I'm a bit of a critic of permaculture as applied to agricultural systems because I do not believe it can feed today's population. This is based mostly on the lack of organic, bio-available nitrogen for crops, as well as the grossly uneven distribution of agriculturally viable land on the planet (making local food production impossible for many areas of the world).
It's a beautiful set of design principles, and Holmgren and Mollison's book contains many gems of wisdom for designing small-scale agricultural systems, but it cannot feed today's population. For better or worse, we are dependent (at least until we can drastically lower population levels) on synthetic nitrogen and centralized, large-scale farming. This does not mean we are totally screwed, however, as there is much room to improve large-scale farming practices to reduce runoff, protect topsoil etc.
Allow me to give two anecdotes for now: First, Japan, like many countries, is a net food importer. So is Nevada. The midwest of the United States (and to a smaller degree the California Central Valley) feeds not only most of the United States, but exports tons of grain to other countries that do not have enough agricultural land to sustain their own populations. And with the exception of some areas in Latin America, all agriculturally viable land is already being cropped. We would have to deforest or destroy more natural wetlands if we wanted to expand world agriculture, which would have devastating consequences for an already warming planet.
Second, the growth of the organic industry is directly proportional to the growth of feed-lot cattle. This is because the primary source of nitrogen for organic farms is cow manure collected from feed lots. You cannot collect manure from grazed cattle land because it's too cost prohibitive, and besides, you would then have to add more synthetic fertilizer to the grassland in order to keep the grass growing sustainably. In other words, we grow corn with synthetic fertilizer. Then we feed it to cows. Then we collect the cow manure to grow organic crops. Cow manure as organic fertilizer on crops, therefore, actually consumes more synthetic fertilizer than if we simply used the synthetic fertilizer directly on the crops!
These two anecdotes (there are more) underlie the impossibility of small-scale and organic methods to feed 7 billion people.
