Assessment of Struvite Containing Controlled Release Fertilizer as a Source of Phosphorus for Containerized Ornamental Crops

Currently, phosphorus use is being scrutinized by state and national regulatory agencies because of its contribution to the degradation of water quality and eutrophication. Phosphorus in runoff can be harvested from wastewater by local and state water treatment facilities and precipitated to form a phosphorus containing mineral that slowly releases plant available phosphorus. This prilled mineral, struvite, is a magnesium (16%) ammonium phosphate (MgNH₃PO₄·6H₂O) and is available as a 6-month controlled release 6N-29P-0K commercial fertilizer (Crystal Green^TM, Ostara, Vancouver BC), To test if a struvite containing 6-mo controlled release fertilizer can release phosphorus in controlled manner maximizing plant growth, liners of Hydrangea macrophylla ‘Endless Summer’, Juniper horizontalis ‘Youngstown’, and Rhododendron x Girard ‘Girard’s Fuchsia’ were potted into 2-gallon (4L) container and top-dressed with various fertilizer treatments. Plants received a conventional 3N-1P-2K ratio 6-mo CRF (control) and a 6-mo hybrid CRF composed of potassium nitrate, ammonium nitrate, and struvite.  The hybrid CRF was formulated to have varying rate of phosphorus: 0, 0.32, 0.64, 0.94 g P. Each plant was top-dressed at a rate of 4.6 g nitrogen (N) and 2.4 g potassium (K). Each taxon was arranged into a completely randomized design with 10 replications per treatment. Final dry weight data showed comparable results across taxa. The contol treatment produced the greatest total dry weights; followed by the taxa receiving 0.32, 0.64, and 0.94 g P. Total dry-weight decreased 40% to 70% for plants receiving no phosphorus. The increased crop growth may be the result of an increased rate of nitrogen release when using the conventional, homogeneous, fertilizer. Phosphorus did not appear to limit plant growth when applied at varying rates in the form of struvite. Only in the absence of phosphorus did total dry weight significantly decrease. Bronze or red marginal coloration was observed on all struvite fertilized plants. This discoloration may be a symptom of inadequate rates of phosphorus uptake. There was no significant effect of treatment on root:shoot ratio; however it was observed that the ratio decreased with increasing applied phosphorus. Struvite is a potential source of phosphorus for containerized crop production; however more research needs to be conducted to determine needed application rate, release rate, and longevity to ensure plant growth is not limited and is without foliar discoloration.