The basic tenet of organic growing is that you should feed your soil so that it can feed your plants. People unfamiliar with soil biology often wonder what this means, in practice---how do you go about feeding your soil? By contrast, conventional chemical-based growing methods ignore the soil altogether and focus on directly feeding the plant with synthetic fertilizers and protecting it with chemical pesticides, fungicides, and insecticides.
Many people think of soil as dirt, that sandy, muddy, earthy stuff that makes a mess on your floor and stains your clothes. A few remember high school biology classes that looked at all the little critters that live in the soil---mites, springtails, nematodes, viruses, algae, bacteria, yeast, actinomycetes, and protozoa. What most of us forget is what all those life forms actually do, besides eating each other all the time. Just looking at a handful of soil, it is hard to believe that it contains more organisms than there are people in the world.
What Soil Life Does
Every organism living in the soil depends for its existence on everything else that's living in the soil. For example, plant roots attract bacteria for a mutually beneficial trade --- the bacteria make nutrients in the soil available to the roots, and the roots let the bacteria take some stuff they need in return. Nematodes, which are microscopic worms, wriggle around and keep bacteria numbers in check (they share this job with protozoa), occasionally getting snared and eaten themselves by fungal hyphae, the threadlike "roots" of mushrooms. Fungi decompose organic matter, and mites eat fungi. It may sound like an everything-eats-everything massacre, but it is in fact a perfectly balanced system.
One thing that must be present in healthy soil is organic matter --- dead plant material --- that provides the energy needed to keep things going. According to Erv Evans, consumer horticulturalist at North Carolina State University, "The microorganisms' primary role is to break down organic matter to obtain energy. They help release essential nutrients and carbon dioxide, perform key roles in nitrogen fixation, the nitrogen and phosphorus cycles, denitrification, immobilization and mineralization. Microbes must have a constant supply of organic matter or their numbers will decline. Conditions that favor soil life also promote plant growth."
Feeding the Soil
What this really means is feeding all the billions of little critters that live in the soil. The tastiest treat you can give them is a nice big load of really good compost. You call it compost, they call it food. When they have finished breaking it down to a point where it can decompose no further, it is called humus. In the process of doing that, they create or release everything your plants need to grow, bloom, and produce fruit.
Testing the Soil
Besides feeding compost to the microorganisms, test the soil to find out what's there and what's missing. Your local county cooperative extension office will help you find a soil-testing laboratory and tell you how to take soil samples. When you get the report, follow the recommendations. If necessary, adjust the soil pH---the measure of soil acidity or alkalinity---so it ends up around pH 6.5 to 6.8. If the pH is really out of balance, you can have soil filled with nutrients, but your plants won't be able to access them.
Protecting Soil Life
After you have fed and cared for all that valuable soil life, the worst thing you can do (almost as bad as using chemicals that kill everything) is to rototill the soil all the time. Perhaps once a year a rototiller is good for adding compost and preparing a nice seedbed, but it wreaks havoc on the soil food web. Earthworms, springtails and nematodes get chopped up, and even tiny bacteria and protozoa don't enjoy getting whacked with a steel tine traveling at several hundred RPM. It is better to use hand tools that do the same job but do less damage.