Relative Efficiency of Different Extractants for Available Boron Estimation in Alkaline Calcareous Soils

Seven extractants widely used to determine plant-available boron (B) from either acidic or alkaline soils were tested for 20 alkaline and calcareous soil series from cotton belt of Punjab, Pakistan. The hot-water-soluble boron (HWS-B) extraction procedure was used as a benchmark to see any relative variation in available B in comparison with alternate extractants [0.05 M hydrochloric acid (HCl), 0.018 M calcium chloride (CaCl2), 1 M ammonium acetate (NH4OAc), 0.25 M sorbitol-diethylenetri-aminepentaacetic acid (DTPA), 0.05 M mannitol prepared in 0.01 M CaCl2, 0.005 M ammonium bicarbonate (AB)-DTPA]. The soil series used in this study belonged to four soil orders (Aridisols, Entisols, Alfisols, and Inceptisols). The greatest mean B concentration (0.32 +/- 0.11 mg kg(-1)) was obtained by hot water extraction followed by 0.05 M HCl extraction (0.31 +/- 0.12 mg kg(-1)), whereas the lowest was obtained by 0.25 M sorbitol-DTPA (0.24 +/- 0.06 mg kg(-1)) extraction. The comparison between soil series showed that the greatest HWS B and 0.05 M HCl B were 0.53 and 0.56 mg kg(-1), respectively, in the Rustam series while the lowest were 0.11 and 0.12 mg kg(-1), respectively, in the Thal series. Rasulpur, Rangpur, Sultanpur, Thal, and Kasur series had the lowest (<0.20 mg B kg(-1)) HWS-B concentrations. Although there was significant correlation for available B among all the extractants tested, the strongest correlation (r = 0.99** at P < 0.01) was observed between HWS B and 0.05 M HCl-extractable B. This study revealed that the 0.05 M HCl extraction method may substitute the hot water extraction method for plant-available B in alkaline and calcareous but modest fertile soils.