Growth and Foliar Nutrition of Douglas-Fir Seedlings Provided with Supplemental Polymer-Coated Fertilizer

Polymer-coated fertilizer (PCF) provides a potential means to optimize nutrient delivery for plant uptake, while minimizing leaching. Coating technology varies by manufacturer, which may alter patterns of nutrient release over time among comparable products. Three months following sowing, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings were transplanted into containers with Osmocote Plus (OS) (15-9-12) and Apex (AP) (16-5-9) PCF (each with 5–6 month longevity) applied at four rates (0, 600, 1200, and 1800 mg total N per seedling) as a supplement to periodic addition of conventional water-soluble fertilizer. Seedlings fertilized with OS had 11% greater stem diameter growth during the first 4 months after transplant than those fertilized with AP, but differences were negligible after 9 months. After 4 months, foliar concentrations were 11% higher for N and 10% higher for P in seedlings fertilized with AP, although contents were the same. These results suggest that different PCF types have variable temporal patterns of nutrient release, which may affect seedling development over time. Differences among products must be understood by growers to help formulate fertilizer prescriptions that optimize plant response. Seedling response varied marginally among PCF rates, suggesting little benefit to PCF when used in conjunction with water-soluble fertilizer. West. J. Appl. For. 20(1):58–63.