and non-Andisols) from the Pacific coastal plain of Guatemala by X-ray diffraction, oxalate and pyrophosphate extractions, and thermal analysis, with four selected soils further examined by Mössbauer spectroscopy (MBS). Only soils with andic character (high SRO Si–Al content) contained considerable amounts of imogolite, with allophane in the clay fraction. Overall, soil SRO Al–Si phases were strongly correlated with clay B adsorption capacity (R = 0.65, p < 0.001) and clay specific surface area (R = 0.88, p < 0.001) suggesting that SRO Al–Si phases are the dominate influence on B behavior in these soils. Fe mineral composition was dominated (39%–71% of total Fe) by SRO Fe(III) oxyhydroxides (nanogoethite and ferrihydrite) of various crystallinities. In low allophane soils, SRO Fe phase abundance correlated with b_B as did the abundance of low crystallinity kaolinite phases, suggesting these phases are important when SRO Si–Al content is low. Any efforts to predict B behavior or plan soil fertility treatments in volcanic-influenced soils needs to consider the impact of SRO phases.