It may be coincidence, fate or a shift in world consciousness, but recently, I have noticed a large number of articles written on the subject of how agriculture might fare once cheap oil is no longer in its present abundance. It is often argued that the most practical diet would be vegetarian, for one reason that it takes less land area to grow crops than to provide an equal number of calories from animal husbandry. It is also argued e.g. by the Vegan Society that such a diet is more healthy. However, adopting an entirely vegetarian diet comes with the caveat that deficiencies in certain essential dietary components, e.g. vitamin A, vitamin B12, iron, calcium and fat might be incurred, whereas they are provided by eating at least some meat. Nonetheless, there are many who live well without consuming animal products, and so these should not be considered an absolute daily necessity.
Modern farming depends on fossil resources in a variety of ways. Fertilizers and pesticides depend on gas and oil as raw chemical feedstocks for the industries that make them. Farm machinery is fuelled by oil, and so it is oil that drives the tractors, combine harvesters, and so on, and which enables the ultimate carriage of crops and meat to their end points of use, which for us in the West is mostly supermarkets. To protect the products from spoilage, food items are wrapped in unrelentingly robust packaging (made from oil at that), most of which ends-up on landfill sites, along with about one third of all the food bought in the UK which is thrown away too. The problem of saturating landfill sites is such that most local authorities here have implemented recycling programmes, and while there is periodic concern raised that much of "recycled" waste actually finds its way onto landfill sites too, we are assured that in the Borough of Reading in Berkshire, where I live, the papers, tins and so on, really are used to produce new goods, and this saves resources both of raw materials and energy.
In his book "The Long Emergency", James Kunstler has stressed the vulnerability of urban America to the impending loss of cheap oil. Emplaced along highways, such accommodation settlements are relatively remote from its residents' places of work, and other than living space they offer very little. As the price of oil soars while its supply declines, Kunstler argues that such societies will undergo a steady collapse. Consequently, such personal calamities as businesses failing and jobs being lost are to be expected, while the financial sector roller-coasts up-and-down in perpetuity, thus constituting a protracted crisis that he calls "the long emergency". According to his argument, those living in cities will fare little better, since it is not clear how such densely populous conurbations might be deconvoluted into sustainable local societies. Perhaps they cannot. I believe that it is at least in principle possible to redistribute large populations across entire states, counties or entire nations, but most likely large swathes of people will be drawn out from cities and inner-cities when essentials of fuel and food can no longer be brought in sufficiently from further afield. The latter is not a pretty prospect and I hope that such relocation is not simply left to the prevailing market forces to sort out, as most other things are these days, in a hangover from pseudo-monetarism, discredited though that financial philosophy now is.
If people are no longer able to travel long distances to work (or that those enterprises that employ them have meanwhile gone bust), and neither is it feasible to bring essential goods from afar to supply them, the only practical action is to provide the essential means for living at the local level - hopefully in a planned strategy - and this will inevitably pivot around farming. It is an important consideration then, to decide exactly what crops these farms should grow. If we were to try and live on green vegetables alone we would slowly starve, and it is grains that provide most of the calories we need to fuel our bodies. Wheat, rice, maize, barley, rye, oats, sorghum and millet are prime such examples. We need proteins, carbohydrates, vitamins and minerals, many of which are provided adequately from grains. Winter (not summer) squashes are high in calories, as are parsnips, while carrots, turnips, rutabagas and beets yield somewhat less energy. Beans score well in terms of calories too, and provide the best source of vegetable protein, particularly if they are consumed along with maize and other grains which contain complementary amino acids.
It is also essential to try and do some accounting of the amount of land that would be necessary to support populations at particular levels. Exactly how much land is needed depends on the type of crop, the soil and the prevailing climate of its particular circumstance. Those crops that yield the most in terms of food are not necessarily the most resistant to disease, nor the best that can be grown according to the climate and soil-type for a given location. Changes in weather conditions can have a devastating effect on particular crops too, e.g. onions, which normally are grown sufficiently in the UK to keep us supplied throughout the year, are now predicted to run-out within 6 weeks or so because of the severe flooding last year, which has rotted them in the ground. They will thence need to be imported from New Zealand. QED!!
David Pimental referred to one study of maize production using slash-and-burn farming methods in Mexico, which produced 1,944 kg of maize per hectare - equivalent to 6.9 million kcal of energy. Since we will be far less sedentary in the Oil Dearth Era, it is reasonable to assume that an average hard-working adult might need 5,000 kilocalories per day to keep them going; hence it can be concluded that one hectare of maize can support 4 people. There is around 15 million km^2 (1.5 billion hectares) of arable land on Earth, and so we might naively conclude that this is enough to support 6 billion of us, or close to the present world population. This is however rather more generous than the roughly 3 billion that I predicted it could provide for, in a recent posting, but I did allow that the Earth should support other animal species than just humans and that not all those other species should be only those that live entirely in the service of humans. Roughly one quarter, 65,000 km^2, of the UK mainland is available for growing crops (arable) , and about another half of it for grazing animals (pasture). The rest of the almost 250,000 km^2 is fen-land, forest and so on, not suitable for farming.
Clearly, keeping animals should not impede on arable land if they can all graze on pasture, but that is not of course how modern agriculture works at all, and we feed crops to animals to improve their weight etc. Nonetheless, I think we will still need animals both for food (meat, eggs, milk etc.) and for their muscle-power, e.g. horses and oxen. I do not envisage any immediate return to a society driven purely by human and animal muscle-power, but there will most likely be an increasing agrarian component arising within our efforts to achieve a sustainable scheme for living. Use of GM is proposed to enhance crop yield and help to feed a world population that is predicted to rise from its current 6.5 billion to about 9 billion by 2050. However, by then we will be severely restricted in terms of oil supplies, and the carrying capacity of the planet without present levels of fossil materials (oil and gas) will be considerably reduced from the artificial bubble of plenty we have created with ample such resources.
The worst scenario is a "die-off" which some believe will involve a reduction in the world population to under one billion, through wars over resources and unchecked epidemics of disease. Hence in the absence of plentiful oil and gas supplies, even with GM, we cannot feed everyone, let alone provide the developing nations with a "Western Lifestyle" that even for the West has become untenable. I do not believe that some new brand of technology (e.g. hydrogen) will come to our aid within 10 years, by when world oil supplies will have fallen to perhaps 90 - 95% of current levels and the world economy is reeling in panic. Holding-onto our global consumer lifestyle is a doomed prospect, and we might as well get used to the idea and to that of a return to village-life.
 "Agriculture in a Post-Oil Economy," By Peter Goodchild. http://www.countercurrents.org/goodchild220907.htm
 D.Pimental and C.W.Hall eds. "Food and Energy Resources," Academic Press, Orlando, Florida, 1984.
 J.H.Kunsler, "The Long Emergency," Atlantic Books, London, 2005.