Plain dirt, where nothing's ever been grown, is just a random collection of minerals. It can be clay, like mine, or silt, or sand. Clay is very dense, holds water, turns into brick when it dries out, and is generally unmanageable. I thought the way to fix it was to add sand -- but my reading tells me that the cure for clay and sand is the same: add organic matter.
That's also the cure to nutrient-deficient soil, either because it hasn't been grown in, or because it's been used up by a lot of past gardening. Whatever your problem, working in organic matter is the cure. It aerates the soil and provides nutrients. Organic matter might include dead leaves, kitchen scraps, animal waste, seaweed, and bones, all properly prepared. These provide the nutrients that plants need: nitrogen, potassium, phosphorus, and trace minerals. Plants use these minerals to produce all the vitamins and amino acids they need, and which we so gratefully obtain from them.
In the wild -- say, a forest floor, or an open meadow -- the soil cares for itself. Plants shed leaves and branches. Earthworms aerate the soil. Animals poop on the soil, and later die on it. Microbe activity is high. Everything dropped on the ground eventually decays, adding its nutrients to the soil. It's the CIRCLE of LIFE! (Sorry, couldn't help myself!) If you look at a cross-section of wild soil, you'd see organic matter, like leafmold, on top, slowly decaying into humus underneath. After some nice topsoil of mixed humus and dirt, there is the original packed clay, silt, or sand. Building topsoil is a very slow process in the wild -- it could take as long as five hundred years to make an inch! (1)
With careful stewardship, we humans can build topsoil much faster. On rocky islands near Ireland and Scotland, topsoil has to be built completely from scratch. Hardworking farmers spread seaweed, which is an incredibly rich fertilizer, over their fields from year to year until they build up sufficient soil over the rocky ground. In other places, manure and compost are used. Rotating cropland and pasture can build up topsoil quickly -- as fast as an inch per year!
There have been times in man's history where we haven't had the knowledge to produce good topsoil, and land would be depleted in a few years. Yields would be good the first year, middling the second, and completely insufficient the third. Nitrogen, in particular, gets depleted very fast and is hard to restore the soil. Then farmers discovered that legumes, like beans or clover, improved the soil. This is because they harbor bacteria which "fix" nitrogen out of the air and into the soil to make it available to plants. It became common to rotate crops -- one field of wheat, one of legumes, and perhaps one fallow (wild). Plowing the stubble under also helped build topsoil, as the stubble rotted underground.
This method is highly preferable to the one in use by some South American farmers, who use the soil until it is depleted and then burn a few more acres of the Amazon rainforest, which has extremely rich soil. It's also preferable to the modern industrial farming method, which depletes the soil freely and then fertilizes with nitrogen, phosphorus, and potassium chemical fertilizers. Yes, those three vital nutrients are being returned to the soil, but not all of the necessary trace minerals are. Produce you find in the store may have as much as 75% less of the various trace minerals. Also, these chemical fertilizers pollute the rest of the environment in runoff, causing poisonous algae blooms in the ocean, contamination of the water table, release of methane and nitrous oxide into the atmosphere, and numerous other problems. I've just been reading the Wikipedia article on fertilizer and am quite shocked at the amount of documented harm that chemical fertilizers do!
In nature, animals and plants complement each other, each trading off what the other needs. Animals inhale oxygen and exhale carbon dioxide, while plants need carbon dioxide and emit oxygen. Plants use nitrogen to make amino acids; animals use amino acids to make protein, but eventually return the nitrogen to the air and soil. Bringing chemicals into the picture is a shortcut, but one that doesn't lend itself to a perfect, self-sustaining, closed system like nature does.
I used to think "organic" produce was just a label. After all, aren't all living things "organic"? The term organic simply means fertilized with organic matter (plant waste, seaweed, bone meal) rather than non-organic matter (chemical fertilizers). Organic produce is more sustainable than conventional produce: it doesn't require factories or petrochemicals, and it doesn't pollute the environment. It also results in a crop that is richer in trace minerals than conventional produce. And that's without even considering potential pesticide contamination.
Well, what's the catch? Some people believe it would not be possible to feed the world with only organic methods. We have a much larger population than we had 100 years ago, before the advent of chemical fertilizers. This growth has been made possible in part by an easy, cheap food supply. Unfortunately, we cannot keep it up indefinitely -- not only is the soil becoming more and more depleted, but our sources for chemical fertilizers (petroleum and mineral mines) are not renewable. We're going to have to find new ways to grow food that are sustainable.
On the other hand, some believe it is possible to feed the world organically. Definitely we wouldn't be able to have feedlot beef anymore -- so a burger a day would no longer be possible. A book of mine on Roman cookery, written in 1936, predicts that we will "soon" no longer be eating so much beef: "Today lives a race of beef eaters. Our beef diet, no doubt, is bound to change somewhat. Already the world's grazing grounds are steadily diminishing ... With the increasing shortage of beef, with the increasing facilities for raising chicken and pork, a reversion to [ancient Roman] methods of cookery and diet is not only probable, but actually seems inevitable." (2) The invention of the feedlot has staved off this eventuality, but if we weaned ourselves off this food-raising method, we'd be back to eating chicken, pork, mutton, and goat like ancient Europeans often did.
We would also have to spread farming education to areas where food is limited. Cry, the Beloved Country describes the poverty of South African farmers, not because the land is poor, but because the farmers don't know how to work it. They deal with erosion and drought due to poor planning. Those South American farmers who are burning off the Amazon need to be stopped, not with fiercer laws protecting the rain forest (which there already are), but education about how to preserve the soil they already have. Currently, the world relies heavily on a few food sources, while often not having sources close to home. Thus, when civil unrest breaks out (as it so often does, especially in Africa), the people have no way to get food. They are relying on a complicated infrastructure to feed them, and when the infrastructure fails them, they have no recourse. A family cow, a garden plot, a couple of acres of farmland would be much harder to deprive them of. (This is why I love charities like Heifer International.)
So, can organic feed the world? I don't know. I do know, however, that I feel a lot better about nourishing my own soil, growing my own crops, and eating my own handiwork than I do about supporting conventional agriculture. That's one big reason why I want to garden.
(1) Wikipedia, "Topsoil"
(2) Apicius, Cookery and Dining in Imperial Rome, edited and translated by Joseph Dommers Vehling