It has been said that the difference between soil and dirt is the amount of organic material present. Dirt represents the nonliving material, such as silicone, granite dust, clay particles and other minerals. It takes the addition of decomposing organic material; microorganisms such as bacteria, algae, fungi and protozoa; and worms and other living inhabitants to make it soil.
Providing a Hospitable Environment
One sign of a healthy soil is the number of living creatures that inhabit it. A handful of rich soil will have more microorganisms within it than the number of people on the earth. There can easily be more than 100,000 types of microorganisms within that handful.
These live off from the decomposing organic material in the soil and eventually help form humus, which promotes strong root growth and vigorous plants.
Much of the nutrients, such as nitrogen, that are necessary for plant growth are locked into various unusable forms within organic matter. Microorganisms and worms decompose the organic material through digestion and convert it to usable chemical compounds. They provide a "nutrient soup" that the roots of the plant can then feed from for a long time.
Soils that are primarily sand or clay have problems retaining water. Water runs through sand and the plants suffer from lack of moisture. Clay either retains too much water, which can drown the plant, or when it is very dry, the water can run off the top without leaving any available for the roots.
Organic material in the soil solves both problems by holding many times its weight in water. This keeps sandy soil moist for a longer period, and it soaks up the excess water in clay and holds it through dry periods, preventing the soil from shedding water.
A handful of good soil that is a little damp should crumble easily when compressed into a ball. If it is too sandy, it will crumble immediately; if it is too clayey, it will retain its shape under pressure. Organic material solves both problems by giving the sand particles something to cling to and allowing the clay particles to separate from one another.
Soil with good friability gives roots the opportunity to easily grow and stretch out for sources of nutrients and oxygen.
Renewing Organic Matter
Nature renews the organic matter in the soil every time something dies and falls to the ground, whether it is a leaf, a flower or even an animal. All are eventually decomposed and the nutrients and minerals returned to their original source.
You can do the same by directly adding organic matter to the top of the soil or digging it in. Compost, peat moss, manure, leaf mold and cover crops are all options available to the homeowner. This is important because otherwise the existing organic material will eventually be used up, with the soil becoming less fertile.