If a field is harvested too efficiently it needs to be left fallow to rebuild lost nutrients. Leaving some in the ground is not waste by any definition.

Well... You do need to account for the lost inputs, e.g. fertilizer that runs off instead of being sequestered, water pumped in for irrigation, gasoline to power a tractor, and so forth.

A cover crop (e.g. clover) specifically for building soil that grows without aid of fertilizer or irrigation is a better story in this regard.

No, pointing out that what some people call waste is not actually being wasted, it is serving a useful purpose. "Waste" means it is serving no useful purpose at all.

It's fairly difficult to do something that serves no conceivable purpose. I could buy stuff and throw it straight into the trash and we could say it's not waste as I'm supporting manufacturing and the garbage industry.

If you care about limiting some wasteful practice, you have to pick some definition of what counts as waste.

It's it essentially composting? How is that waste?

If I buy a tomato, and throw it in my garden, eating none of it, most people would say I wasted the tomato.

There are far cheaper methods of fertilizing my garden than tossing food into it, which makes it "wasteful".

At a certain point you have to pick a definition of what counts as food waste, and stick with it. Fiddling with the definition isn't going to make the world a better place.

Plowing it back into the fields means less fertilizer is needed for future crops. How is that waste?

That's like a perpetual motion machine. Energy is wasted, there is no way around it.

The sun is a great source of free energy.

But not necessarily nutrients/fertilizers and most tractors and transportation today run on ICEs. So it's still not a free lunch.

It needed that fertilizer to grow to even be plowed back in to start. It's just a waste of space for a "good product". It's basically a free pass for next year. No money gained means no profits means profits loss.

If the produce is lost, it's tax deductible

If the produce goes back to soil, it's an expense and next time it will result in higher output.