The parts of the country with the most intensive animal farming have so much manure they don’t know what to do with. Manure is bulky — it has a low value per ton. This is a problem. The reason Iowa farmers are putting chemical fertilizers on their cornfields instead of manure is that it’s a lot more concentrated, so it’s cheaper to ship. In fact, it’s cheaper to ship oil halfway around the world, make fertilizer out of it, and truck the fertilizer to the farm belt than it is to truck free manure to the farm belt.