I'm a bit surprised that we can't turn plants and crops back into fertilizer. IRL this is pretty common, and in a wasteland where you're trying to survive I would think essential. My suggestion is one of two ways to handle this:

1. The same right click method used for fish and guts, but maybe less per plant. 1:1 maybe.

2. A bin or structure you can put the crops into and when they degrade enough they turn into fertilizer. Possibly both, a bin that would work opposite to the food storages, where instead of delaying food decay, it would speed it up. Second something similar to the water barrel, not able to be picked up, but built next to your farm land. You could put plants and crops in and they would produce over time as the materials decay. A compost bin/barrel.