Evaluating the Validity of a Nitrate Quick Test in Different Chinese Soils

Because laboratory tests are expensive and time-consuming and may not be available to farmers, soil nitrate quick tests are required for optimal nitrogen management strategies in China to increase nitrogen use efficiency and to reduce nitrogen losses. A total of 328 soil samples were collected at different soil depths from 225 sites in China, which covered a wide range of climatic and geographic regions, soil types, croplands and soil textures, to evaluate the suitability of a quick reflectometer test method for analysing soil NO^-₃-N in a wide range of soil NO^-₃ concentrations, soil types and cropping systems in China, mainly by comparison of soil NO^-₃-N assessed by a quick-test method (a reflectometer) and a standard laboratory method, i.e., high-performance liquid chromatography (HPLC). The reflectometer showed excellent agreement with the laboratory HPLC method with regard to soil nitrate contents for all analysed soil samples. The linear regression had slopes of 1 ± 0.08 and intercepts of ± 1.38 mg NO^-₃-N L^-1 among different soil types and croplands. Compared with the 1:1 lines, the regression analysis for each soil type showed statistically significant but small differences in slope; the relative difference between the values measured using the two analytical systems varied from -8% to 6%, and there were no differences in intercept except for paddy soil. The reflectometer showed adequate, statistically significant precision in determining soil nitrate contents, and it could therefore be directly used instead of the laboratory methods for soil NO^-₃-N measurement in China.