Agricultural productivity and the use of nitrogen-based fertilizers have increased substantially with increased global food requirements over the last century. Controlled release fertilizers (CRF) have been developed to increase synchronization between fertilizer application and plant nutrient demand. Despite the benefit of reduced nutrient losses, microplastics from the fertilizer coatings, which are not biodegradable, accumulate in soil. A potential alternative is to use self-immolative polymer (SIP) coatings, which go through end-to-end depolymerization to small molecular by-products in response to cleavage of their end-caps from selected stimuli. Poly(ethyl glyoxylate) (PEtG) is a SIP that degrades into innocuous by products. We developed PEtG coatings that selectively degrade to release nitrogen in response to decreased pH within the vicinity of plant roots, thereby improving nutrient delivery and reducing microplastic accumulation in soil. PEtG was blended with other biodegradable polyesters and was applied to urea pellets as a coating using heat pressing (at 60°C), and solvent spraying (dichloromethane at 10 mg/mL). Coated fertilizer pellets using PEtG were characterized based on degradation, release rates, and nutrient delivery to plant species.
Results/Conclusions
Coatings that diminished early release of urea pellets were successfully developed using PEtG. Root zone acidification was assessed using Agrostis stolonifera roots (creeping bentgrass) in agar (pH 7) with bromocresol purple dye for visualization. The PEtG coatings degraded in a pH sensitive and temperature dependent manner, with more rapid degradation occurring at higher temperatures (30 °C) and lower pH (5). Creeping bentgrass root stimuli were sufficient to trigger film degradation, in which 50% mass loss was achieved by 30 days at 30 °C, but at 22°C the effect was more subtle. Urea release studies in solution showed that temperature had a greater effect on release than pH did and 100% release was achieved by 20 days. For a range of different grass species and soil substrates, PEtG blended coatings promoted plant growth to a similar degree as currently marketed CRF and performed better than the non-coated controls. Our results establish a positive first step for the use of a SIP as a potential controlled release fertilizer coating to increase nutrient synchronization, while reducing the amount of microplastic accumulation in agricultural soils.