In areas that remain unaffected by industrial pollution soil acidification is mainly caused by the release of protons (H ÷) during the oxidation of carbon (C), sulphur (S) and nitrogen (N) compounds in soils. In this review the processes of H ÷ ions release during N cycling and its effect on soil acidification are examined. The major processes leading to acidification during N cycling in soils are: (i) the imbalance of cation over anion uptake in the rhizosphere of plants either actively fixing N 2 gas or taking up NH 4 ions as the major source of N, (ii) the net nitrification of N derived from fixation or from NH 4 and R -N H 2 based fertilizers, and (iii) the removal of plant and animal products containing N derived from the process described in (i) and losses of NO3-N by leaching when the N input form is N2, NH 4 or R -N H 2. The uptake of excess cations over anions by plants results in the acidification of the rhizosphere which is a "localized" effect and can be balanced by the release of hydroxyl ( O H -) ions during subsequent plant decomposition. Nitrification of fixed N 2 or NH 4 and R -N H 2 based fertilizers, and loss of N from the soil either by removal of products or by leaching of NOa-N with a companion basic cation, lead to 'permanent' acidification.