Phosphorus Requirement and Chemical Fate in Containerized Nursery Crop Production

Environmental contamination issues related to phosphorus (P) in surface waters substantiates the need to identify minimally-sufficient P fertilization amounts for production of containerized nursery crops and better understand the effect of routine amendments (i.e., dolomite [DL] and micronutrient fertilizer [MF]) added to pine bark substrates on chemical fate of P fertilizer. Four studies were conducted to accomplish two overarching objectives: 1) determine the minimum P fertilization amount and corresponding pore-water P concentration needed to achieve maximal growth of common containerized nursery crops and 2) determine the effect of DL and MF amendments in pine bark on P retention during irrigation and P fractions in substrate pore-water. In a fertigation, greenhouse study, calculated lowest P-fertilizer concentration that sustained maximal growth in Hydrangea paniculata ‘Limelight’ (panicle hydrangea) and Rhododendron ‘Karen’ (azalea) was 4.7 and 2.9 mg·L⁻¹ , respectively, and shoot growth Ilex crenata ‘Helleri’ (holly) was the same when fertilized with 0.5 to 6.0 mg·L⁻¹ P. Porewater P concentrations corresponding with treatments that sustained maximal growth of panicle hydrangea, azalea and holly were as low as 0.6, 2.2 and 0.08 mg·L⁻¹ P, respectively. In a separate study, utilizing low-P controlled-release fertilizers (CRFs), shoot growth of Hydrangea macrophylla ‘P11HM-11’ (bigleaf hydrangea) produced in two ecoregions was maximal when fertilized with as little as 0.3 g CRF-P per 3.8-L container, a 50% P reduction from the industrystandard CRF. Holly required 0.2 or 0.4 g CRF-P depending on ecoregion. Mean pore-water P concentrations that corresponded with highest SDW were 0.8 and 1.2 mg·L⁻¹ for hydrangea and holly, respectively. When irrigating fallow pine bark columns containing CRF for 48 d, amending pine bark with DL and MF reduced orthophosphate-P (OP-P) leachate concentrations by ≈ 70%, most of which was retained within the substrate. In a greenhouse study, containerized Lagerstroemia ‘Natchez’ (crape myrtle) were grown for 91 d in pine bark containing CRF. In pine bark amended with DL and MF, pore-water OP-P and total P concentrations, measured approximately weekly, were reduced by, on average, 64% and 58%, respectively. Total dry weight values of plants grown with DL plus MF or MF-only were 40% higher than those grown with no amendments; however, tissue P amounts and relative P uptake efficiency were the same among plants in these three treatments. Therefore, sorption of OP-P by DL and MF reduced water-extractable OP-P but did not limit P uptake by plants.