A modified Kjeldahl procedure for determining strongly fixed NH ₄ ⁺ ‐N

Summary This paper reports a procedure for determining the content of strongly fixed NH 4 + in soil. The procedure consists of a Kjeldahl digestion followed by an acid attack of the residue with a 5 m HF:1 m HCl solution. Distillations after each of the two treatments recover different forms of NH 4 + . The procedure was tested on fine earth (< 2 mm) and skeleton (> 2 mm) fractions of two forest soils developed on sandstone parent material. In both soil fractions we evaluated three different forms of NH 4 + ‐N: (i) Kjeldahl, (ii) non‐exchangeable and (iii) micaceous. The last is located in the interlayer of mica flakes larger than 50 μm that resist the Kjeldahl digestion and is considered strongly fixed. The total NH 4 + ‐N content of a soil is obtained by the summation of the Kjeldahl and the micaceous NH 4 + ‐N. In the soils under consideration, the micaceous form prevails in the skeleton because this fraction is richer in micas of sand size (> 50 μm). Following the proposed procedure, we found that micas (muscovite and biotite) contain about 3000 mg kg –1 of NH 4 + ‐N in the interlayer. The presence of micaceous NH 4 + ‐N in soil is generally ignored because the skeleton is usually excluded from analyses, and the micas larger than 50 μm cannot be dissolved by the Kjeldahl treatments. The micaceous NH 4 + is the least extractable form of NH 4 + ‐N, and we infer that it is the least available to plants.