Memmel
I have been likewise thinking (and trying small-scale cultivation) in spare time for 25 or more years in northern England.
Is it worth, however, now backing up a bit, and looking at possible different scenarios? I wonder about North America where there is vastly more land.
Having previously regarded growing biofuels as useless for substituting for current total transport fuel usage (biofuels are a really ludicrous idea in the UK as soon as one checks the numbers), I wonder nevertheless about farms in mostly rural communities growing fuel mostly just for the farm. (The trend round here has been to use ever larger more efficient machines - economies of scale.) If a large farm needs 2 guys (which is normal where I live), how much home-grown fuel could they need to service the farm? The fuel growing would not export soil nutrients away from the farm. (edit. Not 'draining' the soil fertility from farms is critical if NPK imports are limited. There are historical analogies with large estates served by vast human labor having a small per capita food surplus for 'export' or 'tax', or similarly with present 2000 year old Chinese villages served by a high density of persons who are sustained by and renew their high-yielding presently 12t/ha/yr grain systems using these days some supplementary synthetic N input.)
In an alternative 'post-modern' scenario, non-farming populations local to large farms could grow their own vegetables and fruit of very high nutritional value very efficiently in gardens that were sustainably manured by compost toilets, collecting particularly the high NK urine fraction. Their main calories needs, however, would be served by the mechanized big farms. The big farms would need to retrieve (with machinery) a proportional fraction of the composted humanure to balance the farm's soil nutrients budget.
One problem is to figure out what total non-farming population might be carried by a region supported in this way. It is hard to imagine the region supporting large urban populations.
[ So far I (we), in our very large garden have been able to maintain or enhance easily cultivated soil and maintain (just) soil fertility for garden produce, but we still need to 'parasitise' the local large scale farm. (He does not need up to now too many 'symbionts'). So far though, on prime garden soil, I cannot improve on moving swiftly with timely cultivations using a large '3rd-world' hoe, coupled with straw mulches ex my large farm neighbour!]