Different soil boron fractions were evaluated from 29 superficial (0-20 cm) soil samples in Ceará, Brazil. Relationships were established between soil fractions and soil properties and soluble boron, which has been used as an index of boron availability to the plants. Fractional boron analyses of the soils were carried out following a sequential procedure. The following determinations were carried out: the water soluble boron (B-Sol), the not specifically adsorbed boron (B-NEsAd), the boron associated to manganese oxide (B-OxMn), the boron associated to Fe and Al amorphous oxides (B-OxFeA), the boron associated to Fe and Al crystalline oxides (B-OxFeC) and the residual boron (B-Res). The total boron (B-Total) was assessed as well as the available boron by means of three extractors: hot water (B-H2Oq), 0.05 mol L-1 hydrochloric acid (B-HCl) and 0.05 mol L-1 mannitol+ 0.01 mol L-1 CaCl2 (B-Man). B-Sol, B-NEsAd and EsAd have exhibited amounts ranging from 0.05 to 0.79 mg kg-1, which represents 0.3 to 4.43% of the B-Total. Among the oxides there was a predominance of B-OxFeC with fractions 1 to 2 times higher than those of B-OxFeA. In all soil samples, the B-Res (occluded in silicate minerals) was the predominant fraction, with amounts ranging from 19.6 to 70.7% of the B-Total. The available B, values removed by the three extractors, were highly correlated among themselves and with B-H2O, B-NEsAd and B-EsAd fractions. Clay and organic matter were the properties which best correlated with the B-Sol, B-NEsAd and B-EsAd.
available boron; soil properties; micronutrient; fractionation