Evaluation of Soil Tests for Plant-available Mercury in a Soil-Crop Rotation System

A reliable soil test is needed for estimating mercury (Hg) availability to crop plants. In this study, four extraction procedures including 0.1M hydrochloric acid (HCl), 1M ammonium acetate (NH4OAc) (pH7.0), 0.005M diethylenetriaminepentaacetic acid (DTPA), and 0.1M calcium chloride (CaCl2) (pH5.0) were compared for their adequacy in predicting soil Hg availability to crop plants of a rice-cabbage-radish rotation system. The amounts of Hg extracted by each of the four procedures increased with increasing equilibrium time. The optimal time required for extraction of soil Hg was approximately 30min, though it varied slightly among the four extractants. The amounts of Hg extracted decreased with increasing soil/solution ratio, and a soil/solution ratio of 1:5 appeared to be adequate for soil Hg availability tests. The amounts of Hg extracted increased in the order of NH4OAc CaCl2 DTPA HCl in silty loam soil (SLS) soil, and the order was NH4OAc CaCl2 DTPA HCl in yellowish red soil (YRS) soil. Significant positive correlations among the four extractants were obtained in SLS soil. In contrast, the correlations were poor in YRS soil, especially for HCl. There were significant correlations between concentrations of Hg in edible tissue of three plants and the amounts of soil Hg extractable to the four extractants for soil-rice system and soil-radish system, but not for soil-Chinese cabbage system. The 0.1M HCl extraction overall provided the best estimation of soil-available Hg and could be used to predict phytoavailability of Hg in soil-crop systems.