SENSITIVITY OF THE NITRATE-ION MEMBRANE ELECTRODE IN VARIOUS SOIL EXTRACTS

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Abstract
Measurement of NO3 concentration in a series of soils with a specific-ion membrane electrode varied with the amount of dispersed colloidal material present in the soil extract. Nitrate-N values with the membrane electrode were in good agreement with those by colorimetric (Phenoldisulfonic acid, PDS) and reduction-diffusion (NO3 to NH3) methods for filtered or flocculated water extracts, and for filtered or unfiltered salt extracts (saturated CaSO4). In water extracts having dispersed colloidal material (i.e., unfiltered or unflocculated) the measurements were higher (1 to 6 ppm N) with the membrane electrode than by the other two methods, and the discrepancy increased generally with the clay content (< 2 μ size) of the soil. Removal of the dispersed material by filtration (no. 42 Whatman), addition of a polyacrylamide flocculating agent, or extraction with a salt solution reduced the discrepancy. Improved reproducibility was obtained by adjusting the meter to a standard NO3 concentration (e.g., 100 mv with a 10-ppm standard) and using a larger aliquot of the extract at low concentrations (100 ml for concentration < 2 ppm N). The standard deviation for measurements made on a standard solution of 1 to 100 ppm N repeated over a period of several months was 1.74 mv.