COMPARISON OF TOTAL NITROGEN BY 4 PROCEDURES AND SEQUENTIAL DETERMINATION OF EXCHANGEABLE AMMONIUM, ORGANIC NITROGEN, AND FIXED AMMONIUM IN SOIL

Eight soils were analyzed for total N by: (i) steam distillation of NH3 yielded after conventional H2SO4 digestion, (ii) HF-H2SO4 digestion, (iii) HF-HCl treatment in a microwave system, or (iv) a CHN analyzer. Comparison of results showed that the customary Kjeldahl methods underestimate total N in soils. After determination of exchangeable NH4, soil organic matter was destroyed by treatment with a reagent composed of hydrogen peroxide plus pyrophosphate plus NaCl to prevent swelling of clays, buffering at pH 5 to avoid loss of NH3. Organic N was calculated as the sum of NH4 and NO3 determined in the resulting solution produced during organic matter oxidation. Soils residues were analyzed for fixed NH4 using the three above mentioned procedures. It was deduced that underestimation of total N given by the customary Kjeldahl method depends on inefficiencies of acid digestion in releasing NH4 trapped in clay lattices. Values for total N calculated from the sum of exchangeable NH4, organic N, and fixed NH4 are fairly consistent with those determined directly.