Boron fractionation in some Saskatchewan soils

Abstract

Knowledge of B fractions is essential for understanding its chemistry and potential contribution to plant uptake. Four different extraction methodologies to determine readily available B were compared, using a four-step sequential B fractionation procedure, to reveal the nature of soil B in nine selected soils from the Brown and Gray Luvisol zones with different textures and management histories. The four soil extraction techniques were: hot water (HW), 0.01M CaCl2, 1 M NH4-acetate and anion exchange membranes (AEM). The other four fractions, specifically adsorbed, oxide bound, organically bound, and residual B were determined sequentially on the same soil samples. On average, HW extracted more B than 0.01M CaCl2, 1 M NH4-acetate, and AEM. In almost every soil, readily soluble B represented only a small proportion of the total B content, regardless of the extraction method used. Most soil B existed in the residual or occluded form, which accounted for between 92 and 99%, with an average of 97%, of the total soil B. The concentration of organically bound B was higher than that of oxide bound B and specifically adsorbed B, irrespective of the readily soluble B extracting solution used. Cation exchange capacity (CEC) appeared to be an important characteristic to predict the B pools. No significant correlation was found between soil organic carbon content and the different B pools, except for hot water soluble (HWS) B. Correlation coefficients between the B pools and particle size distribution were poor and correlation between carbonates and B pools were insignificant. The findings suggest that in Saskatchewan soils, readily soluble B is only a small proportion of the total B, and the majority of B exists as residual or occluded form. Hot water soluble B appears to be a good method to estimate available B. We found this method rather simple, efficient, and consistent. Key words: Boron fractionation, exchangeable B, extractable B, organically bound B, soluble B