The term "organic farming" is a recent innovation, as opposed to its practices per se, which were those of the world's agriculture prior to the post WWII period, when chemical fertilizers were introduced to the soil. Until then, all farming was "organic" and was done without the employment of artificial "nitrogen" from ammonia or involving the routine use of synthetic pesticides. Modern "intensive farming" methods, such as we rely on in the industrialised nations, have been costed to consume 10 calories of energy in the form of fossil fuel (to provide fertilisers, pesticides and transportation fuel) for each calorie of energy that is recovered from the food itself.
Now, a strategy of localisation will inevitably reduce the contribution from transportation fuel, which is significant, but if pesticides and fertilisers are cut-out too, crop yields fall appreciably, meaning that fewer people can be fed per acre or hectare of arable land. It is a truth that "organic" farming is far more intensive in terms of land, if not in terms of energy. A major driver for the development of "chemical" farming methods was the plenty of chemical materials produced with the intention of military use during the war, and which it was decided could be put to benefit by turning them into agrochemicals.
I have mentioned Thomas (Robert) Malthus previously, who predicted more than 200 years ago that because population grew at a geometric rate (i.e. 2, 4, 8, 16...) but food production increased arithmetically (i.e. 1, 2, 3, 4...), the rate of reproduction would outstrip that of its sustenance, leading to mass starvation and an effective die-off scenario. This did not happen, in consequence of the "green revolution", which ironically is the opposite of the modern "green movement" since it refers to the many developments in agricultural technique that have been implemented since the 1960's, including the use of chemical additives to soil and to the produce grown on it. In consequence, world food production swayed-in with an increase of 250%, from greater absorption of nitrogen than occurs naturally, the growth of selected high-yielding crops like wheat and corn, and a greater mass of grain in the plants overall. For example, in 1950, an acre of land produced around 400 kg of wheat, but by 1950 this had risen to around 2000 kg/acre in South Asia but 4000 kg/acre in Europe and the US.
The downside of this is that it is necessary to provide more irrigation and hence an intensive infrastructure of dams and water-channels are necessary, especially to provide sufficient water during the winter period in order to grow an additional annual crop. Additionally, because more of the plant is consumed by humans, there is less residue left from it for animal feed. A mean energy intake for a human adult is reckoned at 2500 Calories (kilocalories) per day. A balanced diet is believed to correspond to about 60% carbohydrates, 12% protein and 28% fat. It is significant that during the green revolution the world has eaten more meat, meaning that the per capita land requirement is greater than would be the case to feed vegetarians. It has been estimated that 20 people can live entirely without animal products on the same area of land required by a typical meat eater. This may be a considerable overestimate, but certainly the carrying capacity of the earth is reduced if many of its inhabitants eat much more meat than they once did.
According to one calculation , the amount of land required to feed a single human is about one acre, following a mainly agrarian lifestyle, i.e. on the basis of pre-green revolution farming, without chemical enhancers. Since the total land area of the earth is about 150 million square kilometers, of which 10% is suitable for growing grains, another 10% for grazing animals on and a further 20% in the form of forests where animals can be raised, it may be deduced as a simple total that the sustainable world human population is:
150 x 10^6 x 100 hectares^2/km^2 x 40% x 2.47 acres/hectare x 1 acre/person = 14.8 billion.
However, the primary energy (food) input is surely the growing and grazing on a total of 20% of the planetary surface (we can't eat trees, although animals such as pigs can grub around the forest floor), suggesting a maximum sustainable population of nearer 7.4 billion, which is way short of the 8 - 9 billion presumed by 2050 and that contemporary farming methods will continue in perpetuity. Certainly there are other species on the planet, that do not exist purely in the interests of supporting the human race and the earth must support them too. So, would 30% of that land resource available for humans be a reasonable estimate? That leaves us with about
2.2 billion as the carrying capacity.
I am depressed. Either we will need to maintain the basic "forced methods" for crops by some means other than oil (and gas), to keep the present level of agriculture going (how?? coal??), or there will be a die-off in the world population, presumably through famine and wars over declining resources. Probably we will need to provide more of our diet directly from crops, rather than processing it through animals first, but even then, that only saves us perhaps a quarter-acre (from the per capita one acre), meaning the planet might support a maximum 3 billion, or less than half the present number. However, can we thus provide sufficient daily calories to fuel a population living far less sedentary lives, by grains etc. alone? There are just too many of us.
 "The World's Expected carrying capacity in a Post Industrial Agrarian Society." http://www.theoildrum.com/node/3090
 "Human Appropriation of the World's Food Supply