... in the modern era. It's a given that fabric and metals are high cost of production from raw inputs, and pre-mechanisation would be far too valuable to just leave in the fields. Post mechanisation the problem is logistics. (The factory war era pretty much begins with the Napoleonic war: Marc Brunel (Isembards father) made boot making machines and block making machines for the british war supply chain. You can see the blockworks in Portsmouth harbour and bits of the machines are in the science museum, London)

I think "battlefield pick-over" is not a busted trope. Logistics means you use the stuff to hand, be it 155mm shells you captured taking a Russian trench system, or arrows left over from a stupid french knight charge over muddy ground towards english archers.

The point here, is that a vaguely disgusting re-use of the consequences of war, is that dead bodies turn out to be valuable, not just the grave goods around them. You want phosphates for fertilizer enough that digging up bones to burn to make it, is worthwhile. You would think that the slaughter of cattle and sheep provided enough but a few thousand buried soldiers is a pretty good deposit.

In times past soldiers piss has been used to make gunpowder, dung was used in leathermaking. What's the difference here to using urine, and digging up dead mans bones?