Boron Adsorption and Desorption on Volcanic Ash–Derived Soils

Abstract

Core Ideas Soil B availability in volcanic soils is governed by SRO minerals. Analyzing whole‐soil reactivity based on mineral surface abundance is proposed. Volcanic soils adsorb as much as 18‐fold higher than non‐volcanic soils. The reversibility of adsorption reactions decreases as andic character increases. pH in NaF can be used to reflect field B availability and application rates. Many crops in volcanic regions exhibit symptoms of B deficiency but do not respond to B fertilizer rates that solve deficiency problems in non–volcanic‐influenced soils. The US recommended rates are routinely adopted in these regions without accounting for differences in soil B dynamics. Because of the paucity of investigations and the resulting lack of understanding of B adsorption/desorption dynamics in volcanic ash–derived soils over the past 40 yr, it would be incorrect to extrapolate the few data available to all volcanic ash–derived soils because they have not been analyzed as a function of andic character. This paper aims to create an understanding of what soil properties govern B availability in volcanic‐influenced soils and to establish if there are low‐cost testing methods that can help estimate appropriate field B application rates in these soils. Boron adsorption/desorption isotherms were constructed for a group of volcanic ash–derived and non‐volcanic ash–derived soils, and the derived parameters were compared with various standard soil tests. Our results show that soil B availability in volcanic‐influenced soils is governed mainly by strong and stable B complexation with short‐range‐order (SRO) minerals. Boron adsorption maxima exhibit a wide variability not directly correlated with clay content but rather with reactivity quantified by SRO mineral and organic matter contents. Organic matter is not a significant variable explaining specific surface area for the overall group of soils, but it becomes significant for soils containing <200 g clay kg‐1, suggesting that a new approach to analyzing whole‐soil reactivity based on mineral surface abundance is needed. The maximum adsorption capacities of volcanic‐influenced soils can be as much as 18‐fold higher than for non‐volcanic soils, with the reversibility of adsorption reactions decreasing as andic character increases. These findings explain why standard B fertilizer rates fail to solve deficiency problems in volcanic‐influenced soils. Because pH in NaF is strongly related with acid‐oxalate Al (SRO minerals) and B desorption index, this low‐cost test can potentially be used as a measure of soil andic character to better reflect B availability and corresponding field application rates.