Field measurement of soil nitrate concentrations

Information on the nitrate concentration of soil at the time of sowing crops is useful in estimating the optimal amount of fertilizer nitrogen (N). A field measurement of soil nitrate overcomes the delay and cost of laboratory analysis. The accuracy of Merckoquant test strips used with a hand-held Nitrachek reflectometer was tested with a wide range of soil samples. We have found that the system gave accurate results provided two precautions were taken. First, samples should be compared with standard solutions analyzed at the same temperature, because the method was found to be temperature dependent. Second, the extractant should be 0.5M K2SO4 or water, since the usual soil extractant, KCl, interferes with the analysis. The use of water for extraction reduces the cost of analysis and makes it feasible to analyze kilogram amounts of soil, thereby reducing the error of subsampling. We report a procedure which minimizes the time of extraction and hence the chance of denitrification, by filtering the suspension of soil in water through a 0.8-mm filter attached to a syringe.