Evaluation of Extractants and Critical Limits of Sulphur in Rice Soils of Bangladesh
An investigation was undertaken to evaluate suitable extractant(s) for available sulphur and critical limits of sulphur for wetland rice soils of Bangladesh. Some 22 soils from 0-15 cm depth were collected from different locations of Old Brahmaputra Floodplains of the country. Sulphur in the soils was extracted with four different extractants, MCP (500 ppm P), CaCl2 (0.15%), NH4OAc (0.5 M) and NaHCO3 (0.5 M). Rice plants were grown in pots treated with and without S for eight weeks. At harvest dry matter of rice was recorded. The critical level of S was determined by both graphical and statistical methods. The extractable S of the soils varied considerably with the soils and the extractants used. The ability of the extractants to extract S followed the order: 0.5 M NH4OAc > 0.5 M NaHCO3, >0.15% CaCl2 > MCP. The MCP extractable S showed significant and positive correlation with organic matter, available P and exchangeable K contents but was significantly and negatively correlated with soil pH. The amount of extractable S by other methods did not show any significant correlation with soil properties. The extractable S by any pair of extractants viz., CaCl2 vs NH4oAc, CaCl2 vs NaHCO3 and NH4oAc vs NaHCO3 were significantly and positively correlated. The critical levels of MCP, CaCl2, NaHCO3 and NH4OAc extractable S were 9.3, 9.7, 15.8 and 17.8 mg kg-1, respectively in both graphical and statistical methods for rice. The critical limit for plant S was found to be 0.12% at 56 days of crop growth.
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