Evaluation of extractants for predicting availability of boron to mustard in arid soils of India

Greenhouse and laboratory studies were undertaken to evaluate the suitability of extractants for determining the boron (B) status of arid soils of Rajasthan (India) using mustard (Brassica juncea L.) as a test crop. Mustard is an important oilseed crop of northern India, which is widely grown in Rajasthan. This crop is sensitive to B deficiency. For predicting response of mustard to B application, several extractants have been tried with variable results. Seven extractants, commonly used for estimating the availability of B, were evaluated and B in soil extract and in plant tissue were assayed using the azomethine-H colorimetric method. The suitability of seven extractants for determining available soil B was assessed by correlating the amount of extractable B in untreated soils with Bray's percent yield, plant tissue B concentration and B uptake by mustard. The results indicate that hot 0.01 M CaCl2 and hot water extracted comparatively more B than other extractants. The amount of B extracted by these extractants was found to be in the following order: hot 0.01 M CaCl2 > hot water > 1.0 M NH4OAc (pH 4.8) > sorbitol + NH4OAc + TEA = mannitol + NH4OAc + TEA > mannitol + CaCl2 > salicylic acid + NH4OAc + TEA. Simple correlation coefficients of the amount of B extracted by different extractants and soil characteristics were calculated. Based on present studies, the suitability of extractants for mustard crop in arid soils of Rajasthan may be arranged as follows: hot 0.01 M CaCl2 > hot water > sorbitol + NH4OAc + TEA = mannitol + NH4OAc + TEA > 1.0 M NH4OAc (pH 4.8) > salicylic acid + NH4OAc + TEA> mannitol + CaCl2 soluble B. The critical values in respect to sufficiency of extractable B for mustard plants were: 0.52 for hot 0.01 M CaCl2, 0.33 for hot water, 0.31 for mannitol + CaCl2, 0.34 for 1.0 M NH4OAc (pH 4.8), 0.19 for sorbitol + NH4OAc + TEA, 0.24 for mannitol + NH4OAc + TEA and 0.19 mg kg(-1) for salicylic acid + NH4OAc + TEA. The critical B concentration in mustard plants was 28.05 mg kg(-1) on dry weight basis.