Inability of the Kjeldahl Method to Fully Measure Indigenous Fixed Ammonium in Some Soils

In the course of some N fractionation studies in soils, inconsistencies resulted between the summation of the N fractions and the total soil N. In some, but not all, soils the summation figure exceeded by 25% the Kjeldahl figure. This inconsistency was especially noted in one soil that contained as much as 55% of the total N as indigenous fixed ammonium. A question was thus raised about the accuracy of the Kjeldahl procedure to measure indigenous, clay‐fixed ammonium. When soils were treated with HF before Kjeldahl analyses, the difference between the summation figures and the modified Kjeldahl results was < 5%. These results leave little doubt that on soils which contained from 11 to 55% of the total N as fixed ammonium the HF pretreatment increased the total N recovered.