Controlled release of one or more agricultural chemicals is provided by microcapsules adapted to rupture in a magnetic field. The microcapsules, which may be applied to soil, seeds and/or plants, each have a shell that encapsulates an agricultural chemical, such as a fertilizer, herbicide or insecticide. One or more organosilane-coated magnetic nanoparticles is/are covalently bound into the shell of each microcapsule. For example, (3-aminopropyl)trimethylsilane-coated magnetite nanoparticles may be incorporated into the shell of a urea-formaldehyde (UF) microcapsule during in situ polymerization. In one embodiment, microcapsules encapsulating a fertilizer are applied during seed planting. Controlled release is subsequently triggered after an appropriate period of dormancy by positioning a magnetic field generating device proximate the microcapsules to generate a magnetic field sufficient to rupture the microcapsule shells through magnetic stimulation of the organosilane-coated magnetic nanoparticles. The ruptured microcapsule shells release the fertilizer.