A modified Kjeldahl procedure for determining strongly fixed NH4+-N

This paper reports a procedure for determining the content of strongly fixed NH4+ 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 NH4+. 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 NH4+-N: (i) Kjeldahl, (ii) nonexchangeable and (iii) micaceous. The last is located in the interlayer of mica flakes larger than 50 mu m that resist the Kjeldahl digestion and is considered strongly fixed. The total NH4+-N content of a soil is obtained by the summation of the Kjeldahl and the micaceous NH4+-N. In the soils under consideration, the micaceous form prevails in the skeleton because this fraction is richer in micas of sand size (>50 mu m). Following the proposed procedure, we found that micas (muscovite and biotite) contain about 3000 mg kg(-1) of NH4+-N in the interlayer. The presence of micaceous NH4+-N in soil is generally ignored because the skeleton is usually excluded from analyses, and the micas larger than 50 mu m cannot be dissolved by the Kjeldahl treatments. The micaceous NH4+ is the least extractable form of NH4+-N, and we infer that it is the least available to plants.