Comparative effects of three nitrogen fertilizers applied in fall and spring to a 29-year-old Douglas-fir plantation

Stand growth and mortality were monitored for 13 years after six fertilizer treatments were applied in a 29-year-old Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) medium site quality plantation on Vancouver Island. The equivalent of 224 kg N/ha was added in either fall or spring as ammonium nitrate, urea, or urea - ammonium sulfate, which additionally supplied 56 kg S/ha. These treatments and a control were replicated three times. Foliar analyses indicated insufficient available N initially and increased available N after fertilization. Average, adjusted gross and net cubic volume growth in fertilized plots, however, did not differ from that of controls in any of the intermediate periods or in the total measurement period. This field experiment was not sensitive enough to detect real differences in 13-year growth among the seven treatments. Based on the experimental error measured in this study, seven replications would have been necessary to detect a 20% response in 13-year gross growth at the 10% confidence level; with seven replications, this 20% response would be detected in 90% of all experiments. Suggestions for improving field trials are included.