Assessing Efficiency of Soil Sulfur Extractants for Fragrant Rice Production

Various chemical extractants have been used for assessing soil sulfur (S) availability for fertilizer recommendations. Currently there is no specific extractant associated with fragrant rice production although S fertilization enhances productivity and aroma intensity as characterized by 2-Acetyl-1-Pyrroline (2AP) content. Field trials were established in five locations to evaluate chemical extractants for plant-available S in acid soils, which represent most fragrant rice production in the north and northeast Thailand. Fragrant rice cultivar KDML105 was grown under a range of S fertilizer rates. Soil samples were collected before planting and at 7 days after S fertilization and extracted with 0.01M Ca(H2PO4)2, 0.25M KCl (40°C), 1N NH4OAc, and distilled H2O followed by turbidimetric S detection. The extracted S was related to rice grain yield, 2AP content and S in leaf. The 0.01M Ca(H2PO4)2 extracted the most S from these acid soils followed by H2O, 0.25M KCl (40°C), and 1 N NH4OAc, but all highly correlated with each other (r = 0.95-0.99, P < 0.01). The 1N NH4OAc, 0.25M KCl (40°C), and distilled H2O, had significantly positive correlations with grain 2AP content at three out of 5 locations, while 0.01M Ca(H2PO4)2 had two. The 1N NH4OAc, which generally had higher correlation coefficients with grain 2AP (r = 0.63-0.83, P < 0.05) and S in leaf (r = 0.79-0.81, P < 0.01), was selected for assessing S deficiency for fragrant rice production in lowland acid soils with a critical soil test S level of 13.9 mg/kg. Rice grain 2AP content was highly and linearly related to soil organic matter (Y = 1.65X+2.40, R2 = 0.84, P < 0.01). The enhancement for fragrant grain yield by S fertilization was found more likely in low organic matter (OM) soils rather in OM-rich S-deficient soils.