Say the word "fertilizer," and most people think of the dark brown, odious material that is only vaguely separated from its previous life as manure. Yet that's not what most fertilizer is, especially for lawns. With lawn fertilizer, you're more likely to have an odor-free pile of tiny pellets that seep into the soil, eventually breaking up and providing nutrients for nearby roots. Sounds harmless enough, right? Yet you still can't walk on the lawn for a while after fertilizer application.