Comparing calcium chloride, barium chloride, and hot water extractions and testing activated charcoal plus azomethine-H dosage for boron determination in Brazilian soils

ABSTRACT

Most acid soils have low boron (B) contents which hampers the efficiency of B extraction. The setting of a suitable B extraction method for Brazilian acid soils is important to improve the evaluation of the status of this nutrient in acid soils from Brazil. The extraction of boron with hot water, the standard method, is a hard work method with operational difficulties, alternative methods using saline solutions and microwave oven heating have been tested.

The efficiency of CaCl₂ 5 mmol L⁻¹ as an extractant for determination of B by colorimetry with azomethine-H was evaluated. Boron concentration in extracts from ten Brazilian soils was determined based on the color generated in the reaction of B with azomethine-H. The soil samples were extracted with hot water, barium chloride BaCl₂ 5 mmol L⁻¹, and calcium chloride (CaCl₂) 5, 10, 20 mmol L⁻¹, with and without activated charcoal. For each extractant it was set a completely randomized block design with the treatments arranged in a complete factorial 10 × 2 (soils × presence and absence of activated charcoal in the extraction) with 3 replications. The evaluation of organic matter interference was performed by analysis of variance and extractants were correlated by Pearson's correlation. Regression analysis was performed for selection of the best concentration of CaCl₂ for extraction. The extraction with CaCl₂ did not remove organic matter interference in the determination of B with azomethine-H. Activated charcoal was required in the B dosage with azomethine-H. The 5 mmol L⁻¹ CaCl₂ solution is as effective as BaCl₂ in the extraction of B from Brazilian soils.

Acknowledgments

G.B.F. extends thanks to CAPNS (Coordenadoria de Aperfeiçoamento de Pessoal de Nível Superior) for financial support.