A rapid and sensitive analysis of inorganic and organic phosphorus (P) is needed to analyze water and soil extracts at submicromolar concentrations. The proposed method, based on the complexation of malachite green with phosphomolybdate under acidic conditions, was adapted to a 96-well microtiter plate format, and was tested for matrix interferences using 15 soils and some common extractants, including water, KCI, CaCl2, NaOH, and HCl. The accuracy of P determination was affected when CaCl2 and HCl concentrations were greater than 0.1 M and when NaOH concentration exceeded 0.4 M. Potassium chloride concentration up to 1 M did not interfere with P determination. The molar absorptivity was 46 841 M(-1) cm(-1) and the reagent blank absorbance was 0.071+/-0.003 (n = 10). At the 99% confidence limit, the method detection limit was calculated to be 0.006 mg P L(-1). Recovery of added inorganic P in different types of soils and extracts ranged between 95 and 112% with an average of 102%. The proposed microplate method allows P to be determined rapidly in a wide range of soil types and extracts and requires limited volume (20-200 microL). The procedure uses limited quantities (40 microL) of two stable reagents (>1 yr), and generates low amounts of hazardous waste.