GROWTH-RESPONSE TO DIFFERENT TYPES OF NPK-FERTILIZER IN NORWAY SPRUCEPLANTATIONS IN WESTERN DENMARK

In Norway spruce planted on former heathland and fertilized to increas e production, a series of experiments (72 sample plots) was establishe d throughout Central and Western Jutland (Denmark) during the springs of 1978 and 79. The sample plots were all fertilized with 120 kg nitro gen in each of two five-year periods. Different types of mixed nitroge n/potassium/phosphorus fertilizer were applied. According to results f rom fertilization trials in the 1950s and 60s a gain of 3-4 m(3) ha(-1 ) year(-1) was expected. In contrast to these earlier findings, only 0 , 76 m(3) ha(-1) year(-1) was gained over the two periods as an averag e of all sample plots. The initial assumption that nitrogen is the mai n limiting factor for tree growth in Western Denmark no longer seems v alid. The results might indicate that due to increasing nitrogen depos ition during the 1970s and 80s, potassium and maybe phosphorus are dev eloping into new minimum factors, limiting tree growth and devitalizin g the forest ecosystem. Hence, fertilization on these soils should mai nly be considered as a way to compensate such nutrient imbalances.