2018 ASHS Annual Conference

The essential mineral nutrient phosphorus (P) must be supplied throughout production of containerized ornamental crops because of low anion exchange capacity of soilless substrate paralleled with frequent irrigation. The resulting crop P use efficiency can be as low as 20% resulting in wastefulness of a finite resource that is increasing in price and under regulatory scrutiny because of its ability to degrade surface waters via eutrophication. Additionally, P fertility research is lacking two-fold: (1) reducing P below the recommended low rate in containerized woody taxa of 5 mg·L^-1 and (2) P availability or fate in soilless substrates. This led to five-years of research focused on optimizing P fertility rates for seven taxa and laboratory experiments to determine the fate of P in substrate solution. Liquid fertilization studies with constantly applied 0.5 to 6 mg·L^-1 P determined the minimum P concentration required to sustain maximal growth was 5 mg·L^-1 for ‘Limelight’ hydrangea, 3 mg·L^-1 for ‘Karen’ azalea, and 1 mg·L^-1 for ‘Helleri’ holly. Studies using controlled release fertilizers (CRF) with varying rates of P in a zone 6 or 8 ecoregion determined maximal growth of Bloomstruck hydrangea could be attained with a 50% reduction in P compared to a conventional 15N-2.6P-10K CRF, regardless of hardiness zone, when produced with 18N-1.3P-10K. Growth of Knockout rose was equivalent when comparing two CRF, 18-1.7P-6.6K (conventional) versus 18N-1.3P-10K (low P), across zones 6 and 8. Green Giant’ arborvitae, Encore azalea, ‘Natchez’ crape myrtle, and ‘Annabelle’ hydrangea required the 18-1.7P-6.6K (conventional) CRF to maximize growth. This may be in part because the conventional fertilizer formulation used in this specific trial had a higher N:P ratio (10:1) compared to the 6:1 ratio historically used in conventional fertilizer; however, this was not specifically investigated. In laboratory experiments we found that common substrate amendments reduced availability of the P inherent to the bark as well as that which is released by the CRF. The immediate availability and subsequent leaching of this initial P can be reduced 35% with lime, 54% with micronutrients, or 78% when amending pine bark with both dolomite and micronutrients. Once the CRF released (≈ 20 days), available or leachable P can be reduced by more than 50% for > 30 days when amending with dolomite with or without micronutrients. These data can be used to further refine P fertility when producing containerized woody nursery crops.