This is a great thread and already has a number of smart, informative replies. I don't know how smart and informative I can be but let me throw in a few comments from the age of 75 and a lifetime of farming, hunting, and fishing all over the world.
For starters, there are just FIVE crops that feed the entire world! No kidding. They are wheat, rice, potatos, beans, and corn. Those five plants and their modern hybrids and deviritives are all high in caloric density, relatively easy to store for fairly long periods of time, and humans in some parts of the world subsist almost entirely on just two of them. One of the 'magic' pairs that comes to mind is beans and corn. Beans have lots of protein, however, we can't assimilate much of it unless the enzymes from corn are combined with the beans---as healthy Mexican peasants have done for centuries. Another healthy pair is rice combined with very small quantities of meat or fish and millions of Asians have lived for eons eating little else. BTW, China is NOT and never has been a major rice producing nation. The principal food crop in China is wheat and guess where noodles and pasta came from?
I spent my childhood through 1950 on a dirt poor farm in the hills of western Arkansas. There were several years when the ONLY things we bought at the grocery store in town were coffee and salt. Everything else we ate came off the land. Yeah, we worked hard but, kid you not, we ate like kings! Fresh milk, eggs, produce, fish from the creeks, animals from the trap lines, our own pork, beef, and poultry, and some hunting (when we could afford a box of 22 shorts, 25 cents per 50 round box in those days). Added to that was the 'truck' crops we raised plus all the other neat things foraged from the bordering Quachita National Forest---wild grapes, blackberries, persimmons, nuts, polk salad, on and on. We even made our own wine and grew and dried our own tobacco.
Later in life, I never lost my love of gardening. I've experimented with several methods of 'small plot' gardening, the French Intensive method, raised beds, and now 'Earth Boxes' (Google them). Presently, I garden solely with an array of Earth Boxes in my front yard. Each box takes about one gallon of water per day. Also no weeds and few bugs. The plastic covers over the boxes retain the water and prevent weeds and burrowing insects. If you're mechanically inclined or a DIYer, there's some good articles on the Web for making your own. Anyway, with minimal effort, I raise more than I can consume. I also take three or four boxes in the house in the fall and place them on my enclosed back veranda facing due south, then enjoy fresh tomatos, green beans, okra, and herbs all winter. Point is, a subsistence farmer could, I believe, easily raise the veggies with something like Earth Boxes in a very small area and use other available land for animal forage and woodlot.
I'll also mention there are a host of designs on the Web for producing your own methane gas from animal and plant waste 'digesters' to help with the fuel problem. As far as seeds go, if you're preparing for a possible long term survival situation, it's cheap to stock up on seeds especially with quantity buys. How long will seeds keep? Ha! They found seeds in the pyramids over 5,000 years old that were still fertile. Oh, almost forgot, don't forget honey bees along with goats, etc. Nice to have some high calorie sweet stuff also and the bees basically feed themselves.
And really what city in their right mind would want the publicity of some guy on tv saying "we are just trying to feed our family in these trying times but the city is mowing down our crops and fining us."
