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"Forget the pig is an animal - treat him just like a machine in a factory" | Hog Farm Management magazine, 1976
(i.imgur.com)
EDIT: changed title to reflect that the original place saying the quote was the Hog Farm Management magazine rather than the Washington Post. The photo itself is from an article in the Washington post
The first issue we need to take is using synthetic fertilizer.
We already recognize oil is more of a source of problems than anything else: runaway methane leaks at the wells, soil polution, water polution, spills during transport, high energy consumption for processing, etc.
Manures are already available elements that only need to be reintegrated into the soil.
Composting operations also greatly benefit by having manures added to it (and manures technically require composting before use) as the bacteria from the animals digestive tract help breakdowm the material.
And yes, crop rotation and field management are essential but the more tried and tested techniques and resources we can use to shake away our dependency of oil, the better.
One lesser known source of nitrogen and phosphorous very under used: waste water management plants muds. Many countries are still sending precious resource for landfills.