Synthetic ammonia is produced from natural gas, which is not otherwise very important for producing food. If the question is simply feeding the world today with no concern for long term environmental impact (global warming), then synthetic nitrogen production doesn't take from available food. It doesn't require significant land use, etc. it only has a resource cost, but a resource that is currently plentiful and not a limiting factor.
When we're asking environmental questions of total greenhouse gas output, that's another issue.
For that, we would be interested in things like green manure and intercropping. We might be looking at using something like 2x the land to do that, to grow enough green manure to sustain cereal crops.
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex133
For example, a cereal crop requires 45 lb/acre of nitrogen for optimum growth. The cereal crop can obtain 12.5 lb/ac of nitrogen from the ploughed-down legume crop if the legume residue contains 62 lbs/acre of nitrogen. Assuming an additional 9 lbs/acre of mineral nitrogen is released from the soil reserves, then the cereal crop will require only 23.5 lbs/acre of fertilizer nitrogen. In most cases, as in this example, legume green manuring does not supply all the nitrogen for optimum crop growth but can reduce the amount of nitrogen fertilizer required.
That's a speculative example from crop rotation.
It's very likely that just by eating more legumes (as a higher ratio of diet than grains, which vegans should be doing for protein in replacement of meat anyway) there would be enough nitrogen fixation for the cereal crops we eat, particularly if we can do waste stream recovery (
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5274620/ ) which is really the priority here when it comes to closing the loop (animals certainly don't help).
Canastenard wrote: ↑Tue Oct 17, 2017 11:35 am
Variables to calculate that would be how much energy loss from animal agriculture comes in the form of manure that can be reused to fertilize crops, how much of this energy from manure is used by plants to grow, and finally whether those plants are fed directly to humans or fed back to farmed animals.
Fungus can use the energy in manure to grow, but plants do not (beyond possibly saving the trouble to fix as much nitrogen for nitrogen fixing plants). Nitrogen just makes them grow faster because it's a limiting factor. They only use the minerals (mostly phosphorous, which can also be provided as ground up rock or other sources, or by better farming methods) and the fixed nitrogen.
Plants need something like 50 lb or nitrogen per acre, which equates to thousands of gallons of liquid manure (something like 4,000).
We can compare the nitrogen in manure to synthetic and look at the energy savings, but they're still not 100% identical.
Canastenard wrote: ↑Tue Oct 17, 2017 11:35 amAlso there's something I'm wondering about: how do we get nutrients from nature to agricultural lands in a sustainable way?
We need to recover nutrients from the human waste stream. We don't have to completely close the cycle, we do get some nitrogen fixing from legumes we already grow, and we get phosphorous and potassium and other minerals from the soil (particularly from trees with their deep roots), alley cropping is a way to take advantage of deep rooted plants pulling these minerals up.
Canastenard wrote: ↑Tue Oct 17, 2017 11:35 amOf course nutrients can be mined but that doesn't seem sustainable due to the fact we extract them faster than their reserves can be restored.
Doesn't really matter, most of the earth's crust is made of these things. The only reserves that are running out in any meaningful way is the most concentrated forms which are the cheapest to mine, transport, and spread on fields. There's still a virtually unlimited quantity, it'll just cost a little more.
Of course we should recover these thing from the waste stream, and will as soon as it costs less than mining it fresh.