How Compost Is Produced
The road from raw organic material to finished compost is a complex one, because both chemical and microbial processes are responsible for the gradual change from one to the other.
Decomposition of compost is accomplished by enzymatic digestion of plant and animal material by soil microorganisms. Simultaneously, the chemical processes of oxidation, reduction, and hydrolysis are going on in the pile, and their products at various stages are usedby microorganisms for further breakdown.
Bacteria use these products for two purposes: (1) to provide en ergy to carry on their life processes and (2) to obtain the nutrients they need to grow and reproduce. The energy is obtained by oxidation of the products, especially the carbon fraction. The heat in a compost pile is the result of this biological “burning,” or oxidation. Some materials can be broken down and oxidized more rapidly than others. This explains why a pile heats up fairly rapidly at the start. It is because the readily decomposed material is being attacked and bacterial activity is at its peak. If all goes well, this material is soon used up, and so bacterial activity slows down—and the pile begins to cool. Of course, if the mass of the material is big enough, it acts as an insulator to prevent heat loss, and the high temperature may thus persist for some time after the active period is over, especially if the pile is not turned.
Persistent high temperatures are the result of uneven breakdown.
The raw materials that you add to your compost heap will have to be of biological origin in order to decompose down to finished com post. Wood, paper, kitchen trimmings, crop leavings, weeds, and manure can all be included in the heap. As compost is broken down from these raw materials to simpler forms of proteins and carbohydrates, it becomes more available to a wider array of bacterial species that will carry it to a further stage of decomposition.
Carbohydrates (starches and sugars) break down in a fairly rapid process to simple sugars, organic acids, and carbon dioxide that are released in the soil. When proteins decompose, they readily break down into peptides and amino acids, and then to available ammonium compounds and atmospheric nitrogen. Finally, species of “nitrifying” bacteria change the ammonium compounds to nitrates, in which form they are available to plants.
At this stage of decomposition, the heap is near to becoming finished compost, with the exception of a few substances that still resist breakdown. Through complex, biochemical processes, these substances and the rest of the decomposed material form humus. There is some evidence that humus is largely the remains of microbial bodies.
The microorganisms of the compost heap, like any other living things, need both carbon from the carbohydrates, and forms of nitrogen from the proteins in the compost substrate. In order to thrive and reproduce, all microbes must have access to a supply of the elements of which their cells are made. They also need an energy source and a source of the chemicals they use to make their enzymes. The principal nutrients for bacteria, actinomycetes, and fungi are carbon (C), nitrogen (N), phosphorus (P), and potassium (K). Minor elements are needed in minute quantities.
These chemicals in the compost pile are not in their pure form, and certainly not all in the same form at the same time. For example, at any given moment, nitrogen may be found in the heap in the form of nitrates and nitrites, in ammonium compounds, in the complex molecules of undigested or partly digested cellulose, and in the complex protein of microorganism protoplasm. There are many stages of breakdown and many combinations of elements. What’s more, microorganisms can make use of nitrogen and other elements only when they occur in specific forms and ratios to one another. The carbon cycle. Green plants use carbon dioxide gas, water, and sunlight to make sugars and other carbon-containing compounds that animals use as food. Carbon compounds in plant and animal wastes provide food for decomposers in the compost pile. Materials that have passed through the decomposers’ bodies and the microbial bodies themselves contain nutrients used by plants to continue the carbon cycle.
Nutrients must be present in the correct ratio in your compost heap. The ideal C/N ratio for most compost microorganisms is about 25:1, though it varies from one compost pile to another. When too little carbon is present, making the C/N ratio too low, nitrogen may be lost to the microorganisms because they are not given enough carbon to use with it. It may float into the atmosphere as ammonia and be lost to the plants that would benefit by its presence in humus. Unpleasant odors from the compost heap are most often caused by nitrogen being released as ammonia. Materials too high in carbon for the amount of nitrogen present (C/N too high) make composting inefficient, so more time is needed to complete the process. When added to the soil, high-carbon compost uses nitrogen from the soil to continue decomposition, making it unavailable to growing plants. See chapter 6 for more
on balancing the C/N ratio.
Affecting the interwoven chemical and microbial breakdown of the compost heap are environmental factors that need to be mentioned here.
Composting can be defined in the terms of availability of oxygen. Aerobic decomposition means that the active microbes in the heap require oxygen, while in anaerobic decomposition, the active microbes do not require oxygen to live and grow. When compost heaps are located in the open air, as most are, oxygen is available and the biological processes progress under aerobic conditions. Temperature, mois ture content, the size of bacterial populations, and availability of nutrients limit and determine how much oxygen your heap uses.
The amount of moisture in your heap should be as high as possible, while still allowing air to filter into the pore spaces for the benefit of aerobic bacteria. Individual materials hold various percentages of moisture in compost and determine the amount of water that can be added. For example, woody and fibrous materials, such as bark, saw dust, wood chips, hay, and straw, can hold moisture equal to 75 to 85 percent of their dry weight. “Green manures,” such as lawn clippings and vegetable trimmings, can absorb moisture equaling 50 to 60 per cent of their weight. According to longtime composting advocate and researcher Dr. Clarence Golueke in Composting, “The minimum content at which bacterial activity takes place is from 12 to 15 percent.
Obviously, the closer the moisture content of a composting mass approaches these low levels, the slower will be the compost process. As a rule of thumb, the moisture content becomes a limiting factor when it drops below 45 or 50 percent.”
Temperature is an important factor in the biology of a compost heap. Low outside temperatures during the winter months slow the decomposition process, while warmer temperatures speed it up. During the warmer months of the year, intense microbial activity inside the heap causes composting to proceed at extremely high temperatures.
The microbes that decompose the raw materials fall into basically two categories: mesophilic, those that live and grow in temperatures of 50° to 113°F (10° to 45°C), and thermophilic, those that thrive in tempera tures of 113° to 158°F (45° to 70°C). Most garden compost begins at mesophilic temperatures, then increases to the thermophilic range for the remainder of the decomposition period. These high temperatures are beneficial to the gardener because they kill weed seeds and diseases that could be detrimental to a planted garden.
The nitrogen cycle. Shortage of available nitrogen is often a limiting factor in plant growth, since plants can’t make use of abundant atmospheric nitrogen gas. (So-called nitrogen-fixing plants rely on symbiotic bacteria.) Composting plant and animal wastes exposes the nitrogen they contain to nitrogen-fixing microorganisms and decomposers that break it down into forms available to plants
The bacterial decomposers in compost prefer a pH range of be tween 6.0 and 7.5, and the fungal decomposers between 5.5 and 8.0. Compost must be within these ranges if it is to decompose. Levels of pH are a function of the number of hydrogen ions present. (High pH levels indicate alkalinity; low levels, acidity.) In finished compost, a neutral (7.0) or slightly acid (slightly below 7.0) pH is best, though slight alkalinity (slightly above 7.0) can be tolerated.
Lime is often used to raise the pH if the heap becomes too acid. However, ammonia forms readily with the addition of lime, and nitrogen can be lost.
