Short-term assay of soil urease activity using colorimetric determination of ammonium

Summary A rapid assay for soil urease in the absence of bacteriostatic agents has been developed. The method comprises incubation of soil with an aqueous or buffered urea solution, extraction of ammonium with 1 N KCl and 0.01 NHCl and colorimetric NH₄ ⁺ determination by a modified indophenol reaction. The method is characterized by high sensitivity and stability of the coloured complex formed. Measurements obtained by this method showed that no change in urease activity occurred when field-moist samples of soils were stored at –20°C for as long as 5 months. Air-drying of field-moist soil samples may lead to an increase in urease activity.