Compost Biodiversity

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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 primarily the bacteria, and specifically the ther-mophilic bacteria, that create the heat of the compost pile.

Although considered bacteria, actinomycetes are effectively intermediates 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 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 known species of fungi, the overwhelming majority of them being microscopic.31 Most fungi cannot grow at 500C because it's too hot, although thermophilic fungi are heat tolerant. Fungi tend to be absent in compost above 600C and actino-mycetes tend to be absent above 700C. Above 820C 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 600800 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

Compost Microorganisms "Sanitize" Compost

A frequent question is, "How do you know that all parts of your compost pile 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:

  • competition for food from compost microorganisms;
  • inhibition and antagonism by compost microorganisms;
  • consumption by compost organisms;
  • biological heat generated by compost microorganisms; and
  • antibiotics produced by compost microorganisms.

For example, when bacteria were grown in an incubator without compost at 500C and separately in compost at 500C, 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 600C (1400F). 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 500C (1220 F), if maintained for twenty-four hours, is sufficient to kill all of the pathogens, according to some sources (this issue is covered in Chapter Seven). A lower temperature will take longer to kill pathogens. A temperature of 460C (1150F) may take nearly a week to kill pathogens completely; a higher temperature may take only 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 47-550C (116-1300F) 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 55-600C (131-1400F) 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 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 can be a bit overblown at times. People may believe that it's only the heat of the compost pile 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 outright killing the other inhabitants of the compost, which include fungi and actinomycetes as well as the bigger organisms that you can actually see.

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 mat ter 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 caused by sterilization due to excess heat, 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 (800C), only about 10% of sewage sludge solids could be decomposed in three weeks, whereas at 50-600C, 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 55-590C (131-1390F) 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 450C (1130F).40 Another researcher asserts that maximum biodegradation occurs at 45-550C (113-1310F), while maximum microbial diversity requires a temperature range of 35-450C (95-1130F).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, however, is probably not 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 500C 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.

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 since 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. Such labs are listed in Chapter Six.

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 confident that their thermophilic compost can be safely returned to the soil, 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?


To turn or not to turn: that is the question

What is one of the first things to come to mind when one thinks of compost? Turning the pile. Turn, turn, turn, has become the mantra of composters worldwide. Early researchers who wrote seminal works in the composting field, such as Gotaas, Rodale, and many others, emphasize turning compost piles, almost obsessively so.

Much of compost's current popularity in the West can be attributed to the work of Sir Albert Howard, who wrote An Agricultural Testament in 1943 and several other works on aspects of what has now become known as organic agriculture. Howard's discussions of composting techniques focus on the Indore process of composting, a process developed in Indore, India, between the years of 1924 and 1931. The Indore process was first described in detail in Howard's 1931 work, co-authored with Y. D. Wad, The Waste Products of Agriculture. The two main principles underlying the Indore composting process include: 1) mixing animal and vegetable refuse with a neutralizing base, such as agricultural lime; and 2) managing the compost pile by physically turning it. The Indore process subsequently became adopted and espoused by composting enthusiasts in the West, and today one still commonly sees people turning and liming compost piles. For example, Robert Rodale wrote in the February, 1972, issue of Organic Gardening concerning composting humanure^ "We recommend turning the pile at least three times in the first few months, and then once every three months thereafter for a year."

A large industry has emerged from this philosophy, one which manufactures expensive compost turning equipment, and a lot of money, energy and expense go into making sure compost is turned regularly. For some compost professionals, the suggestion that compost doesn't need to be turned at all is utter blasphemy. Of course you have to turn it — it's a compost pile, for heaven's sake.

Or do you? Well, in fact, no, you don't, especially if you're a backyard composter, and not even if you're a large scale composter. The perceived need to turn compost is one of the myths of composting.

Turning compost potentially serves four basic purposes. First, turning is supposed to add oxygen to the compost pile, which is supposed to be good for the aerobic microorganisms. We are warned that if we do not turn our compost, it will become anaerobic and smell

Compost Anaerobic

bad, attract rats and flies, and make us into social pariahs in our neighborhoods. Second, turning the compost ensures that all parts of the pile are subjected to the high internal heat, thereby ensuring total pathogen death and yielding a hygienically safe, finished compost. Third, the more we turn the compost, the more it becomes chopped and mixed, and the better it looks when finished, rendering it more marketable. Fourth, frequent turning can speed up the composting process.

Since backyard composters don't actually market their compost, usually don't care if it's finely granulated or somewhat coarse, and usually have no good reason to be in a hurry, we can eliminate the last two reasons for turning compost right off the bat. Let's look at the first two.

Aeration is necessary for aerobic compost, and there are numerous ways to aerate a compost pile. One is to force air into or through the pile using fans, which is common at large-scale composting operations where air is sucked from under the compost piles and out through a biofilter. The suction causes air to seep into the organic mass through the top, thereby keeping it aerated. An accelerated flow of air through a compost mass can cause it to heat up quite drastically; then the air flow also becomes a method for trying to reduce the temperature of the compost because the exhaust air draws quite a bit of heat away from the compost pile. Such mechanical aeration is never a need of the backyard composter and is limited to large scale composting operations where the piles are so big they can smother themselves if not subjected to forced aeration.

Aeration can also be achieved by poking holes in the compost, driving pipes into it and generally impaling it. This seems to be popular among some backyard composters. A third way is to physically turn the pile. A fourth, largely ignored way, however, is to build the pile so that tiny interstitial air spaces are trapped in the compost. This is done by using coarse materials in the compost, such as hay, straw, weeds, and the like. When a compost pile is properly constructed, no additional aeration will be needed. Even the organic gardening pros admit that, "good compost can be made without turning by hand if the materials are carefully layered in the heap which is well-ventilated and has the right moisture content." 45

This is especially true for "continuous compost," which is different from "batch compost." Batch compost is made from a batch of material that is composted all at once. This is what commercial com-posters do — they get a dump truck load of garbage or sewage sludge from the municipality and compost it in one big pile. Backyard com-posters, especially humanure composters, produce organic residues daily, a little at a time and rarely, if ever, in big batches. Therefore, continuous composters add material continuously to a compost pile usually by putting the fresh material on the top. This causes the ther-mophilic activity to be in the upper part of the pile while the ther-mophilically "spent" part of the compost sinks lower and lower, to be worked on by fungi, actinomycetes, earthworms and lots of other things. Turning continuous compost dilutes the thermophilic layer with the spent layers and can quite abruptly stop all thermophilic activity.

Researchers have measured oxygen levels in large-scale windrow composting operations (a windrow is a long, narrow pile of compost). One reported, "Oxygen concentration measurements taken within the windrows during the most active stage of the composting process, showed that within fifteen minutes after turning the windrow — supposedly aerating it — the oxygen content was already depleted." 46 Other researchers compared the oxygen levels of large, turned and unturned batch compost piles, and have come to the conclusion that compost piles are largely self-aerated. "The effect of pile turning was to refresh oxygen content, on average for [only] 1.5 hours (above the 10% level), after-which it dropped to less than 5% and in most cases to 2% during the active phase of composting . . . Even with no turning, all piles eventually resolve their oxygen tension as maturity approaches, indicating that self-aeration alone can adequately furnish the composting process . . . In other words, turning the piles has a temporal but little sustained influence on oxygen levels." These trials compared compost that was not turned, bucket turned, turned once every two weeks and turned twice a week.47

Interestingly enough, the same trials indicated that bacterial pathogens were destroyed whether the piles were turned or unturned, stating that there was no evidence that bacterial populations were influenced by turning schemes. There were no surviving E. coli or Salmonella strains, indicating that there were "no statistically significant effects attributable to turning." Unturned piles can benefit by the addition of extra coarse materials such as hay or straw, which trap extra air in the organic material and make additional aeration unnecessary. Furthermore, unturned compost piles can be covered with a thick insulating layer of organic material, such as hay, straw or even finished compost, which can allow the temperatures on the outer edges of the pile to grow warm enough for pathogen destruction.

Not only can turning compost piles be an unnecessary expen diture of energy, but the above trials also showed that when batch compost piles are turned frequently, some other disadvantageous effects can result (see Figure 3.6 on page 49). For example, the more frequently compost piles are turned, the more agricultural nutrients they lose. When the finished compost was analyzed for organic matter and nitrogen loss, the unturned compost showed the least loss. The more frequently the compost was turned, the greater was the loss of both nitrogen and organic matter. Also, the more the compost was turned, the more it cost. The unturned compost cost $3.05 per wet ton, while the compost turned twice a week cost $41.23 per wet ton, a 1,351% increase. The researchers concluded that "Composting methods that require intensification [frequent turning] are a curious result of modern popularity and technological development of composting as particularly evidenced in popular trade journals. They do not appear to be scientifically supportable based on these studies . . . By carefully managing composting to achieve proper mixes and limited turning, the ideal of a quality product at low economic burden can be achieved." 48

When large piles of municipal compost are turned, they give off emissions of such things as Aspergillus fumigatus fungi which can cause health problems in people. Aerosol concentrations from static (unturned) piles are relatively small when compared to mechanically turned compost. Measurements thirty meters downwind from static piles showed that aerosol concentrations of A. fumigatus were not significantly above background levels, and were "33 to 1800 times less" than those from piles that were being moved.49

Finally, turning compost piles in cold climates can cause them to lose too much heat. It is recommended that cold climate com-posters turn less frequently, if at all.50

Do you need to inoculate your compost pile?

No. This is perhaps one of the most astonishing aspects of composting.

In October of 1998, I took a trip to Nova Scotia, Canada, to observe the municipal composting operations there. The Province had legislated that as of November 30, 1998, no organic materials could be disposed of in landfills. By the end of October, with the "ban date" approaching, virtually all municipal organic garbage was being collected and transported instead to composting facilities, where it was effectively being recycled and converted into humus. The municipal garbage trucks would simply back into the compost facility building (the composting was done indoors), and then dump the garbage on the floor. The material consisted of the normal household and restaurant food materials such as banana peels, coffee grounds, bones, meat, spoiled milk and paper products such as cereal boxes. The occasional clueless person would contribute a toaster oven, but these were sorted out. The organic material was then checked for other contaminants such as bottles and cans, run through a grinder, and finally shoved into a concrete compost bin. Within 24-48 hours, the temperature of the material would climb to 700C (1580F). No inoculants were required. Incredibly, the thermophilic bacteria were already there, waiting in the garbage for this moment to arrive.

Researchers have composted materials with and without inocula and found that, "although rich in bacteria, none of the inocula accelerated the composting process or improved the final product. . . The failure of the inocula to alter the composting cycle is due to the adequacy of the indigenous microbial population already present and to the nature of the process itself. . . The success of composting operations without the use of special inocula in the Netherlands, New Zealand, South Africa, India, China, the U.S.A, and a great many other places, is convincing evidence that inocula and other additives are not essential in the composting of [organic] materials." 51 Others state, "No data in the literature indicate that the addition of inoculants, microbes, or enzymes accelerate the compost process. "52


It is not necessary to put lime (ground agricultural limestone) on your compost pile. The belief that compost piles should be limed is a common misconception. Nor are other mineral additives needed on your compost. If your soil needs lime, put the lime on your soil, not your compost. Bacteria don't digest limestone; in fact lime is used to kill microorganisms in sewage sludge — it's called lime-stabilized sludge.

Aged compost is not acidic, even with the use of sawdust. The pH of finished compost should slightly exceed 7 (neutral). What is pH? It's a measure of acidity and alkalinity which ranges from 1-14. Neutral is 7. Below seven is acidic; above seven is basic or alkaline. If the pH is too acidic or too alkaline, bacterial activity will be hindered or stopped completely. Lime and wood ashes raise the pH, but wood ashes should also go straight on the soil. The compost pile doesn't need them. It may seem logical that one should put into one's com post pile whatever one also wants to put into one's garden soil, as the compost will end up in the garden eventually, but that's not the reality of the situation. What one should put into one's compost is what the microorganisms in the compost want or need, not what the garden soil wants or needs.

Sir Albert Howard, one of the most well-known proponents of composting, as well as J. I. Rodale, another prominent organic agriculturist, have recommended adding lime to compost piles.53 They seemed to base their reasoning on the belief that the compost will become acidic during the composting process, and therefore the acidity must be neutralized by adding lime to the pile while it's composting. It may well be that some compost becomes acidic during the process of decomposition, however, it seems to neutralize itself if left alone, yielding a neutral, or slightly alkaline end product. Therefore, it is recommended that you test your finished compost for pH before deciding that you need to neutralize any acids.

I find it perplexing that the author who recommended liming compost piles in one book, states in another, "The control of pH in composting is seldom a problem requiring attention if the material is kept aerobic. . . the addition of alkaline material is rarely necessary in aerobic decomposition and, in fact, may do more harm than good because the loss of nitrogen by the evolution of ammonia as a gas will be greater at the higher pH."5 In other words, don't assume that you should lime your compost. Only do so if your finished compost is consistently acidic, which would be highly unlikely. Get a soil pH test kit and check it out. Researchers have indicated that maximum thermophilic composting occurs at a pH range between 7.5 to 8.5, which is slightly alkaline.55 But don't be surprised if your compost is slightly acidic at the start of the process. It should turn neutral or slightly alkaline and remain so when completely cured.

Scientists who were studying various commercial fertilizers found that agricultural plots to which composted sewage sludge had been added made better use of lime than plots without composted sludge. The lime in the composted plots changed the pH deeper in the soil indicating that organic matter assists calcium movement through the soil "better than anything else," according to Cecil Tester, Ph.D., research chemist at USDA's Microbial Systems Lab in Beltsville, MD.56 The implications are that compost should be added to the soil when lime is added to the soil.

Perhaps Gotaas sums it up best, "Some compost operators have suggested the addition of lime to improve composting. This should be done only under rare circumstances such as when the raw material to be composted has a high acidity due to acid industrial wastes or contains materials that give rise to highly acid conditions during composting." 57

What NOT to compost? You can compost almost anything.

I get a bit perturbed when I see compost educators telling their students that there is a long list of things "not to be composted!" This prohibition is always presented in such an authoritative and serious manner that novice composters begin trembling in their boots at the thought of composting any of the banned materials. I can imagine naive composters armed with this misinformation carefully segregating their food scraps so that, God forbid, the wrong materials don't end up in the compost pile. Those "banned" materials include meat, fish, milk, butter, cheese and other dairy products, bones, lard, mayonnaise, oils, peanut butter, salad dressing, sour cream, weeds with seeds, diseased plants, citrus peels, rhubarb leaves, crab grass, pet manures, and perhaps worst of all — human manure. Presumably, one must segregate half-eaten peanut butter sandwiches from the compost bucket, or any sandwich with mayonaisse or cheese, or any left-over salad with salad dressing, or spoiled milk, or orange peels, all of which must go to a landfill and be buried under tons of dirt instead of being composted. Luckily, I was never exposed to such instructions, and my family has composted every bit of food scrap it has produced, including meat, bones, butter, oils, fat, lard, citrus peels, mayonnaise and everything else on the list. We've done this in our backyard for 26 years with never a problem. Why would it work for us and not for anyone else? The answer, in a word, if I may hazard a guess, is humanure, another forbidden compost material.

When compost heats up, much of the organic material is quickly degraded. This holds true for oils and fats, or in the words of scientists, "Based on evidence on the composting of grease trap wastes, lipids [fats] can be utilized rapidly by bacteria, including actinomycetes, under thermophilic conditions." 58 The problem with the materials on the "banned" list is that they may require thermophilic composting conditions for best results. Otherwise, they can just sit in the compost pile and only very slowly decompose. In the meantime, they can look very attractive to the wandering dog, cat, raccoon, or rat. Ironically, when the forbidden materials, including humanure, are combined with other compost ingredients, thermophilic conditions will prevail. When humanure and the other controversial organic materials are segregated from compost, thermophilic conditions may not occur at all. This is a situation that is probably quite common in most backyard compost piles. The solution is not to segregate materials from the pile, but to add nitrogen and moisture, as are commonly found in manure.

As such, compost educators would provide a better service to their students if they told them the truth: almost any organic material will compost — rather than give them the false impression that some common food materials will not. Granted, some things do not compost very well. Bones are one of them, but they do no harm in a compost pile.

Nevertheless, toxic chemicals should be kept out of the backyard compost pile. Such chemicals are found, for example, in some "pressure treated" lumber that is saturated with cancer-causing chemicals such as chromated copper arsenate. What not to compost: sawdust from CCA pressure treated lumber, which is, unfortunately, a toxic material that has been readily available to the average gardener for too many years (but now largely banned by the EPA).

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