Urea release from silicate- and polymer-coated urea in water and a simulated wetland soil

Abstract

Urea release rates in water (38°C) and in a simulated wetland soil system under greenhouse conditions from silicate- and polymer-coated urea (SPCU) materials have been studied. The SPCU materials were prepared by the Korea Advanced Institute of Science and Technology (KAIST), Seoul, Korea, in cooperation with the International Fertilizer Development Center (IFDC). Average urea release ($\overline {UR}$) in water was mainly a diffusion-controlled, concentration-gradient dependent process. It was markedly influenced by the extent of swelling of the coatings of granules. Under unrestricted swelling condition,$\overline {UR}$ versus time gave an s-shaped curve with three distinct periods: namely, a delayed period, a maximum-rate period, and slow-rate period.$\overline {UR}$ versus time curves for SPCU after placement at 0- to 2-, 5- to 10- and 10- to 15-cm depth in a simulated wetland soil system were, in general, sigmoidal but different from those in water. They were influenced by placement depths of SPCU granules in the soil, and the order of$\overline {UR}$ at the soil depths studied was: 0–2 cm, < 5–15 cm ⩽ 10–15 cm.