Quantifying Winter Discharge of Controlled Release Fertilizers to Determine Environmental Impact and Plant Uptake

It is widely believed in the nursery industry that controlled release fertilizers (CRF) release nutrients only when temperatures are greater than ≈4ºC (40ºF).  Research recently conducted in Southern California reported CRF continued to release nitrate and phosphorus throughout the winter months. We suspect that CRF used in the Pacific Northwest also release throughout the winter months since atmospheric temperatures remain above freezing and soilless substrates are able to warm quickly on sunny days. Though release may be less than in California, environmental impact may be greater since reduced plant growth rate may reduce nutrient uptake resulting in high phosphorus and nitrate concentration in runoff. With reduced plant uptake and increased rainfall, nitrate and phosphorus from CRFs have greater potential to leave nursery sites and degrade water quality. The three objectives of the project were to 1) determine the extent of nutrient release from CRF throughout winter months in Oregon’s Willamette Valley, 2) evaluate uptake of nutrients of deciduous and evergreen plants, and 3) assess environmental impact on experimental nursery sites located in Aurora, OR. In October 2007 one-gallon (4L) deciduous and evergreen Ilex and Rhododendron were potted in three-gallon (11L) containers using a Douglas-fir bark substrate. A 12 month complete, homogenous CRF was applied at the recommended medium rate (48g 19N-2.6P-10K). Plants were grown under normal irrigation conditions until experiment was initiated on November 1, 2007. At this time nutrient content of plants, CRF, and substrate were determined by grinding dry plant tissue and extracting nutrients from CRF and substrate. Nursery leachate/runoff was measured weekly from simulated nursery sites to determine nitrogen and phosphorus concentration. In April 2008 plant root and shoots were harvested and ground to determine plant nutrient concentration.  Nitrogen and phosphorus were extracted from CRF to determine remaining nutrient content. Substrate daytime temperatures remained higher than ambient air temperature. In January substrate daytime temperatures were 14ºC (58ºF) while daytime air temperature remained around freezing. These elevated substrate temperatures in the container contributed to nutrient release detected in the runoff. There was negligible net nutrient uptake by either deciduous or evergreen plants regardless of taxa. Runoff nitrogen and phosphorus concentration averaged 4.0 and 0.4 mg L^-1, respectively. Ammonium accounted for ≈20% of total nitrogen. Maximum nitrate concentration detected was 7 mg L^-1. Over the course of the five month period average effluent nutrient content per container was 254 mg N, 67 mg P, and 890 mg K.