Cover Story
STATE OF MATTERS
Fertilizer Coating Technology By Max Schlossberg, PhD.
No more than zero artificial intelligence resources participated in the composition of the following chronicle.
In the beginning… By the time NASA successfully landed their unmanned ‘Surveyor 1’ spacecraft on the moon (1965), numerous slow-release, granular N fertilizers were available in the US. Natural organic fertilizers derived from sewage sludge or livestock manure/ litter were the original slow-release N carriers. Next came the urea-formaldehyde reaction products; e.g., methylene urea, isobutylidene diurea, and the eponymous ureaform, all characterized as synthetic organic carriers. However, neither these natural nor synthetic organic N sources completely satisfied the Green Industry’s need for a dependably long-lasting, controlled-release, N fertilizer. But by the time Jacobsen released the original ‘Greens King’ triplex putting green mower (1968), agricultural and material scientists had developed coated fertilizers. Their purpose was to improve the utilization efficiency of nitrogen fertilizer, most notably urea (460-0). This effort improved the granular fertilizer at its core by adding one (or more) layer(s) of a persistent coating that limited nutrient diffusion from inside to outside the coating/membrane. Alkyd resins, synthetic polymers derived by reacting polyhydric alcohols with polybasic acids or anhydrides, were the original fertilizer coatings. Yet the high cost of this added value precluded widescale adoption of the urea fertilizer featuring it. Subsequently, scientists of the Tennessee Valley Authority patented a more economical process of coating urea with elemental sulfur. In 1970, sulfur-coated urea (SCU) became the first, mass-produced, controlled-release fertilizer by which
10 Pennsylvania Turfgrass • Fall 2023
plant-availability of urea-N was initiated by physical rupture or ‘failure’ of the sulfur coating. The prolific marriage of urea and sulfur was hardly a proverbial crapshoot, given sulfur’s already established role as a plant essential macronutrient, fungicide, and additive mitigating the caking tendency of many fertilizers. Moreover, elemental sulfur comprises a biodegradable and inexpensive acidulent that facilitates plant recovery of urea-N in neutral to alkaline soil. Relative to granular urea and confirming all coatings remained intact through application, numerous University studies showed SCU to significantly improve fertilizer N recovery by treated turfgrass. This is likely why the next generation of coated fertilizer built upon, rather than replaced, the sulfur component. While the controlled release of urea N from SCU is governed by the sulfur-coat thickness, the addition of a thin thermoplastic or resin envelope around the SCU prill improved the coating integrity without further N dilution. These polymer-sulfur-coated urea (PSCU) or polymer-coated/sulfur-coated urea (PC/ SCU) hybrids featured a more linear release rate than SCU and were widely adopted in the late 1980’s. Yet the 21st century ushered in a new era of reduced polyurethane/polyolefin cost and a specialty fertilizer marketplace heralding granules coated by a thin layer of polymer as the preferred technology (Figure 1). Owing to their thinner coat, synthetic polymer-coated fertilizers possess a higher grade than their sulfur coated counterparts. Polymer-coated urea (PCU) is minimally affected by microbial activity and remains a highly popular blending source within the turfgrass, landscaping, and horticultural production industries.
