With mulch and a little fertilizer, soil building is possible even on bare rock.
If you want to raise your own food, you can—even if the soil you have to work with is poor or non existent. It's quite possible to build rich, fertile earth on top of rock if need be and harvest good crops in the meantime.
The raw material for your new topsoil can almost anything that will decompose: leaves, lawn clippings, paper, garbage, cornstalks, even sawdust and shavings. (In the southern part of the U.S. it's long been believed that the turpentine in pine by-products inhibits plant growth. I understand, though, that recent research shows the real cause of such failures to be the lack of nutrients—especially nitrogen—in wood wastes. Here in Montana turpentine is no problem in any case, and I've produced good potato harvests in 15 to 18 inches of fresh shavings with the help of fertilizer. Do be wary of pine needles, however: They make an acid mulch that's good for blueberries and evergreens but not for most other garden species.)
Suppose your homestead contains an infertile area you want to improve, and suppose you also want that patch to produce food while you're building the earth. Start your soil building program in the fall, by spreading one side of the field with 15 to 18 inches of mulch mixed with plenty of manure or sludge. Some people think the organic matter should then be turned under. Nature, however, leaves it on the surface of the ground, and our best soil is found where wastes have accumulated for a number of years.
Winter the covering to give it time to settle down. Then, the following spring, apply commercial fertilizer if you wish. I use plenty of 16-16-16 and ammonium nitrate myself but whatever source you choose, you must provide at least 50 pounds of actual nitrogen per ton of mulching material. (Fertilizer formulas are expressed as percentages by weight of nitrogen, phosphorus and potassium, always in that order. Ammonium nitrate analyzed as 30-0-0 thus yields almost a third of its weight in available nitrogen, and every 100 pound bag of Edgar's "triple 16" should add 16 pounds of nitrogen to his mulch. The calculation isn't always that simple for the other two components, which may be listed by percentage either of the elements themselves or of their oxides. In the latter case, one pound of phosphate—P205—equals 0.44 pounds of phosphorus, and one pound of potash—K20—provides 0.83 pounds of actual potassium. The analyses of various manures and other natural fertilizers can be found in Rodale's Encyclopedia of Organic Gardening. —MOTHER EARTH NEWS)
After fertilizing, let the rain fall on the patch once or twice before you plant (or keep the area wet for about a week by watering it yourself).
I prefer to use potatoes as the first crop in heavy mulch, because their habit of growth allows them to thrive in loose material whereas corn, for instance, might topple for lack of a firm soil around its roots. Although I haven't tried any of the smaller garden vegetables as initial plantings, I suspect that the leaves or paper or whatever would be too coarse a medium for them.
Planting is easy: Just punch holes in the mulch and drop in the seed potatoes. Since you needn't make hills, the rows can be two feet apart instead of three. You needn't weed, either, because the surface material holds down the growth of unwanted plants, even quack grass. The tubers will form as usual, only in the top layer instead of in the soil. I once grew 4 1/2 pound specimen in about 15 inches of leaves.
The material that produces all those potatoes for you will, of course, be turning itself into good soil at the same time. In the second year you might move the spuds on to another freshly mulched area and try beans or peas or carrots on the first strip.
I continue to use fertilizer at this stage. As the mulch breaks down, however, there's a danger that some elements will build up excessively in the ground. It's safest to use a slow release type to minimize this possibility. My own choice is Migorganite, an organic product made from sewage and often used on lawns because it won't burn the grass. Manures are also useful, but if fresh they should first be composted to kill weed seeds. "Hot" types like chicken or horse droppings should be mixed with plenty of sawdust, leaves or other organic material, and the pile turned over every time it reaches a high temperature.
The unmulched portions of your garden, of course, can be steadily improved with compost. I also get extra value from my big piles of waste by using them to grow heat-loving plants like squash, tomatoes, peppers, and eggplants. The warmth of the fermentation serves as a protection from light frost—a help in these parts, where the growing season is none too long. I can't turn the material, of course, with a crop flourishing on top of it, but the wastes in the interior of the heap break down anyhow and can be removed after harvest.
The same method that lets me raise good garden vegetables on my poorest soil also produces beautiful flowers. I've grown the nicest glads you could ask for on rock using a mulch of paper covered with lawn clippings to keep it from blowing around, and the dahlias in my front yard thrive in a deep layer of ground bark.
I know from experience that time and waste material can help any gardener rebuild his land's barren spots and, if he's willing to make the necessary investment in fertilizer, he won't have to go hungry while he waits.