Chapter 7. Compost


Hot composting is a method for recycling residues rapidly and with the possibility of descroying pathogens and weed seeds. The result is a concentration of plant nutrients and organic byproducts.

Cold composting is a slower alternative which does not kill weed seeds or pathogens but should be more consertive of humus nitrogen.

This chapter manifests more than the others of a possibly irrational personal bias, so I should reveal it now. For those composting for their own use, the best residues are those of especially low nutrient value and inappropriate to spread directly. The use of high quality residues such as manure, hay and straw risks the loss of nutrients and are better managed in other ways. This judgement strongly favors cold over hot composting.

The predicted nutrient value of self-produced compost is possible, based on averages.

Hot Composting

Conditions and Requirements

Composting is the decomposition of waste products by aerobic means, that is, through microorganisms which require oxygen. It first became popular through the work of Sir Albert Howard [48]. Briefly, raw materials are gathered into a pile and so managed as to generate enough heat to reach an initial temperature somewhere between 100 and 160 degrees F. At this temperature rapid breakdown of organic residues occurs, along with the destruction of weed seeds and parasites. The pile is turned regularly in order to introduce additional oxygen and to bring undecomposed material from the outside into the center1.

The process was further developed at the University of California at Berkeley as a means of sanitizing municipal wastes [42].

In the U.S. the process is commonly referred to either as the Berkeley composting method or as hot composting.

The result of the decomposition of organic matter is humus, which is a complex mixture of the byproducts of hundreds of varieties of organisms plus the remains of those which have expired. Furthermore, the byproducts are subject to further attack in order to gain whatever energy they may furnish. There is a continual sequence of stages of decomposition; each stage liberates additional carbon, the carbon/nitrogen ratio drops, and the residues become increasingly resistant to further attack.

Two essential requrements are moisture and air; adequate moisture is necessary for any form of life, and oxygen is needed to release the energy from carbon in the residues. The two threats to adequate oxygen are excessive moisture and compaction of material. Fresh, succulent green residues require particular care in order to avoid compaction; most likely the best way of working with them is with a variety of shapes.

Nitrogen is necessary for the growth of organisms and consquently to maximize the generation of heat. But it is not always necessary to obtain the temperature range for hot composting.

Temperature is a measure of the state existing when the flow of heat away from the pile equals the heat generated within it. The extent of the flow depends on the amount of material and its insulating value. Fallen tree leaves, for example, with a C/N ratio of about 100, constitute a fire hazard if gathered in a sufficiently large pile: they are known to spontaneously catch on fire from the heat generated.

Nitrogen, however, is necessary where only a small amount of material is available. The C/N ratio should be such as to maximize biological activity in order to compensate for the greater heat loss from the larger ratio of surface to volume.

In most situations, the minimum size of a compost pile should be about four feet on each side and 3 - 4 feet high. With a higher pile, the material should vary enough to avoid compaction. The residues should be just wet enough that no excess moisture can be wrung out from a handful. If the pile is too wet, turning will help dry it out. Insulating the surface with hay or leaves should reduce heat loss2.

The usual criteria for maximum activity is a C/N ratio in the range of 20 - 30; it seems reasonable inasmuch as it correlates with the C/N ratio of the organisms themselves3.

If left alone, the carbon/nitrogen ratio of a compost pile drops from its initial value to a point somewhere between 15 and 20. But decomposition, although it slows considerably, doesn't stop, as denitrification becomes a major activity4. Eventually, life ceases as organisms die out for lack of food. The end result is a lot of minerals, a small, stable humus fraction, and a minimal amount of life.

So when should the result of hot composting be spread? Weather permitting, it may be best applied as soon as possible after most of the material has lost its original appearance; as soon as it begins to look like soil. Doing so offers the best opportunity to minimize unnecessary losses, and it gives the soil organisms a chance at the available energy.

Limitations of Hot Composting

The two inevitable losses are nitrogen and humus (through the loss of carbon). Nitrogen is lost by

Humus is lost by

For municipalities and businesses that produce compost commercially, these losses are either unimportant or limited by advanced technology. For them hot composting is an ideal process for quickly generating large quantities of safe and valuable soil amendments from waste products. For municipalities at least, conservation of nitrogen and humus is not important in processing wastes; nor is it for some commercial producers, who make up losses with fertilizer additions.

But losses are important to most farmers and many gardeners. They should consider other options before doing it themselves: not only does it require time and labor but also the proper equipment to assure that the entire pile reaches the minimum temperature necessary to kill undesireable organisms. Furthermore, it is wasteful, probably of organic matter but certainly of nitrogen.

As discussed in chapter 6. Unprocessed Residues nitrogen losses have three causes: leaching of nitrates, escape of ammonia, and denitrification.

Leaching losses can be prevented by keeping the pile covered; a layer of hay or straw may be enough to cause water to roll off the surface. Ammonia losses are unavoidable in hot composting of a small pile unless its pH is monitored and controlled, not easily done without the proper equipment. Denitrification losses depend on the degree to which oxygen is lacking; without adequate control it is extensive in hot compost owing to the high demand for oxygen.

Furthermore, not only disease organisms are destroyed by the high temperature inherent in hot composting, but beneficial organisms as well, and a period of time is necessary before hot compost achieves the biological diversity already existing in cold compost. For example, compost produced by hot composting is not as effective as cold compost in preventing damping-off of seedlings [7].

Finally, without knowing the actual C/N ratio of the materials in a small pile, the tendency is to add an excess of nitrogenous material in order to assure that it will heat up. The consequence is an even greater loss of nitrogen.

There is always a price to pay for doing something quickly. In this case it is a loss of nitrogen additional to the loss occurring in a slow process of breakdown; it is also a loss of humus owing to the decreased availability of nitrogen and probably to the less efficient metabolism of those micro-organisms which survive the high temperature.

If rapid production of a concentrated mineral fertilizer with a broad range of trace elements is the primary goal, then hot composting is a good choice; otherwise alternatives are preferable. Whatever the decision, the reward is not worth the risk in composting diseased plants with the expectation that a small home operation will kill all pathogens.

A counter argument is that the price is acceptable: the creation of growth-enhancing humus components is worth the loss of nitrogen and total humus. That may be so, but other methods of composting also produce a variety of components with less loss. Furthermore, alternatives permit the activity of a larger number of organisms during the entire process, not just those that can withstand high temperatures. They include not only a greater range of micro-organisms but also bigger animals such as earthworms and spiders.

Some of these objections may not apply to commercial operations using modern methods. Those methods include an elaborate system for controlling air flow through the pile. The air not only supplies oxygen, but it cools the pile, and by this means, compost can be produced at an optimum temperature range which kills pathogens and weed seeds but nevertheless preserves some beneficial organisms. Furthermore, waste products handled by some businesses are such that disease organisms and weed seeds are unlikely5.

Cold Composting

An obvious alternative to hot composting is cold composting - gather materials into a pile without regard to the C/N ratio, adding to it as materials accumulate, and let it sit. In time, anywhere from a few weeks to a few months (or maybe years with difficult materials), it takes on the appearance of soil and can be spread. Better yet is to add soil in order to absorb excess moisture and improve aeration by reducing compaction.

Nitrogen losses will also occur in cold compost. Leaching of nitrates is probably more extensive than in hot compost because the pile is exposed longer to the environment. Owing to the slow rate of decomposition, however, ammonia loss should be minimal, unless animal manure is a major component. Although the demand for oxygen is much less than it is in hot compost, denitrification is still likely but to a lesser extent. Demtrification can, however be further reduced by adding soil to increase aeration.

Cold composting has several variations. One is the traditional above-ground pile, but there are at least two possibilities for direct incorporation underground, which may be more effective:

Finally, even though cold composting does not destroy disease organisms directly, its results probably help to control them, owing to the ecological balance resulting from the increased diversity of organisms.

A final note illustrates the difference between the two processes. Liming a cold compost pile to a pH near 7 is advisable because it will encourage the development of a greater variety of micro-organisms. Fungi will proliferate at almost any pH, but bacteria tend to prefer a neutral environment.

Never, however, lime a hot compost pile and probably not any pile containing a substantial amount of fresh animal manure: doing so will accelerate the escape of ammonia and subsequent loss of nitrogen.

Figure Figure 2. Loss of Humus is my view of the expected results of hot and cold composting. It is not meant to support the discussion but only to show the results if you accept it. The first chart shows the percent of carbon remaining if spread when the final C/N ratio is 20 and 30, and the expected loss of nitrogen is 50%. This is intended to represent the effect of hot composing; the expected loss of nitrogen appears to be based on anecdotal evidence but is reasonable considering the various causes.

The second chart is similar, except for an expected nitrogen loss of 30%. This is meant to represent cold compost. It is, however, a guess and could be more or less depending on aeration and the nature of the materials.

The curves are not completely fair; for instance, the loss of nitrogen from a compost pile spread when the C/N ratio is 30 will be less than if it were spread at a C/N ratio of 20; but noone to my knowledge has measured the loss as the C/N ratio drops from 30 to 20. So it may be best to accept the error rather than add another assumption. In any event, the curves come from equations derived in appendix D. Changes in Compost , so you are free to modify them.

Figure 2. Loss of Humus
Humus retained vs. C/N ratio if N retained = 70% Humus retained vs. C/N ratio if N retained = 50%

Manure In Compost

Is there a difference between hot compost made with and without manure? According to followers of Biodynamic agriculture, there is a difference, but no one makes clear what it is. One experiment does show a difference: according to work done in Germany with watery mixtures of compost, that made with manure has a natural fungicide not comparably present in vegetable compost [13].

One advantage of manure in compost is its convenience. It is a natural buffer, supplying whatever may be missing. Otherwise, compared with the two alternatives of managing manure (rotting, direct application in advance of planting), composting, especially hot composting, is likely to cause the most loss7.

A decision on whether to compost manure is likely to depend on three factors:

Nutrient Value of Compost

Compost concentrates nutrients because of the loss of bulk. This loss depends upon the initial C/N ratio of the residues; the higher the ratio, the greater the loss of bulk and the greater the concentration of nutrients. On the other hand, the loss of bulk means less humus.

The nutrient content of well-prepared compost may be approximately 15 to 30 lbs of nitrogen/ton of compost, about 5 to 10 lbs of phosphate/ton, and about 30 lbs or more of potash/ton. Relative to calcium, magnesium is usually moderate to high, but it may be low compared to potassium. Sulfur and the trace elements should be high. A cubic yard of soil-free compost weighs about 700-1000 lbs.

Approximately half of the compost breaks down in the soil during the first year after application, and so about half of the nitrogen and sulfur should become available the first season. Much of the phosphorus in compost is in inorganic form; although a significant amount should be readily available, depending upon the pH, the actual quantity is impossible to predict.

Most of the calcium, and magnesium in the original residues is no longer in organic form and will become available immediately. Most of the potassium is never in organic form and is always available; indeed leaching losses can be significant.

1 For further information on the technology of composting, see [41].    [return to text]

2 For additional suggestions especially applicable to a vegetable compost, see [3].    [return to text]

3 For example, the C/N ratio of fungi is about 10. When a fungus attacks an organic substance, some of the carbon is oxidized, releasing energy; some is utilized as body tissue; and the remainder is passed off as a waste product. Approximately 40% of the carbon attacked by fungi is used for tissue development. With these assumptions, the C/N ratio of the organic material must be at least 25 in order that carbon not be limiting.    [return to text]

4 The possibility of significant denitrification at a C/N ratio near or below 17 was pointed out to me by William Brinton.    [return to text]

5 A discussion of compost produced commercially for use in plant containers can found in [17, chapter 12].    [return to text]

6 This was developed by the Henry Doubleday Research Association, a unique organization which carries out a number of research activities on gardening on its own grounds and among its members. For a subscription, which includes a quarterly newsletter, contact the Henry Doubleday Research Association, Ryton-on-Dunsmore, Coventry, CV8 3LG, England.    [return to text]

7 this may not be so, however, if the urine is treated separately from the solids, because it is the main source of the loss of ammonia.    [return to text]

© 2013 Robert Parnes

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