Rename grass into weeds to clearly indicate that these plants are unwanted
Introduce concept of weed roots under ground

These subterranean roots are not noticeable by looking at the surface but
help weeds grow.  You can reduce amount of the roots by plowing, but
you cannot completely remove them by doing so only a few times.
Make overgrown corns do not produce money

You can't sell overgrown, degraded crops to the market.
They'll be discarded, but still you need working power to clean up
the soil.