Growth, Foliar Nutrition, Photosynthesis, and Nutrient Leaching of Container-Grown Landscape Trees Produced with Conventional and Organic Fertilizer

Organic production of greenhouse and nursery crops has risen dramatically in the past decade.  One of the largest challenges for organic nursery production is meeting crop nutritional needs without the use of inorganic fertilizers.  In this study we grew shade trees (‘Bloodgood’ planetrees, Platanus x acerifolia ‘Bloodgood’) and conifers (Colorado blue spruce, Picea pungens and Fraser fir, Abies fraseri) for two years in a pot-in-pot nursery at the Michigan State University Horticulture Teaching and Research Center.  ‘Bloodgood’ planetrees were grown in 104L (#25) containers.  Conifers were grown in 25L (#7) containers.   The potting media was a mix of pine bark and peat moss (80:20, v:v) and trees were irrigated daily via a micro-sprinkler system to achieve a target leaching fraction of 20%.   Trees were fertilized with either a conventional controlled-release fertilizer (Osmocote Plus 15-9-12 (N-P₂O₅-K₂O) or an Organic Materials Review Institute (OMRI)-listed organic fertilizer (NatureSafe 10-2-8 and NatureSafe 5-6-6 (N-P₂O₅-K₂O)).  The organic fertilizers were blended to provide approximately the same nutrient ratios as the conventional source.  Fertilizers were applied at an annual rate of 60 grams N per container for trees in the #25 containers and 25 grams of N per container for trees in the #7 containers.  We measured height and caliper growth each year.  Photosynthetic gas exchange was measured periodically with a portable photosynthesis system (LI-6400, Li-Cor, Inc.).  Foliar nutrient samples were collected in August (planetrees) and October (conifers).  Leachate losses from containers were continuously recorded by a series of automated collection systems equipped with tipping bucket gauges connected to a central datalogger.  We collected leachate samples weekly for nitrate, ammonium and phosphorus analysis. Through the first year of the study, stem caliper growth, net photosynthesis, and foliar nutrition of ‘Bloodgood’ planetrees was similar with either conventional or organic fertilization.    Caliper growth and net photosynthesis of conifers were greater with conventional controlled release fertilizer rather than with organic fertilization.  Fertilizer source (conventional versus organic) did not affect foliar nutrition of trees of either conifer species.  Leachate concentrations of nitrate and ammonium were consistently higher from trees receiving conventional fertilization than those fertilized with the organic product.  Overall nutrient budgets will be presented for both productions systems.