Effect of Fertilizer Rate on Growth of Azalea and Holly in Pine Bark and Pine Tree Substrates

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Recent interest in the use of wood substrates in horticulture crop production has justified the need for determining fertilizer requirements in these substrates compared with traditional pine bark (PB) and peatmoss substrates. The objective was to determine the response of japanese holly (Ilex crenata Thunb. ‘Compacta’) and azalea (Rhododendron obtusum Planck. ‘Delaware Valley’) grown in a pine tree substrate (PTS) (trade name WoodGro™) or milled PB to fertilizer rate. Pine tree substrate is produced from freshly harvested loblolly pine trees (Pinus taeda L.) that are delimbed, chipped, and ground in a hammer mill to a desired particle size. Japanese holly plants were grown in 2.8-L containers in the fall of 2005 and again in the spring of 2007 with the addition of azalea. Plants grown in PTS or PB were fertilized by incorporating Osmocote Plus fertilizer (15N–3.9P–10K) at rates of 3.5, 5.9, 8.3 or 10.6 kg·m−3 for japanese holly and 1.2, 3.5, 5.9, or 8.3 kg·m−3 for azalea. After 3 months, shoot dry weights were determined for japanese holly and azalea. Japanese holly root dry weights were determined for both experiments, and substrate CO2 efflux (μmol CO2 m−2·s−1) was measured on the treatments at the end of the experiment using a LI-6400 soil CO2 flux chamber. In 2005, japanese holly shoot dry weights of PTS-grown plants were comparable to plants grown in PB at the 8.3 kg·m−3 fertility rate, and shoot dry weights of PTS-grown plants were higher than PB at the 10.6 kg·m−3 rate. In 2007, japanese holly and azalea shoot dry weights of PTS-grown plants were comparable to PB plants at the 5.9 kg·m−3 fertilizer rate. Both japanese holly and azalea achieved shoot growth in PTS comparable to shoot growth in PB with ≈2.4 kg·m−3 additional fertilizer for PTS. Substrate CO2 efflux rates were higher in PTS compared with PB indicating higher microbial activity, thereby increasing the potential for nutrient immobilization in PTS.