Evaluation of rapid analysis systems for on-farm nitrate analysis in vegetable cropping

Abstract

Small analytical systems enabling rapid on-farm analysis may be useful for monitoring crop and soil nutrient status. Two commercially available, rapid analysis systems were evaluated for analysis of (NO3-) in aqueous standard solutions (AS), and in soil solution (SS), plant sap (PS) and fertigation nutrient solutions (NS) obtained from tomato crops. The systems were the Merck RQFlex reflectometer and the Hanna Instruments C115 Grow Master colorimeter. (NO3-) in AS covered the analytical range of each instrument, being 0.3-3.2 and 0.3-16.1 mmol/L, respectively, for the reflectometer and colorimeter. (NO3-) were 5-14 mmol/L for SS, 2.3-33 mmol/L for NS, and 63-158 mmol/L for PS. Accuracy was evaluated using linear regression against (1) known concentration for AS, and (2) laboratory analysis for SS, PS and NS. For the reflectometer with AS, SS and PS, the linear regression had slopes of 1 +/ - 0.08 and intercepts of <0.6 mmol/L. In NS up to 9 mmol/L, there was a similar slope, with an intercept of 1.3 mmol/L. In the three most concentrated NS samples, there was a larger relative error that was attributed to faulty dilution. Coefficients of determination were always r2 >/= 0.94. The reflectometer was generally accurate for all media examined; it was rapid (1 min per analysis) and simple to use. The colorimeter was generally similarly accurate; however, it was slower, required reagent preparation and was subject to colour interferences. This work demonstrated that the reflectometer can provide rapid and accurate on-farm measurement of (NO3-) in SS, PS and NS.