FAST AND SLOW
Cellulose (the fibrous plant substance that forms the cotton and flax of commerce, for example) decays more readily than do waxes, fats and lignin, but cellulose is more resistant than protein, starches and sugars. The more readily decomposed substances break down quickly into alcohols, aldehydes, amides, amino acids and similar products.
The starches and sugars are almost immediately absorbed by soil bacteria and fungi and used as energy foods. Since these plant forms do not produce chlorophyll, they must get these energy foods from an outside source.
At this stage of breakdown, some humus is formed by an interesting process. Protein is attacked at the same time as starches and sugars but because a more elaborate breakdown process is required for it, there will be some free protein in the soil solution along with lignin. These two substances have a strong attraction for each other and will combine to form humus long before it would otherwise be produced in the end stages of organic breakdown. Any protein which
does not combine at this stage will go to feed bacteria and fungi, except for the portion that rotifers, protozoa and other animal forms manage to steal from them.
Cellulose next breaks down, releasing hydrogen, carbon dioxide and methane as by-products. Only two species of bacteria are known to decompose cellulose, both of which are slowed up by acid (low pH) soil. One reason why adding lime to compost (raising the pH) speeds up decomposition is that the cellulose bacteria are stimulated. However, too high pH is just as bad as too low.
The mention of methane in garden soils may seem strange, since it is a gas usually associated with marshy lands and swampy areas. Actually, while methane is called marsh gas, it is released by many forms of microorganisms that work on carbohydrates, organic acids and proteins. However, in normal soils, it is used as a source of energy by both bacteria and fungi and so does not reach the atmosphere. In swamps, lack of air in the soil favors anaerobic forms of microorganisms that work in the absence of oxygen and do not utilize methane for energy, allowing it to escape into the atmosphere unchanged.
By this time, many series of chemical compounds are in the soil from the tissues of our plant. In addition to those already mentioned, there are sulfates, phosphates, calcium compounds and others. With all of these being released, and processes by the dozens proceeding simultaneously, a seething ferment of infinite complexity results and tremendous amounts of energy are being used. Someone has called organic matter the fuel for bacterial fires in the soil-certainly an apt metaphor.