The Process

Compost is created from the decomposition and recombination of various forms of plant and animal life, such as leaves, grass, wood, garbage, natural-fiber clothes, hair, and bones. These materials are organic matter. Organic matter is only a small fraction of the total material that makes up the soil— generally between 1% and 8%. Yet organic matter is absolutely essential to the sustenance of soil life and fertility. Organic matter refers to dead plant and animal residues of all kinds and in all stages of breakdown or decay. Inseparable from these decaying dead residues are the living microorganisms that decompose, or digest, them.

Microscopic life-forms (bacteria and fungi) in the soil produce the recombining process, which creates the warmth in the compost pile. Most of the decomposition involves the formation of carbon dioxide and water as the organic material is broken down. You can monitor the temperature of your compost pile with a compost thermometer. You can also do this by inserting a 1-inch by 1-inch piece of wood into the pile, removing it periodically and feeling the warmth with your hand. You can judge whether the latest measurement is hotter or cooler than before.

As the available energy is consumed, microbial activity slows down, their numbers diminish—and the pile cools. Most of the remaining organic matter is in the form of humus compounds. Humus is the living and dead bodies of microbial life. As humus is formed, nitrogen becomes part of its structure. This stabilizes nitrogen in the soil because the humus compounds are resistant to decomposition. They are worked on slowly by soil organisms, but the nitrogen and other essential nutrients are protected from too rapid solubility and dissipation. Organic matter includes humus and some undecomposed organic matter.

Humus also acts as a site of nutrient adsorption and exchange for plants in the soil. The surfaces of humus particles carry a negative electric charge. Many of the plant nutrients— such as calcium, sodium, magnesium, potassium, and most trace minerals—carry a positive electrical charge in the soil solution and are thereby attracted to and adhere to the surface of humus. Some of the plant nutrients—such as phosphorus, sulfur, and the form of nitrogen that is available to plants— are not positively charged. Fortunately, a good supply of these nutrients becomes available to the plants through biological transformation in the compost pile and soil.

As plant roots grow through the soil in search of nutrients, they feed on the humus. Each plant root is surrounded by a halo of hydrogen ions that are a by-product of the roots' respiration. These hydrogen ions also carry a positive electric charge. The root actually "bargains" with the humus, exchanging some of its positively charged hydrogen ions for positively charged nutrient ions stuck onto the surface of the humus. An active exchange is set up between humus and roots, the plants "choosing" which nutrients they need to balance their own inner chemistry.

Therefore, humus is the most reliable plant food, and plants pull off whatever combinations of nutrients they choose from its surface. grow biointensive practices rely on this natural, continual, slow-releasing biological process for nutrient release to the plants, rather than making available all the season's nutrients chemically at one time.

The beauty of humus is that it feeds plants with nutrients that the plants pick up on its surface and it also safely stores nutrients in forms that are not readily leached. The humus contains much of the remainder of the original nitrogen that was put in the compost pile in the form of grass, kitchen wastes, and so on. The humus was formed by the resynthe-sizing activity of numerous species of microorganisms feeding off that original "garbage."

The microorganisms in the soil then continue to feed on the humus after the finished compost is spread on the soil. As the microorganisms feed, the core nutrients in the humus are released in forms available to plant roots. Thus, the microorganisms are an integral part of the humus, and one cannot be found without the other. The only other component of the soil that holds onto and exchanges nutrients with plant roots is clay, but humus can hold onto and exchange a far greater amount of these nutrients.

"Give back to the soil as much as you have taken—and a little bit more—and Nature will provide for you abundantly!"

—Alan Chadwick

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