Automatic methods for the determination of nitrogen, phosphorus and potassium in plant material

Abstract

Methods are described, for use with the Technicon Auto Analyzer, that have been successfully used in the determination of nitrogen, phosphorus and potassium in plant materials. Nitrogen is determined as the indophenol-blue complex on an aliquot of solution after digestion by a micro-Kjeldahl technique. Alternative methods are suggested for the final determination of nitrogen dependent on the range of values expected. The “bias” method is extremely sensitive to small variations in nitrogen levels, and, because of this high sensitivity, the recorder signal-to-noise ratio must be high. Methods are suggested for obtaining a maximum signal-to-noise ratio. Phosphorus and potassium are analysed at the same time on the same sample of ash solution with a single manifold. Phosphorus is determined colorimetrically by the yellow phosphovanadate complex, and potassium by flame photometry by using an internal standard of lithium nitrate. Nitrogen determinations can be carried out at a rate of 40 samples per hour, and phosphorus and potassium together at a rate of 60 samples per hour. The accuracy of these determinations is better than those obtained by other recognised techniques.