We treat nonlinear magnetosonic waves propagating perpendicular to an external magnetic field in a warm multispecies dusty plasma with polytropic pressures. Because it could have a nonnegligible influence on the results, the displacement current in Ampère's law is not neglected. We also do not impose the customary quasineutrality between the different species, although that turns out to be automatically obeyed. The nonlinear evolution equation for the magnetic field is the well known Korteweg-de Vries equation, and its multispecies formulation might be useful not only for dusty plasmas but also for several other astrophysical plasmas.