The Elvelyn photoelectric colorimeter has been employed for quantitative determination of nitrates in soil samples and in culture media. The use of this apparatus enables rapid and accurate measurement of nitrate content by the phenoldisulfonic acid method even in the presence of extraneous colors, semiturbid solns., and minimal amts. of nitrates. Standard nitrate solns. may be dispensed with after preliminary calibration of the instrument. Lack of uniformity of K (calibration constant) when filter No. 420 is employed is not an obstacle to the method reported, in view of the ease of preparation of a calibration curve and the stability of the calibration once made, as well as the rapidity with which nitrate conc. can be read from the curve.