one is that industrial mechanised agriculture is 'efficient' and hence is the only way to produce 'high yields'.  this is only true if we accept that 'efficiency' is measured in wage-labour hours per hectare per season per tonne, and that the tonne is a monocrop tonne, as for industrial food processing or export.  far more food value -- as opposed to tradable/shippable commodity value -- can be obtained per acre w/o mechanisation, with diverse polycultivation and sustainable soil management.

the second assumption (which derives from the first) is that our goal is to Save Mechanised Agriculture, as it is our only hope.  imho -- and I'm not alone in this opinion -- our best survival option at present is to abandon large-scale mechanised monocrop agriculture as quickly as rats fleeing the proverbial sinking ship, and start diversifying, localising, and climbing down a few notches on the food chain ASAP.  feeding grain to cattle -- let alone feeding grain to cars -- is not the smartest use of our resources. shipping tonnes of apples from Canada to USA, while in the same year shipping more tonnes of apples from USA to Canada, is just plain insane.  growing western grains for export in third world countries, rather than using the local land base to grow locally adapted crops to feed the local people, is called, er 'colonialism', and it's not a great idea.  forcing people off their highly adaptive local diets and driving their local cultivars into extinction, so that we can dump agricultural surpluses from fossil-intensive monocrop ag onto them, is a kind of slow-mo genocide.

think for one minute about that statement:  it is illegal to grow for sale many traditional cultivars.  what level of Enclosure are we talking about here, where the most basic human activity -- growing food and  selling or trading the surplus -- has been made illegal?  why is this acceptable for even one second to any resident of the area where this law is in effect?  why are not mobs with torches and pitchforks in the streets?

BTW, the staggering loss of information, diversity, and cultural heritage as traditional cultivars are extirpated by corporate-friendly monocrop commodity substitutes is not limited to the UK.  I think there were something like 5,000 varieties of potato cultivated in the Americas;  the corporate potato is down to about 3 or 4 varieties, and they are a cloned  crop (like bananas and apples).  there were more than 2000 named varieties of apple in the US about 100 years ago, iirc -- Pollan's The Botany of Desire has a whole chapter on apples that provides the history and numbers, read 'em and weep -- and now the abominable Corporate Red Tasteless ["Delicious"] has displaced them all.

this is not merely a matter of gourmet fussiness;  the corporate monocrop "foods" are inferior not only in taste, but in nutritional value and in the energy-cost of their production.  they are not well adapted to any particular biome and hence require extra coddling and care;  they are so pest-vulnerable due to the density of their planting in huge monocrop swathes that they need far more pest control (meaning, regular spraying with neurotoxins) than any of their predecessors.  and the enormous size of the plantations implies the dependence on heavy ag machinery and automation that drives all but the deepest-pocketed capitalists out of the business.  the whole model is insanely vulnerable and precarious -- pick your metaphor, "too many eggs in one basket" or "overcapitalised" or "top heavy and teetering," but it is a very fragile system and likely to fold along any of several fracture lines:  a wave of pests or diseases that could wipe out millions of acres of identical (sometimes clonally identical) plants;  variations in rainfall or temperature when there is zero variety left in the cultivar base to give any resilience against changing conditions;  and worst of all, the complete loss in only 2 generations of the cumulative knowledge and skill of millions of small farmers who knew how to feed their families and their communities.

there has been an Enclosure not only of land, but of knowledge and of the very germ plasm of the plants on which our lives depend.  because dependency of the consumer creates a captive market and therefore a comfortable monopsony (I think that's the right word?) in which a cartel of commodity food barons can set prices and control the market, that dependency has been encouraged and engineered until most people are wholly at the mercy of the corporate food chain, and have absolutely no idea how to grow, or prepare, or preserve any basic foodstuff for themselves.  this is commonly called "progress".  I call it collective suicide...

btw wheat is not a particularly nutritious crop.  it became valuable to humans because it was easily stored and preserved -- thus making it easier to stockpile against lean years, and easier for elites to enclose, control, hoard, and dole out in exchange for obedience.  the nutritional content of today's wheat may actually be inferior to the dominant strain grown in Iron Age Britain (I can come back with a ref on this one);  we've been breeding plants for all kinds of reasons that have everything to do with profiteering for rentiers, packagers, processors, retailers, transporters, etc. and nothing to do with getting necessary nutrients into human mouths -- hence the drop-kickable tomato and the other cardboard vegetables and fruits that masquerade as food in chain supermarkets.  wheat is not the be-all and end-all of crops.  it's just a high-status crop that's culturally central to the wheat-beef marauders of the Caucasus and hence N. Europe.  potatoes are more nutritious pound for pound -- potatoes plus dairy are a very good protein/amino acid mix, each complementing the other.  and many other grains are more nutritious and less resource-intensive (quinoa springs to mind).

my own pessimism is not due to a belief that we can't produce enough food -- I think we could produce a hella lot more food than we are now;  but our food system is choked by the diversion of energy and quality at every turn into the pockets of speculators, monopolists, Enclosers, rentiers, etc.   the goal of our food system is not to produce food -- it is to produce money profit.  the contradiction between these two goals is not felt when there is a surplus of cheap energy to cover the gap.  once the cheap energy runs out, the contradiction is felt and we face the moment when the profiteers, to maintain their control and their profit-taking and the centralised monocrop ag that permits this level of monopoly and Enclosure, will have to condemn even larger numbers of people to starvation than they do today -- perhaps even white people in northern countries -- and this will actually be noticed and cause anger and unrest.

meanwhile the local food movements ferment quietly, with a "food underground" building in most areas of most countries.  I pin my hopes -- slim as they are -- on these movements.

it is worth mentioning that while Mollison et al codified permaculture as a system for western readers, many of the practises they describe have been traditional throughout the world for millennia.  it is symptomatic of the insanity of Euro/Anglo colonialism that a couple of whitefellas have to write it all up in scientific language for simple common sense to be recognised as anything other than "primitive and backward".  what is primitive and backward is the crazy priest/king/monopolist centralised ag we are doing today.  permaculture and the traditional polycultures from which it derives [some indigenous activists would go so far as to say "was stolen"] are far more subtle, advanced, and productive than the factory ag methods we use to farm money rather than food.

and this whole topic makes me so damn angry that I'd better stop here, because my next few thoughts involve piano wire and a list of names.

The difference between theory and practise in practise ...