Polymer-coated controlled-release fertilizers (PCFs) are the most widely used class of fertilizers in the production of container-grown nursery plants. Nutrient release from PCFs is primarily influenced by temperature. The objective of this study was to determine the influences of temperature and time on the nutrient release patterns of three PCFs (each with a rated longevity of 8-9 months), each using a different coating technology: Osmocote Plus™ 15N-3.93P-9.96K, Polyon™ 18N-2.62P-9.96K, and Nutricote™ 18N-2.62P-6.64K. The first three experiments investigated the effects of time on long-term nutrient release. In Expt. 1, each of the three PCFs were placed in flasks of distilled water maintained at 40&176;C 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's™ macronutrient release was generally more gradual than that of the other products. Micronutrient release patterns varied substantially between fertilizers and nutrients. In Expt. 2, pine bark (PB)-filled containers were amended with the three PCFs and irrigated regularly in a greenhouse. PCFs were removed from containers when Osmocote Plus'™ NO3-N supply was ~66% expended and analyzed for EC, NO3-N, NH4-N, and P concentration. 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 Plus™ 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 Plus™-fertilized PB began to decline sooner than that of the other fertilizers. Overall, these three experiments led to the conclusion that Osmocote Plus™ nutrient release declines more quickly than does Polyon™ or Nutricote™, while Polyon™ has the most gradual nutrient release pattern.
The objective of the second set of experiments was to determine the effects of temperature on short-term nutrient release. In Expt. 4, 14 g of each PCF was maintained at 40&176;C until ~33% of the NO3-N content in Osmocote Plus™ was expended. Each fertilizer was then placed in a sand column and leached with distilled water at ~100 mL/h. Columns were then incrementally subjected to a simulated diurnal container temperature change from 20&176;C to 40&176;C and back to 20&176;C over a period of 20 h. Leachate was collected hourly and measured for soluble salts and NO3-N and NH4-N concentrations. For all fertilizers, nutrient release increased and decreased with the respective increase and decrease in temperature. Nutrient release patterns of the three fertilizers were significantly different, with Osmocote Plus™ showing the greatest overall change in nutrient release between 20&176;C and 40&176;C and Nutricote™ the least. In Expt. 5, PCFs were placed in flasks of distilled water in constant temperature baths. Initially, fertilizers were held at 40&176;C for three days and then at temperatures of 22, 28, 34, or 40&176;C for two weeks. Fertilizer solutions were poured off after the first and second weeks. Only solutions from the second week were analyzed for soluble salts and NO3-N, NH4-N, P, and K concentrations. For Osmocote Plus™ and Polyon™, there was a 29% to 86% (depending on the nutrient measured) mean increase in nutrient release between 22&176;C and 40&176;C, whereas for Nutricote™ there was a 345% to 364% (depending on the nutrient measured) mean increase. The overall mean increases in nutrient release in Expt. 4 were between 1032% and 4023%, whereas the mean increases in Expt. 5 were between 29% and 364%. In summary, the second set of experiments found that PCF nutrient release was highly sensitive to diurnal temperature changes.