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. Sulfate-S concentrations varied among soils from 6.0 to 19.2 mg S kg⁻¹ in the surface soil and from 6.0 to 78.7 mg S kg⁻¹ in the subsoil. Three treatments were applied: control, 224 kg N ha⁻¹ as urea, and 224 kg N ha⁻¹ plus 112 kg S ha⁻¹ as a mixture of urea and (NH₄)₂SO₄. Adjusted gross volume growth responses to N alone averaged −1.63 to 6.26 m³ ha⁻¹ yr⁻¹ after 3 to 4 yr; average volume growth gains from application of N with S over application of N alone varied from 0.73 to 4.5 m³ ha⁻¹ yr⁻¹ at four locations. At one location, the N-alone effect was greater than that from N with S. Foliar N concentrations in the N-with-S treatments were generally higher than in the N-alone treatments. Little change in foliar-S content occurred in the N-with-S plots. Mean treatment effects were unrelated to foliar N/S, total S, or foliar SO₄-S on the controls, and the relationship between soil SO₄-S and the differential growth response between N and N with S was highly variable. Treatments were reapplied at a rate of 448 kg N ha⁻¹, 224 kg S ha⁻¹, and 9 kg B ha⁻¹. Four-year growth effects were similar to the first period except at one location where N responses appeared greatly improved by B. Periodic annual growth response to N over the entire study period was inversely related to site index and directly related to foliar N content. Prescriptions involving S will require more efficient S sources and a better assessment of S uptake and S reactions in these soils.