DOES (K+MG+CA+P) FERTILIZATION LEAD TO RECOVERY OF TREE HEALTH IN A NITROGEN STRESSED QUERCUS-RUBRA L STAND

In The Netherlands atmospheric deposition of nitrogen compounds to forest ecosystems has been very high for some decades and has led to severe nutritional imbalances in soils as well as in trees. At this moment legislation is not fully in effect with respect to lowering emission/deposition fluxes, particularly of nitrogen. The trees suffer mainly from severe magnesium, potassium and calcium and sometimes phosphorus deficiencies. In this study it was investigated whether fertilization with potassium, magnesium, calcium and phosphorus could prevent a Quercus rubra stand from further decline by restoring the nutritional balance, though the detrimental input of nitrogen compounds still continues. Three fertilization treatments were applied: (i) a standard dose of 1250 kg ha-1 containing 60 kg P.ha-1 (as P2O5), 100 kg K.ha-1 (as K2O), 80 kg Mg ha-1 (as MgO) and 340 kg Ca.ha-1 (as CaCO3); (i) two times the standard dose; and (iii) three times the standard dose. Soil solution chemistry showed that the highest dose led to the most significant results: an improved nutrient balance and an increased availability of nutrients. After one growing season following fertilization, the trees looked much healthier and crown density had increased This revitalization lasted for at least four years. For tree health the mid dose seemed appropriate. After the first growing season potassium is the most mobile nutrient, in the soil and also in the trees, but tended to decrease in the fourth year after fertilization. Magnesium and calcium reached normal, values in the leaves after four growing seasons. No obvious effects of phosphorus were found.