Influence of Temperature and Time on Nutrient Release Patterns

Polymer-coated controlled-release fertilizers (PCFs) are the most widely used fertilizers in the production of container-grown nursery plants. The duration and magnitude of PCF nutrient release varies with product and this variation can significantly affect plant growth. Three experiments were conducted to determine the long-term nutrient release patterns of three PCFs (each with a rated longevity of 8-9 months), each using a different coating technology: Osmocote PlusTM 15N-3.93P-9.96K, PolyonTM 18N-2.62P-9.96K, and NutricoteTM 18N-2.62P-6.64K. In Expt. 1, each of the three PCFs was placed in flasks of distilled water maintained at 40C for 22 weeks. Fertilizer solutions were poured off at bi-weekly intervals and measured for electrical conductivity (EC) and NO3-N, NH4-N, P, K, Fe, Mn, Cu, and Zn concentrations. Overall, nutrient release for the three PCFs was higher and more variable in the first eight weeks than later in the experiment. Polyon'sTM macronutrient release was generally more gradual than that of the other products. Micronutrient release patterns varied substantially between fertilizers and nutrients. In Expt. 2, PB-filled containers were amended with the three PCFs and irrigated regularly in a greenhouse. PCFs were removed from containers when Osmocote Plus’TM NO3-N supply was ~66% expended and analyzed for EC and NO3-N, NH4-N, P and K concentrations. Except for P, the percentage of each nutrient remaining was roughly comparable to those remaining at the corresponding stage of Expt. 1, suggesting that PCF nutrient release behavior in the laboratory method is comparable with nutrient release behavior in PB in the greenhouse. At the end of Expts. 1 and 2, Osmocote PlusTM had expended a higher percentage of its nutrients than the other fertilizers. In Expt. 3 substrate solutions were collected weekly from PB-filled containers (same treatments as in Expt. 2) and EC was determined. The substrate solution EC of Osmocote PlusTM-fertilized PB began to decline sooner than that of the other fertilizers. Overall, these three experiments led to the conclusion that Osmocote PlusTM nutrient