Scientists have actually studied the fluid dynamics of rippling waves of wheat – like the ones referenced in the second line of "America the Beautiful " – with experiments like the one above, in which nylon filaments imitate tall stalks of grass and soap solution the air flowing over them.
FYFD's Nicole Sharp explains:
Flow visualization reveals the strong differences between flow above and between the grass. Vortices recirculate between the filaments at speeds much slower than the flow overhead. The instantaneous interaction of the high-speed freestream, the unsteady vortices, and the resistance of the grass results in familiar synchronous waves of grain.