improved procedure for determining magnesium fertilizer dissolution in field soils

The sequential extraction procedure currently used to measure magnesium (Mg) fertilizer dissolution in soils consists of removing dissolved Mg (step 1), and partially dissolved Mg (step 2), followed by an 18-h extraction with 2 M HCl at room temperature to determine undissolved Mg (step 3). This procedure is satisfactory for soluble and moderately soluble Mg fertilizers but is not an accurate procedure for slightly soluble fertilizers, such as serpentine. When step 3 is replaced by a digestion procedure using 2 M HCl for 4 h at 90-95 degrees C (improved step 3), the total serpentine Mg recovery (dissolved and undissolved Mg) from soil samples, either immediately after serpentine was added to soil or after a 21-day incubation with moist soil, was about 100% compared to 40-50% by the original procedure. The improved procedure also increased the recovery of serpentine Mg applied to field soils. Therefore, this study recommends that the third step of the sequential extraction procedure be replaced by a 4 h digestion using 2 M HCl (90-95 degrees C).