Three-Year Response of Ponderosa Pine Seedlings to Controlled-Release Fertilizer Applied at Planting

Four controlled-release fertilizers (fast release [FR], moderate release [MR], slow release [SR] and slow release with micronutrients extended [ME]) were applied, at rates of 0, 5, 15 and 30 g/seedling, to ponderosa pine seedlings (Pinus ponderosa Doug. ex Laws) immediately after planting. Compared to the controls, the 5 and15 g/seedling of FR or ME fertilizer produced significantly greater caliper growth and the 5 and 15 g/seedling of ME fertilizer and 15 and 30 g of FR fertilizer produced significantly greater height growth after 3 yr. Mortality occurred mainly during the first growing season and varied substantially with fertilizer types and dosage. High dosage (30 g/seedling) generally caused more mortality than other dosage levels. Seventy-eight, 54, 51, and 36% of total nutrients had been released from the FR, MR, SR and ME products, respectively, by late August of the first growing season. Early in the second growing season, the FR product had released 98% of its total nutrients, and the MR, SR, and ME products had released over 90% of their nutrients. The best fertilizer treatment, 15 g of the ME product, produced a 21% diameter increase and a 30% height increase 3 yr after treatment. The relative magnitude of the growth responses is similar to those observed from other adjacent placement, controlled-release, seedling fertilization studies in the Northwest. West. J. Appl. For. 17(3):154–164.