Availability of extractable boron in some acid soils, West Bengal, India

A relatively small range between deficiency and toxic limits of boron (B) necessitates precise evaluation of the availability of extractable boron before applying B in deficient soils. Keeping this in view, laboratory and greenhouse experiments were conducted to assess the availability of native B in soils. For this purpose, 25 acid soils with diverse properties and varying hot water extractable B content, were selected from lateritic and alluvial tracts of Southern West Bengal. A greenhouse pot experiment with four rates of B (0, 0.5, 1.0, and 2.0 mg kg(-1)) was conducted in completely randomized design to study the response of soybean (Glycine max L.) to native and applied B in all 25 soils. The suitability of nine extractants for determining available soil B was assessed by correlating the amount of extractable B in untreated soils with Bray's percent yield, uptake, and tissue B concentration of soybean plants. Optimization of salicylic acid concentration is described and the advantages of this extractant are discussed. The interference of amethyst color (produced by iron and salicylic acid) with the colorimetric estimation of B is studied. Hot CaCl2 was found to be the most suitable extractant for the determination of available B in these soils, followed by hot water, salicylic acid, and ammonium acetate. However, salicylic acid appeared to be the most efficient extractant for routine soil analysis for available B, where a large number of samples are analyzed. The critical values in respect to sufficiency of extractable B for soybean plants were 0.51 for hot water, 0.61 for hot CaCl2, 0.27 for ammonium acetate and 0.45 mg kg(-1) for salicylic acid. The critical B concentration in soybean plants was 18 mg kg(-1) on dry weight basis. Multiple regression equations relating soil properties to native soil B extracted by various extractants were developed. It was observed that organic carbon and clay contributed positively to B extracted by hot water, hot CaCl2, and ammonium acetate, while salicylic acid extractable B showed positive relationships with cation exchange capacity (CEC) and clay. The CEC and Fe2O3 were found to have positive influence on tartaric acid extractable B. Implications of the influences of soil properties on the extractable B content of soils are discussed.