2019 ASHS Annual Conference

Pine bark is the primary substrate component used for containerized nursery crop production in the eastern United States. Poor phosphorus (P) uptake efficiency (PUE) of crops produced in pine bark substrate is a commonly reported issue that can have environmental and monetary consequences. In addition to a complete macronutrient fertilizer (primarily controlled-release), dolomitic limestone and a micronutrient fertilizer are routinely added to pine bark-based substrates to improve fertility. However, the effect of these preplant amendments on PUE of containerized crops has not been thoroughly investigated. Additionally, the influence of dolomitic limestone and micronutrient fertilizer on the form of P in substrate pore-water is not well understood and may provide insight into P lability in the container. The objective of this research was to determine the effect of dolomitic limestone and micronutrient fertilizer amendments on total P (TP), total dissolved P (TDP), and orthophosphate-P (OP-P) concentrations in pour-through extracts and their relative influence on P uptake efficiency (PUE) of containerized Lagerstroemia ‘Natchez’ (crape myrtle). Containerized crape myrtle were grown in a greenhouse for 91 days in a pine bark substrate containing 2.97 kg·m⁻³ of a polymer-coated 19N–2.6P–10.8K controlled-release fertilizer and one of four substrate amendment treatments: no dolomitic limestone or micronutrient fertilizer (F), 2.97 kg·m⁻³ dolomitic limestone (FL), 0.89 kg·m⁻³ soluble micronutrient fertilizer (FM), or both dolomitic limestone and micronutrient fertilizer (FLM). Pour-through extracts were collected approximately weekly and analyzed for pH, electrical conductivity, nutrient concentrations, and three P fractions: TP, TDP, and OP-P. Shoots and roots were harvested at experiment termination to determine dry weight and mineral nutrient concentrations. Amending pine bark with a combination of dolomitic limestone and micronutrient fertilizer reduced pour-through OP-P and TP concentrations by 64% and 58%, respectively, when averaged across sampling dates. Total dissolved P concentrations were similar to TP concentrations in all treatments over the course of the study. In the F, FL, and FM substrates, OP-P contributed >70% of TP at all sampling dates, whereas OP-P in FLM was as low as 41% of TP. Total dry weight of plants grown in FLM or FM was 40% higher than those grown in F; however, tissue P content and PUE were the same among plants in these three treatments. Therefore, sorption of OP-P by dolomitic limestone and micronutrient fertilizer did not limit P uptake by plants.