Two pot trials were conducted on effects of soil acidification and excess ammonium on root and shoot development of juvenile Douglas fir (Pseudotsuga menziesii) on an acid sandy forest soil. Experiment I included a control treatment (without fertilizer application) and different supply rates of NH4. Application of other nutrients to the NH4-fertilized pots was constant, while at one supply level the added N included 50% nitrate in order to study effect of N form. High supply rates of ammonium suppressed root length growth, but did not affect shoot growth during one season of application. Root and stem growth was stronger with a mixture of ammonium and nitrate than with pure ammonium as N source. Experiment II examined balanced fertilization, additional to nitrogen, at low and high NH4-N supply during a two-year period. Second year bud break was retarded and shoot growth depressed at high levels of ammonium supply. In August of the second year nearly all trees died that had received a total NH4-N dose of 230 kg/ha. Addition of base cations and P to ammonium application raised P and K needle concentrations, but could not prevent adverse effects of NH4, and even increased acidification of soil. In both experiments presence of a litter layer tended to increase tree growth, and alleviated adverse effects of ammonium in soil and needles. A corresponding fertilizer application in a mature Douglas fir stand on the same soil improved nutrition of P and K as well.