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Published in Soil Sci Soc Am J 52:1020-1023 (1988)
© 1988 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Comparison of Total Nitrogen by Four Procedures and Sequential Determination of Exchangeable Ammonium, Organic Nitrogen, and Fixed Ammonium in Soil

Livia Vittori Antisari and Paolo Sequi*

Inst. of Agricultural Chemistry, Univ. of Bologna, Viale Berti Pichat n. 10, Via S. Giacomo n. 7, 40127 Bologna, Italy

* Corresponding author.

ABSTRACT

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, buffered 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. Soil 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.

NOTES

Supported by funds of the Italian Ministry of Education and by funds of Agrimont S.p.A., Milan.

Received for publication October 26, 1987.




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