Evaluation of Mehlich III as an extractant for available soil sulfur

Abstract

Among the identified universal extractants, Mehlich III is one of the more reliable multielement extractant for acid soils and soilless mixes. However, the universality of the Mehlich III extractant does not include an important nutrient element, sulfur (S). This study was conducted in order to evaluate the Mehlich III extractant as an index of S availability, and comparing with four other extractants, monocalcium phosphate (MCP), MCP+HOAc, Morgan's Reagent, and calcium chloride. Soil samples representing tea growing areas of several parts of India were collected from selected tea gardens to compare S extractability. These extractants were also evaluated in a young tea plantation (3+ year old) field experiment which is situated near the foot hill soils (Typic Hapludalfs) of Indian western Himalaya. It was found that Mehlich III‐extractable S was significantly correlated with the S extracted by the other extractants from both the top and subsoils of the tea growing region. However, the largest correlation coefficient was found between the Morgan's Reagent and Mehlich III. MCP, MCP+HOAc, and Morgan reagents extracted S mainly from the adsorbed fraction, although some degree of correlation was found with water‐soluble and ester sulfate‐sulfur (SO₄‐S) fractions. In case of the Mehlich III extractant, 83 and 87% of the variability was explained by water‐soluble and adsorbed SO₄ in the respective top and subsoils. Except for the regression equation between Morgan's Reagent‐extractable S and the S content in tea shoots, all other equations were statistically significant. But, only available S extracted from the top soil by Mehlich III was found significantly correlated with S uptake, which was further confirmed by a significant coefficient of regression (r=0.552*). This suggests that the Mehlich III reagent could be a useful index for predicting the S requirement for a tea crop.