Growth, Biomass Allocation and Root Growth Potential of Container-Grown Ponderosa Pine Seedlings with Controlled-Release Fertilizer Incorporated in the Root Plug

Prior to sowing seeds, three controlled-release fertilizers (fast release (FR), moderate release (MR) and slow release (SR� were incorporated into the growing media at rates of 0.8, 1.6 or 3.2 grams as supplements to nursery supplied soluble fertilizer to grow ponderosa pine (Pinus ponderosa Doug. ex Laws) seedlings in the greenhouse. At lifting, the caliper, height and total dry weight of fertilized ""160/90"" seedlings ranged from 114 to 129%, 107 to 121% and 130 to 195% larger than those ofthe unfertilized seedlings, respectively. Needle biomass was affected more than stems or roots by the fertilization treatments. The 0.8 grams ofFR or SR fertilizer treatments significantly increased ponderosa pine seedling root growth potential (RGP) compared to the controls. Toxicity from continuous nutrient release during cold storage resulted in much lower RGP for the 3.2 grams ofMR or SR fertilizer treatments. The best dosage for caliper and height growth was 0.8 grams for FR fertilizer, 2.2 and 2.3 grams for MR fertilizer, and 1.9 and 2.0 grams for SR fertilizer, respectively. Lower application rates and shorter fertilizer release periods should be used with these small containers to prevent root damage when cold storage is required before outplanting