DOUGLAS-FIR GROWTH AND FOLIAR NUTRIENT RESPONSES TO NITROGEN AND SULFUR FERTILIZATION

Nitrogen-fertilizer response in conifer stands of the Pacific Northwest has been related to soil and foliar S, and growth has occasionally been enhanced by the addition of S. In order to test the effect of soil SO4-S concentrations on N response, five stands of Douglas fir, Pseudotsuga menziesii (Mirb.) Franco, with low to moderate quantities of SO4-S in the mineral soil horizon were treated with N and with N plus S. Foliar N concentrations in the N-with-S treatments were generally higher than in the N-alone treatments. Periodic annual growth response to N over the entire study period was inversely related to site index and directly related to foliar N content.