Comparison of methods for determination of nitrogen levels in soil, plant and body tissues, and water

Four different methods to include all forms of nitrogen (N), especially nitrate (NO3), not usually measured in the analysis using the standard total Kjeldahl nitrogen (TKN) digestion procedure, were investigated. The methods compared were: total nitrogen determination by combustion using a LECO FP‐428 Nitrogen Determinator, salicylic acid modification prior to TKN digestion, a method of reduction of nitrate‐nitrogen (NO3‐N) to ammonium‐nitrogen (NH4‐N) before digestion using CrK(SO4)2, and the addition of phenyl acetate to the standard digestion mixture. Obtained results were compared to those obtained by the standard TKN digestion procedure. The tests were performed on air‐dried soil, oven‐dried plant and body issue, and a water sample. The purpose of the investigation was to ascertain which method provides the best recovery and is most easily incorporated into a laboratory setting. Acid digestion was done using a Tecator Block Digester with a forty 75‐mL tube capacity. All digestions beyond pretreatment were identical. Water samples were digested for 30 minutes at 230°C, and then the block temperature was increased to 360°C with the rest of the sample digestion tubes being placed into the block to digest for two hours. The sample N contents were determined by a diffusion‐conductivity method in which the sample passes through a gas‐permeable membrane and ammonia (NH3) is carried to a conductivity cell where it is measured and recorded. The change in conductivity is proportional to the amount of NH4‐N in the digest sample.