Method 56a Determination of soil nitrogen by autoanalysis

If the analysis is to be carried out in a similar way to total N in herbage, by acid-digestion in test tubes in an aluminium block followed by a colorimetric autoanalysis procedure, then take a 0.5-g sample of air-dry soil. If serious frothing occurs, take 0.2 g soil, and adjust the calculation accordingly. Reducing the amount to ^0.1 g may give peaks which are too small to be read with confidence. See Chapter 7 (pp. 138-141) for details of standards, reagents and method.

If the soil contains significant amounts of nitrate, which should be included with the organic and ammonium-N for total nitrogen, it must be reduced to ammonium-N by the following procedure (Method 5.6a.i). Determination of N on duplicates of the same soil sample with and without a reducing step will enable an estimation of the nitrate-N (plus any nitrite-N) by difference. A more precise colorimetric determination of just the nitrate component uses the method described for herbage, except that 10 ml of fresh or freshly thawed soil is used.

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