GROWTH AND NUTRIENT-UPTAKE OF DOUGLAS-FIR SEEDLINGS AT DIFFERENT RATES OF AMMONIUM SUPPLY, WITH OR WITHOUT ADDITIONAL NITRATE AND OTHER NUTRIENTS

Two pot trials were conducted to study the effects of soil acidificati on and excess ammonium on root and shoot development of juvenile Dougl as fir on an acid sandy forest soil. Experiment I included a control t reatment (without fertilizer application) and different supply rates o f NH4. Application of other nutrients to the NH4-fertilized pots was c onstant, while at one supply level the added N included 50% nitrate in order to study the effect of the N form. High supply rates of ammoniu m suppressed root length growth, but did not affect shoot growth durin g one season of application. Root and stem growth was stronger with a mixture of ammonium and nitrate than with pure ammonium as N source. I n the second experiment the effect of balanced fertilization, addition al to nitrogen, was studied at low and high NH4-N supply during a two- year period. In the second year bud break was retarded and shoot growt h 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 23 0 kg.ha-1. Addition of base cations and P to the ammonium application raised P and K needle concentrations, but could not prevent adverse ef fects of NH4, and even increased acidification of the soil. In both po t experiments the presence of a litter layer tended to increase tree g rowth, 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 the nutrition of P and K as well.