Double Coating as a Novel Technology for Controlling Urea Dissolution in Soil: A Step toward Improving the Sustainability of Nitrogen Fertilization Approaches

Abstract

This research introduces a novel technology for reducing ordinary urea (OU) dissolution by developing double-coated urea (DCU) using phosphate rock (PR) as an outer layer to reduce its hydrolysis and sodium thiosulfate (STS) as an inner layer to inhibit the urease enzyme and nitrification process. Due to the double coating, the nitrogen content of DCU was lower than that of the OU (36.7% vs. 46.5%). The ultramorphological analysis using scanning electron microscopy (SEM) indicated that the controlled coating of urea, resulting from the outer layer of PR, was due to the adhesive effect of urea formaldehyde (UF), which was used as a glue. In addition, the transmission electron microscopy (TEM) analysis of the DCU revealed its high degree of agglomeration. The mechanical hardness of DCU was higher compared to that of OU (1.38 vs. 1.08 kgf). The seven-day dissolution rate test showed that OU reached 100% dissolution on the fifth day. The rate of DCU, however, was significantly lower (32% dissolution in the seventh day). Cumulative NO3− and NH4+ losses from a clay soil sample reached 68.3% and 7.6%, respectively, with OU measuring 40.5% compared to 4.9% for DCU 70 days after application. Field experiments showed a significant improvement in the marketable yield and agronomic nitrogen efficiency (ANE) of maize grains and zucchini fruits fertilized with DCU. Furthermore, the macro and micronutrient concentrations in maize grains and zucchini fruits showed an increase in the plants fertilized with DCU. In summary, double coating can be introduced as a novel technique to control urea dissolution in soil.