Biosolids at 750, 1000, and 1500 kg-n/ha and conventional fertilizer at 225 kg-n/ha were applied in each of three seasons (spring, summer, and fall) to a 15-year-old coastal Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) plantation that had recently been precommercially thinned and pruned. Five-year results showed that rate and timing effects were independent. No height-growth response was evident, but annual diameter growth for all biosolids treatments averaged three times higher than for conventional fertilizer. Seasonal application differences were small but statistically significant. All plots, including the controls, experienced extensive top damage from snow and ice. Similar to growth, damage was greatest with biosolids fertilization. The study shows promise for biosolids fertilization as a viable alternative to conventional fertilization, but application in locations prone to snow and ice damage should be avoided.
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Updated April 19, 2007