Microorganisms need a source of energy organic feedstocks to feed on and draw energy. Some of that released energy causes the compost pile to heat. Besides energy, organisms need nutrients, which they also obtain from the feedstocks. In particular, they need nitrogen to build protein for cells and to reproduce. Generally, organisms need about one part of nitrogen for every 12 to 15 parts of carbon used, just for protein synthesis. Other nutrients including phosphorous and potassium are also necessary, but are not usually limiting.
Moisture is critical. The microorganisms need moisture on the surface of all the particles as a medium to live and move in. But moisture also keeps nutrients in solution. The microbes can only use dissolved nutrients. For example, there should be enough moisture to keep ammonia in solution. If not, it becomes a gas, evaporates into the air, and the smell of ammonia is prevalent in your pile.
Composting is predominantly an aerobic or oxygen-requiring process. The microbes need air to function. Even the facultative variety, those organisms that can operate in aerobic and anaerobic modes, should have oxygen if they are going to function in the aerobic mode the mode we want. Under aerobic conditions, the primary breakdown products are carbon dioxide and water. In the anaerobic mode, organic acids are produced. This is where a lot of odor problems come from. So we want to provide enough oxygen for all aerobic organisms to prosper, and for facultative organisms to stay in the aerobic mode.
Finally, the pH has to be in a favorable range 5.5 to 8.5. If the pH is too high or too low, most organisms do not function well or not at all. Some may enter a resting or “spore” stage to protect themselves against pH extremes. You will see the process slow down tremendously as you get outside these boundaries. Plus, at a high pH (above 8.5), more ammonia is in the gaseous, volatile form.
Know What’s Being Produced
For troubleshooting, it is also important to know what is being produced during composting. Ammonia is a natural part of the composting process. It forms as various nitrogen-rich organic compounds decompose. As long as the environment is moist and the pH is close to neutral, most of the ammonia remains in solution, where it can be used by the organisms. If the pile gets too dry or the pH goes too high, the gaseous ammonia forms and is lost from the pile as it evaporates (i.e. volatilizes). This is both a loss of valuable nitrogen and a source of potential odors.
Organic acids are also a natural product of composting, especially when anaerobic conditions exist. A few examples are lactic, acetic, butyric, propionic, and valeric acids. Some of these are volatile and very odorous (also known as “volatile organic acids” or VOAs). Reduced sulfur compounds and some of the longer chain organic acids are very odorous. The smell from rotting waste is largely due to those organic acids.
Another problem with organic acids is that they are toxic to plants at high levels. The phytotoxicity of VOAs depends on the compost use, pH, plant sensitivity, and other factors. Nevertheless, if the total VOA content of compost measures 1,000 to 2,000 or more parts per million (0.1 to 0.2 percent), the compost might harm plants, again depending on how it is used.
Some research has shown that organic acids increase in the compost pile until about 42 days. This is within the most active composting period, when oxygen demand is the greatest and piles are turned most frequently. Eventually, organic acids will break down given time. Food residuals tend to generate a lot of organic acids. With poorly composted food, as much as nine percent of the dry weight can be in the form of organic acids. At this nine percent level, it may take at least six months of curing to reduce organic acids to an acceptable level.
The final product of composting, finished compost, is primarily composed of three different components humus, microbial mass and ash. The humus portion includes lignins, which are organic materials resistant to decomposition, plus the humic and fulvic acids that result from the breakdown. The microbial mass is both the dead and living organisms within the compost. Ash is the mineral content of the compost, largely unchanged by the composting process. These components of compost are relatively stable. They decompose much more slowly than the original and intermediate compounds. Therefore ammonia, VOAs, odors and heat are not a problem to compost handlers or users.
Common Problem — Doesn’t Heat
Usually, if you have a composting problem, you have several because normally problems are interrelated. One of the most common problems is the failure of the pile to heat, that is to reach thermophilic temperatures (greater than 110°). Why is this a problem? Generally, decomposition is slower at lower temperatures. But more importantly, if temperatures do not reach at least 130°F, pathogens and weed seeds are not adequately destroyed at least, it is much less certain. This problem can affect the entire pile or just part of it. A poorly mixed pile that has hot and cold spots is still going to have problems with survival of pathogens and weed seeds.
Piles fail to heat for two reasons there is either too little biological activity, or the heat generated by biological activity is lost as fast as it is produced. The latter situation is not common in large scale composting. It may occur with small piles during cold weather.
As shown in Table 1, there are many possible causes for reduced biological activity. Nearly all of the causes are related to the essential microbial needs discussed earlier. Basically, biological activity slows because of: inadequate available energy (volatile solids low, particles coarse); Some nutrient is scarce (high C:N ratio); Insufficient moisture; Poor aeration (high bulk density, particles small, too much moisture); High or low pH; or Some foreign material inhibits the microorganisms (oils). Lack of nitrogen, often evidenced by a high C:N ratio, is frequently a problem. Factors related to poor aeration are common as well.
Odors can present health and safety issues within contained areas at composting facilities. For example, ammonia concentrations can potentially reach levels that would not pass OSHA standards. However, more frequently, odors are a nuisance problem. Volatile organic acids (VOAs) can cause a lot of odor problems. VOAs are heavier than air, tend to stay close to the ground and can travel for miles. Somebody down wind is going to know when VOAs are being generated in abundance. Ammonia, on the other hand, is lighter than air. Although it may become an on-site problem, it tends to dissipate quickly, usually within the boundaries of the composting site.
Many things can contribute to odor problems (Table 2). Most of the underlying causes lead to the development and persistence of anaerobic conditions and consequently VOAs. Therefore, many of the underlying causes of odor relate to poor air penetration (high bulk density, high moisture, small particles). In many cases, the cause is either excessive moisture or a mix of materials that doesn’t provide enough porosity and structure, even at acceptable moisture levels. Such a mix would tend to be heavy (high bulk density). Odors from anaerobic conditions also occur with materials that decompose rapidly, that use oxygen faster than it can be resupplied. Ammonia problems are not necessarily due to aeration. They are caused by an overabundance of nitrogen (low C:N) and enhanced by low moisture and high pH.
Failure to produce a mature compost product why is that a problem? Depending on when and how it is used, an immature compost can have qualities that hurt its performance and value. Immature composts may contain high levels of VOAs, ammonia, and readily available carbon. Both ammonia and VOAs are harmful to plants, especially seedlings. Furthermore, high ammonia levels can lead to nitrogen loss. An immature compost can also tie up or immobilize nitrogen. The C contained in immature compost is still easily available, and in the process of decomposing that C, microorganisms also use the available N from the compost and surrounding soil. This is more likely to occur in compost as the C:N ratio increases (from 30:1 up).
Furthermore, an immature product will continue to heat even after it goes in a bag. I have heard of composters having several pallet loads returned because the compost was hot. They had to empty the bags and finish curing it before selling the product.
So what causes lead to an immature product? Table 3 lists several causes, but the bottom line is that the compost process was cut short. In most cases it is simply that not enough time is allowed for active composting and/or curing. Otherwise, the process can be slowed or interrupted such that compost is not mature within the normal time frame. Low temperatures plus insufficient moisture, nitrogen, or aeration are common reasons for slow composting.
It is worth noting that Tables 1, 2, and 3 look very much the same. The root causes to each of these problems are much the same. For instance, the failure to reach thermophilic temperatures and the failure to produce a mature product are both a consequence of slow decomposition and the conditions that slow decomposition. Similarly, odors result from poor aeration, which also slows decomposition. Thus many problems associated with composting are closely related.
Bill Seekins is with the Maine Department of Agriculture, Food and Rural Resources, and a faculty member at the University of Maine Cooperative Extension Compost School, an active member of the Maine Compost Team.