Comparison of Total Nitrogen by Four 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 NH 3 yielded after conventional H 2 SO 4 digestion, (ii) HF‐H 2 SO 4 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 NH 4 , soil organic matter was destroyed by treatment with a reagent composed of hydrogen peroxide plus pyrophosphate plus NaCl to prevent swelling of clays, buffered at pH 5 to avoid loss of NH 3 . Organic N was calculated as the sum of NH 4 and NO 3 determined in the resulting solution produced during organic matter oxidation. Soil residues were analyzed for fixed NH 4 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 NH 4 trapped in clay lattices. Values for total N calculated from the sum of exchangeable NH 4 , organic N, and fixed NH 4 are fairly consistent with those determined directly.