Anything that kills or inhibits the growth of the microorganisms should be kept out of the compost material.
The composting process has No affect on inorganic materials in the compost. This includes metals, glass, stones, etc. The composing process has very little decomposition affect on synthesized organic compounds such as most plastics. Some of these materials pose a safety threat to people and therefore should be kept out of the compost material as much as possible.
Reclamation of Nitrogen and Other Nutrients
Two of the most important purposes for home composting of organic wastes are (1) decreasing the amounts of usable organic materials that are being deposited in landfills, and (2) reclamation or conservation of the nutrient and fertilizer values of the organic materials. Of the major nutrients–nitrogen (N), phosphorus (P), and potash K)– nitrogen conservation is the most important. Nitrogen is more difficult to conserve in the compost pile than phosphorus and potash. Nitrogen may be lost by leaching, but the major loss of nitrogen in the compost pile comes from the escape of ammonia or other volatile nitrogenous gases from the compost material to the atmosphere.
Nitrogen loss as ammonia in aerobic composting is affected by the C:N ratio, the pH, the moisture content, aeration, temperature, and the initial form of nitrogen compounds in the organic materials.
A ratio of available carbon to available nitrogen of about 30 or more permits minimum loss of nitrogen. A C:N ratio of considerably over 30 may be necessary to provide maximum conservation of nitrogen. On the other hand, nitrogen losses of around 50% were observed in the University of California studies when the C:N ratio was in the range 20 to 25.
Ammonia escapes as ammonia hydroxide as the pH rises above 7.0. In the later stages of composting the pH may rise to between 8.0 and 9.0. At this time there should not be an excessive amount of nitrogen present as ammonia. Materials which contain large amounts of ash will have a high pH and may be expected to lose more nitrogen.
Some compost operators have suggested the addition of lime to improve composting. This should be done only under rare circumstances, such as when raw material to be composted has a high acidity due to acid wastes or contains materials which give rise to highly acid conditions during composting. It is recommended that when the pH remains above 4.0 to 4.5, lime should not be added. The pH will be increased by biological action and nitrogen will be conserved.
The moisture content of compost affects nitrogen conservation but to a much less extent than the C:N ratio and the pH. Ammonia escape is greater when the moisture content is low. The water serves as a solvent and diluent for the ammonia, thereby reducing the vapor pressure and volatilization. A moisture content range of 50% to 70%, which is also satisfactory for other aspects of composting, will assist in conserving nitrogen.
If ammonia is present, it will escape more easily when the material is disturbed and exposed to the atmosphere. However, if the initial C:N ratio is sufficiently high, the nitrogen losses during turning will be small.
High temperature increases volatilization and escape of ammonia. Since high temperatures are fundamental in aerobic composting and destruction of pathogen, there is little to be done about controlling temperatures other than to avoid temperature above 170°F.
Some materials, such as cellulose and porous fibrous matter, have the capacity to absorb or hold moisture and volatile substances, thereby reducing the tendency to escape. There is considerable evidence that material of this type plays a part in reducing nitrogen loss from compost. Materials containing considerable quantities of horse or cow manure seem to exhibit less nitrogen loss at a low C:N ratio than other materials and should be considered to be nitrogen carriers.
Loss of nitrogen by leaching may occur in rainy weather or if the composting material has too high an initial moisture content and excess liquid drains away. Loss by leaching depends on the amount of soluble nitrogen in the compost and on the amount of rainfall. Leaching may be minimized by arranging the compost piles so that water is prevented from entering the material.
Conservation of phosphorus and potash in composting is not difficult since about the only loss occurs through leaching during rainy weather.
Time
There is no time period after which you can say you have “total”, “perfect”, “complete”, “finished” compost. The current rule of thumb is that organic materials must be composted correctly and aged for at least one year, before the resulting compost can be called “safe for most uses.” Some composting methods will produce “finished compost” before twelve months, and some of the very low maintenance methods may take up to two years, or more, to produce “finished compost.”
How long it takes to turn organic materials into compost depends on many factors including the techniques used, seasonal temperatures (both outside and inside the pile), the balance of brown to green materials or C:N ratio, the size of the material, and moisture levels. In general the more time the compost material has had to “age,” the more stable and fully decomposed it will be. Time is one indicator that the compost is “mature.” With present methods “completely finished” compost is not likely in less than 120 days.
Assuming that the moisture content is in the optimum range, that the compost is kept aerobic, and that the particles of material are small enough to be readily attacked by the organisms present–all factors that can be controlled in the composting operation–the C:N ratio determines the time required for stabilization. Low C:N ratio materials are decomposed in the shortest time because the amount of carbon to be oxidized to reach a stabilized condition is small. Also, in low C:N ratio compost, a larger part of the carbon is usually in a more readily available form, while in higher C:N ratio materials more of the carbon is usually in the form of cellulose and lignin which are rather resistant to attack. As the breakdown process nears completion, cellulose and lignin are attacked last by the changing biological population. When the available C:N ratio is above 30, additional time is required for the recycling of nitrogen present.
(See Carbon:Nitrogen Relationships)
If the material is not kept aerobic so that hightemperatures can be maintained during the active decomposition period, or if the particle size is so large that the bacteria cannot readily attack the material, longer composting periods are required.
Under aerobic conditions at high temperatures when the initial C:N ratio is in the optimum range or below, some of the material in the interior of the pile may take on the appearance and odor of humus after 2 to 5 days of active decomposition. However, active decomposition is not complete at this stage, and the C:N ratio may not have been lowered to the level desired for plant use. Moreover, in the average compost pile, materials on or near the outside of the pile probably would still be uncomposted.
The actual composting time sufficient for pathogens and parasites to be destroyed and for nitrogen to be conserved is not exact. (See Heat-Destruction of Pathogenic Organisms). So long as satisfactory compost can be produced, the turning, time of composting, storage, and other factors is generally flexible for home composting.
