Discussion 55 Determination of available nitrogen

Soil nitrogen reactions


N2 — NH4+ — organic-NH2


NH4+ and NO3- — organic-NH2


organic-NH2 — NH4+


NH4+ — NO2- — no3-


NH/ + OH- s NH3T + H2O

The available nitrogen is equivalent to the mineralized nitrogen, which consists of the soluble nitrate and nitrite, and the exchangeable and soluble ammonium nitrogen. These compounds fluctuate over short periods and are greatly affected by microbial activity; the ammonia gas may escape from the sample by volatilization. The soil sample should be transported in an icebox and transferred to a freezer unless immediately analysed. If just the available nitrate is required, it may be analysed according to the colorimetric auto-analysis method given for nitrate in herbage (Method 7.6). It may also be extracted in the traditional manner using either a saturated or a 0.01 M calcium sulphate solution to reduce cloudiness in the filtrate, followed by analysis using a colorimetric method or a selective ion nitrate electrode. The latter method is given below.

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