Nitrogen release characteristics of polyethylene-coated controlled-release fertilizers and their dependence on membrane pore structure

In this study, controlled-release fertilizers (CRFs) with five different nitrogen release periods were prepared by coating large urea particles with polyethylene (PE) membranes under various experimental conditions. The preliminary and differential solubility rates, release periods, and membrane pore sizes of the obtained CRFs were measured using water immersion, scanning electron microscopy, and mercury porosimetry. For all CRF samples, the median pore diameters of the membranes were equal to 4.5–5.3 nm and pores with sizes smaller than 10 nm accounted for 86–96% of the total pore surface area. The obtained pore diameter distributions differed for the five studied types of CRF, having release periods of 1, 2, 4, 6, and 8 months. Thus, for the CRFs with a 1-month release period, the maximum pore diameter reached a magnitude of 4000 nm, while this value did not exceed 30 nm for the CRFs with a release period of 8 months. Hence, we have established a relationship between the release period of CRFs and their effective maximum pore sizes.