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Compost is normally populated by three general categories of microorganisms: bacteria, actinomycetes, and fungi (see Figure 3.3 and Table 3.6). It is the bacteria, and specifically the thermophilic bacteria, that create the heat of the compost pile.

Although considered bacteria, actinomycetes are effectively intermediate between bacteria and fungi because they look similar to fungi and have similar nutritional preferences and growth habits. They tend to be more commonly found in the later stages of the compost, and are generally thought to follow the thermophilic bacteria in succession. They, in turn, are followed predominantly by fungi during the last stages of the composting process.

There are at least 100,000 species of fungi known, the overwhelming majority of them being microscopic.31 Most fungi cannot grow at 50°C (it's too hot) although some are heat tolerant (thermophilic fungi). Fungi tend to be absent in compost above 60°C, and actinomycetes tend to be absent above 70°C. Above 82°C biological activity effectively stops (extreme thermophiles are not found in compost).32

To get an idea of the microbial diversity normally found in nature, consider this: a teaspoon of native grassland soil contains 600-800 million bacteria comprising 10,000 species, plus perhaps 5,000 species of fungi, the mycelia of which could be stretched out for several miles. In the same teaspoon, there may be 10,000 individual protozoa of perhaps 1,000 species, plus 20-30 different nematodes from as many as 100 species. Sounds crowded to me. Obviously, good compost will reinoculate depleted, sanitized, chemicalized soils with a wide variety of beneficial microorganisms (see Figures 3.4 and 3.5).33


One of the most frequent questions asked of me is, "How do you know that ALL parts of your compost have been subjected to high enough temperatures to kill ALL potential pathogens?" The answer should be obvious: you don't. You never will. Unless, of course, you examine every cubic centimeter of your compost for pathogens in a laboratory. This would probably cost many thousands of dollars, which would make your compost the most expensive in history.

It's not only the heat of the compost that causes the destruction of human, animal, and plant pathogens, it's a combination of factors including:

For example, when bacteria were grown both in an incubator and separately in compost at 50°C, they died in the compost after only seven days, but lived in the incubator for seventeen days. This indicated that it is more than just temperature that determines the fate of pathogenic bacteria. The other factors listed above undoubtedly affect the viability of non-indigenous microorganisms (such as human pathogens) in a compost pile. Those factors require as large and diverse a microbial population as possible, which is best achieved by temperatures below 60°C (140°F). One researcher states that, "Significant reductions in pathogen numbers have been observed in compost piles which have not exceeded 40°C [104°F]." 34

There is no doubt that the heat produced by thermophilic bacteria kills pathogenic microorganisms, viruses, bacteria, protozoa, worms and eggs that may inhabit humanure. A temperature of 50°C (122° F), if maintained for twenty-four hours, is sufficient to kill all of the pathogens, according to some sources (see Chapter Seven). A lower temperature will take longer to kill pathogens. A temperature of 46°C (115°F) may take nearly a week to kill pathogens completely, a higher temperature may only take minutes. What we have yet to determine is how low those temperatures can be and still achieve satisfactory pathogen elimination. Some researchers insist that all pathogens will die at ambient temperatures (normal air temperature) given enough time.

When Westerberg and Wiley composted sewage sludge which had been inoculated with polio virus, Salmonella, roundworm eggs, and Candida albicans, they found that a compost temperature of 47-55°C (116-130°F) maintained for three days killed all of these pathogens.35 This phenomenon has been confirmed by many other researchers, including Gotaas, who indicates that pathogenic organisms are unable to survive compost temperatures of 55-60°C (131-140°F) for more than thirty minutes to one hour.36 The first goal in composting humanure, therefore, should be to create a compost pile that will heat sufficiently to kill all potential human pathogens that may be found in the manure.

Nevertheless, the heat of the compost pile is a highly lauded characteristic of compost that is a bit overblown at times. People think that it's only the heat of the compost that destroys pathogens, so they want their compost to become as hot as possible. This is a mistake. In fact, compost can become too hot, and when it does, it destroys the biodiversity of the microbial community. As one scientist states, "Research has indicated that temperature is not the only mechanism involved in pathogen suppression, and that the employment of higher than necessary temperatures may actually constitute a barrier to effective sanitization under certain circumstances." 37 Perhaps only one species (e.g., Bacillus stearothermophilus) may dominate the compost pile during periods of excessive heat, thereby driving out or just outright killing the other inhabitants of the compost, which include fungi and actinomycetes, as well as the bigger organisms that you can actually see.

[compost microorganisms magnified]

A compost pile that is too hot can destroy its own biological community and leave a mass of organic material that must be re-populated in order to continue the necessary conversion of organic matter to humus. Such sterilized compost is more likely to be colonized by unwanted microorganisms, such as Salmonella. Researchers have shown that the biodiversity of compost acts as a barrier to colonization by such unwanted microorganisms as Salmonella. In the absence of a biodiverse "indigenous flora," such as happens through sterilization, Salmonella were able to regrow.38

The microbial biodiversity of compost is also important because it aids in the breakdown of the organic material. For example, in high-temperature compost (80°C), only about 10% of sewage sludge solids could be decomposed in three weeks, whereas at 50-60°C, 40% of the sludge solids were decomposed in only seven days. The lower temperatures apparently allowed for a richer diversity of living things which in turn had a greater effect on the degradation of the organic matter. One researcher indicates that optimal decomposition rates occur in the 55-59°C (131-139°F) temperature range, and optimal thermophilic activity occurs at 550C (1310F), which are both adequate temperatures for pathogen destruction.39 A study conducted in 1955 at Michigan State University, however, indicated that optimal decomposition occurs at an even lower temperature of 45°C (113°F).40 Another researcher asserts that maximum biodegradation occurs at 45-55°C (113-131°F), while maximum microbial diversity requires a temperature range of 35-45°C (95-113°F).41 Apparently, there is still some degree of flexibility in these estimates, as the science of "compost microhusbandry" is not an utterly precise one at this time. Control of excessive heat is rarely a concern for the backyard composter.

Some thermophilic actinomycetes, as well as mesophilic bacteria, produce antibiotics that display considerable potency toward other bacteria, and yet exhibit low toxicity when tested on mice. Up to one half of thermophilic strains can produce antimicrobial compounds, some of which have been shown to be effective against E. coli and Salmonella. One thermophilic strain with an optimum growth temperature of 50°C produces a substance that "significantly aided the healing of infected surface wounds in clinical tests on human subjects. The product(s) also stimulated growth of a variety of cell types, including various animal and plant tissue cultures and unicellular algae." 42 The production of antibiotics by compost microorganisms theoretically assists in the destruction of human pathogens that may have existed in the organic material before composting.

[graph of fungi and bacteria populations]

Even if every speck of the composting material is not subjected to the high internal temperatures of the compost pile, the process of thermophilic composting nevertheless contributes immensely toward the creation of a sanitary organic material. Or, in the words of one group of composting professionals, "The high temperatures achieved during composting, assisted by the competition and antagonism among the microorganisms [i.e., biodiversity], considerably reduce the number of plant and animal pathogens. While some resistant pathogenic organisms may survive and others may persist in cooler sections of the pile, the disease risk is, nevertheless, greatly reduced." 43

If a backyard composter has any doubt or concern about the existence of pathogenic organisms in his or her humanure compost, s/he can use the compost for horticultural purposes rather than for food purposes. Humanure compost can grow an amazing batch of berries, flowers, bushes, or trees. Furthermore, lingering pathogens continue to die after the compost has been applied to the soil, which is not surprising as human pathogens prefer the warm and moist environment of the human body. As the World Bank researchers put it, "even pathogens remaining in compost seem to disappear rapidly in the soil." [Night Soil Composting, 1981] Finally, compost can be tested for pathogens by compost testing labs.

Some say that a few pathogens in soil or compost are ok. "Another point most folks don't realize is that no compost and no soil are completely pathogen free. You really don't want it to be completely pathogen free, because you always want the defense mechanism to have something to practice on. So a small number of disease-causing organisms is desirable. But that's it." 44 Pathogens are said to have "minimum infective doses," which vary widely from one type of pathogen to another, meaning that a number of pathogens are necessary in order to initiate an infection. The idea, therefore, that compost must be sterile is incorrect. It must be sanitary, which means it must have a greatly weakened, reduced, or destroyed pathogen population.

In reality, the average backyard composter knows whether his or her family is healthy or not. Healthy families have little to be concerned about, and can feel pretty confident that their thermophilic compost will be safe for their garden, provided the simple instructions in this book are followed regarding compost temperatures and retention times, as discussed in Chapter Seven. On the other hand, there will always be those people who are fecophobic, and who will never be convinced that humanure compost is safe. These people are not likely to compost their humanure anyway, so who cares?

Source: The Humanure Handbook. Jenkins Publishing, PO Box 607, Grove City, PA 16127. To order, phone: 1-800-639-4099.

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weblife.org | library | Humanure Handbook | Chapter 3: Compost Biodiversity