Abstract Physical and chemical changes in surface peat layers after afforesting virgin peat lands are described for two sites, one with pine and one with spruce forest. Over a 27-year period the cation exchange capacity (pH7) of the pine site peat (0–40 cm depth) increased 32–38% after drainage and 37–43% after additional fertilization. Base saturation (pH7) was reduced 4% and increased 0.8% for the two categories of plots. Shifts in pH were moderate on the pine plots (0.1–0.2 units). On the spruce plots, however, pH fell from 4.33 to 3.73, and base saturation from about 20% to 8.1%, which indicates a rather strong acidification. Atmospheric deposition of H + on the fertilized pine plots has been about 700 mol ha −1 year −1 and hydronium production could have been about 520 mol ha −1 year −1 in the actual period. The 0–62 cm peat layer of the pine site subsided about 22 cm from 1957–1983, with an estimated dry matter loss of 5.8 tons ha −1 year −1. Budget calculations showed that future fertilizations with K, and to a lesser extent also P, will be crucial for balanced nutrition. Magnesium, Mn, and Cu might be depleted, while atmospheric deposition and mineralization of organically fixed N should be adequate for the years to come.