Determination of ammonium and nitrate in soils by digital colorimetry

A method of digital colorimetric determination of ammonium and nitrate in soils is proposed. The method is based on corresponding photometric procedures of ammonium and nitrate determination after potassium chloride extraction from soil samples. Ammonium is determined as an indophenol dye, and nitrate is determined as an azo dye. The original procedures were modified to overcome the lower sensitivity of the digital colorimetric method. For ammonium determination, the time required for the reaction to proceed completely was studied. Along with the use of a 96-well microplate protected from ambient light by a special frame, mathematical correction of scattered radiation using black ink and taking the images by a scanner in transmission mode without any post-processing, the resulting colorimetric methods proved to provide accuracy and sensitivity close to those of the spectrophotometric method, and the overall analysis speed for tens of samples was even higher. Limits of detection and quantitation for NO₃^- were 0.42 and 1.4 mg/kg, and for NH₄⁺, they were 1.1 and 3.7 mg/kg, which is lower than for standard methods. The methods' validity was proven by the analysis of standard samples and by the analysis of soil samples collected in several districts of the Moscow region.