The Composting Process
Backyard composting replicates nature's system of breaking down organic materials found on the floor of a forest. In nature organic debris such as leaves, dead plants, cones, twigs--even stumps and trees--decompose over time to eventually become a rich, black humus that resembles, in appearance at least, the black potting soils sold in bags in the garden center. The decomposition process is essentially the same whether it takes place in the woods or in your backyard.
Compost is the product of a natural process that involves the activity of enormous numbers of tiny organisms which utilize the two main chemical components of organic matter, carbon and nitrogen, in their life process.
Bacteria, fungi, actinomycetes (microscopic plants), and many invertebrates, including mites, sowbugs, and earthworms, have the key role in decomposing organic materials.
They eat the carbon in the organic materials, converting it to carbon dioxide, water, and humus (the portion they can't digest).
They eat the nitrogen in the organic materials to get the energy they need to grow and reproduce.
The carbon and nitrogen content of the compost pile is so important because it fosters the activity of these billions of microscopic garbage disposers.
Composting involves a chain of events involving the interrelationship of the feeding patterns of the various microbes and bugs which all coexist in a compost pile. The humic byproducts resulting from the digestion of one type of organism becomes the food source for another type. The organic material undergoes progressive decomposition as it moves through the food chain of a succession of different types of microbes. Eventually most of the digestible biodegradable material is consumed and transformed, the remaining substance representing the indigestible parts. What's is left is a dark brown or black humus-like material called compost.
The microorganisms that decompose the organic materials must have a certain environment if they are to thrive and multiply, so to continue the breakdown process. If you create that environment in your compost pile, you produce beautiful compost. If you don't create and maintain that environment, you will eventually still get compost, but in the process you may experience some undesirable side-effects such as bad odors. Fortunately the composting microorganisms aren't terribly fussy, so there is considerable leeway in how you build a compost pile. They do need, of course, some carbon-containing materials (such as dried leaves or straw) and some nitrogen-containing materials (such as fresh grass clippings, weeds or kitchen garbage). They also need oxygen, and sufficient but not excessive moisture. These four things, carbon, nitrogen, oxygen and water, are essential.
Let's take a closer look at the components of a compost pile. While a pile with some carbon, some nitrogen, some oxygen and some moisture will certainly decompose, there are a number of variables that homeowners can control which will increase the efficiency of the composting operation. Awareness of these variables will also help prevent problems with the pile.
Virtually any organic material, alone or in combination with other forms of organic materials, is appropriate in a compost pile. However, mixing different materials or changing the proportions of various materials can make a difference in the rate of decomposition of the pile, Leaves by themselves will decompose. Leaves mixed with grass clippings and kitchen waste will decompose more quickly and more thoroughly. Kitchen waste by itself will decompose, though it is likely to smell pretty bad and could attract rodents and other pests. Mixed with leaves or straw it will decompose quickly and without the odors. It is balancing the mix of organic materials, some with high carbon content and others with a high nitrogen content, that creates the ideal composting recipe. While many have attempted to express this ratio as a technical, scientific formula, others, ourselves included, have come to feel that this aspect of composting is as much an art as a science.
For successful backyard composting it is important to have a general understanding of what is technically called the "carbon/nitrogen ratio" of a compost pile. The C/N ratio reflects the ratio of the weight of carbon and the weight of nitrogen contained in the materials used in the compost pile. As we noted above, the carbon is critical because the microorganisms digest or oxidize carbon as an energy source and the nitrogen is ingested by these microbes for protein synthesis.
For the home composter, understanding the carbon/nitrogen ratio is more an art gained through experience than a scientific or technical rule. The scientist tells us that the ideal C/N ratio for the composting process is between 25 and 30 parts of carbon to 1 part of nitrogen by weight, or 30 to 1. While the composting engineer can figure out the actual C/N ratio for various materials, in practice the home composter simply needs to understand in very rough, imprecise terms how the C/N ratio affects how the compost pile is going to work so he can adjust the mix in his pile accordingly.
If there is too much carbon in a pile (high C/N ratio of over 100/1) then the pile will decompose very slowly. The large amount of carbon material will require a greater number of micro-organism generations for its degradation. It will take time for them to reproduce to generate the population to accomplish this. The material will still decompose, but it will take a long time, maybe years.
If there is too much nitrogen in a pile (low C/N ratio of under 10/1) then nitrogen will be turned into a foul smelling mass which produces ammonia gas. The relative lack of carbon makes it difficult for normal microbial activity to take place. The pile may become anaerobic (without air) and become a putrid mess.
So the challenge for the home composter is to learn how to successfully manage the C/N ratio without having to worry about the numbers. He wants to know how to achieve roughly the right ratio of carbon materials (leaves, sawdust, straw) and nitrogen materials (grass clippings, kitchen garbage, weeds) so that the microbes have a healthy balanced environment. Then they can feed efficiently and reproduce happily and the pile decomposes in a reasonable time and does not smell bad. Although you may never achieve the perfect 30:1 C/N ratio, you will learn how to come close enough to be successful.
Most home composters naturally think about materials in terms of volume; e.g. bags of leaves, piles of brush, containers of garbage, etc. Getting the right C/N ratio requires that you develop a sense of what these units of volume represent in terms of carbon and nitrogen. We've found that roughly 3 large plastic leaf bags of whole leaves mixed with one bag of grass clippings produces a compost pile that heats up nicely and will decompose without any bad odors. That mix probably has a C/N ratio of something around 35:1, well within the ballpark for an acceptable compost operation. See page ### in Chapter ## for some more information about C/N ratios.
It is not necessary to go to the trouble to weigh the materials and calculate the total mixture's C/N ratio. With a little experience you will be able to estimate when you have about the right amount of carbon material and about the right amount of nitrogen material to have effective decomposition with no odors. Basically you want to have lots of carbon (dry) materials and not so much nitrogen (green) materials. Fine tuning is not necessary. By the way, most finished compost that has been produced from a good mix of carbon and nitrogen materials will have a final ratio of carbon to nitrogen of about 15:1. Good garden loam or humus from the forest floor ranges from 10:1 to 15:1.
Proper Particle Size
The microbes that break down the organic material need oxygen to live. Therefore, they feed on the organic materials in the pile whose surfaces are in contact with air or oxygen. It follows then that if the organic materials in the pile are in small pieces, having more surface area exposed to the air, the microbes will work more efficiently and reproduce more readily. Therefore the smaller the particles or pieces of organic material, the faster the microbial activity will decompose that material. Chopped leaves will decompose in one year while the same volume of whole leaves will take almost two years to decompose. So, chopped, shredded, split, or even bruised organic materials, having increased surface area for microbial activity, will always decompose faster than whole ones. See chapter ## for details on tools for shredding organic material.
Organic materials, such as leaves, will decompose whether they are spread out in a relatively thin layer on the forest floor or they are piled up or stuffed into an enclosure that provides access to air. Material in the pile, box or bin, however, will decompose more quickly and more thoroughly than the material left in a thin layer over the soil. By putting organic material into a pile, you increase its mass which, in turn, stimulates the generation of billions more microorganisms than would be generated, for example, on the more exposed surface of the forest floor. That is why it is advisable to build a compost pile of some sort.
Make compost either in a pile that sits in the open or one that is in a container or enclosure of some kind. If you are not concerned about having a pile achieve fairly high internal temperatures, then the size and shape of the pile is not important. The volume or size of the pile or container becomes important if you are looking for high internal temperatures in the pile. Research shows that the minimum volume for effective decomposition at high temperatures is one that is about 3 x 3 x 3 feet in size. A pile smaller than this does not have the critical mass to generate the amount of microbial activity necessary to efficiently decompose the material in high heat conditions. The upper limit of the pile size for home composting is about 5 x 5 x 5 feet. If the pile is any larger than that there is reduced access of air in the middle of the pile which significantly slows the decomposition process and reduces the internal temperatures. See Chapter ### for further discussion about the design and dimensions of compost bins.
Effective decomposition requires air, actually oxygen, in fairly large volumes. Material will decompose without air (anaerobic conditions) but the process is very, very slow. One of the benefits of having organic materials of different sizes, textures, and coarseness is that lots of little air pockets are available within the pile to store that important oxygen. Those who want to hurry the composting process turn the pile a few times over the weeks. This introduces enormous amounts of oxygen to the interior of the pile. The added oxygen stimulates microbial activity and thus speeds up the decomposition process. If you build a pile and never turn it, the air gets slowly used up and the decomposition process is taken over by bacteria that function in low or no air conditions. The process still works, but it takes much longer. Later we will be talking about ways to add air to a pile.
Water is essential to the decomposition process. The trick is having the right amount of water. Too little, and the process slows. Too much, and it fills the air spaces, forcing the air out of the pile. This makes the pile anaerobic and it will begin to smell. The proper level of moisture in a pile is about 40 to 60% moist. That is about the same as a sponge that has been soaked and then wrung out so it is just damp. The test is to pick up a handful of compost material and squeeze it tightly in your fist. If water drips out it is too moist. If it feels dry, rather than damp, it needs water. Later in this chapter we'll tell you how to achieve the right level of moisture for effective composting.
While all the variables mentioned above are important, no decomposition will take place without the micro-organisms which actually do the work of breaking down the organic material into finished compost. The nitrogen, carbon, air and moisture provide an environment that will support them. Now it is time to introduce them in a very brief summary of what is, in fact, a magnificently complex food chain. The main players in the compost process are the microorganisms, including various kinds of bacteria, fungi, and actinomycetes, and the bigger invertebrates, including springtails, sowbugs mites, and earthworms. The combined efforts of the tiny micro-organisms and the larger invertebrates as they feed produce compost.
Bacteria - All kinds of bacteria are found on the surfaces of organic material, most in a dormant state just waiting for the proper conditions to allow them to begin multiplying and doing their particular job in the decomposition project. There are three types of bacteria that do most of the work to digest the materials in a compost pile. They each work best at a particular temperature range.
The psychrophiles like cool temperatures--even as low as 28F--so they do most of the work during the winter months. As they digest carbon in the organic matter, they generate heat. When the surrounding temperature rises to 60 to 70F, the mesophilic bacteria take over. They are responsible for most of the decomposition in the home compost pile. If you start a compost pile during mid-summer, the mesophiles may start the process, bypassing the psychrophiles. These are the most efficient of the decomposers, unless you are trying to kill pathogens and weed seeds which requires higher temperatures, then it is not necessary to get a pile to heat up beyond 100F in order to produce nice compost.
If the mesopyhiles have optimum food, air, and water, they work so hard at digesting carbon that they raise the temperature above 100F and are replaced by the thermophilic bacteria. It is these bacteria that can raise the temperature high enough to kill disease-causing organisms and weed seeds. Three to five days of about 155F is long enough for the thermophiles to do their best work. Then the temperature starts to drop, and when it declines to around 90-100F, the mesopyhiles pitch in again to clean up what is left.
Enzymes - Enzymes are natural organic substances produced by the bacteria which break down the complex carbohydrates into simpler forms that the bacteria can use as food. Enzymes, therefore, must be present before bacteria can work. When you add some kind of compost activator product to your pile to speed up the composting process, you are probably adding some enzymes to the pile; more on that later.
Fungi - These microorganisms break down the cellulose and lignin, after the faster-acting bacteria make their initial inroads on these resistant materials.
Actinomycetes - These microorganisms comprise a transitional group between bacteria and fungi. They share characteristics of both bacteria and fungi and activate to break down organic matter in the later stages of decay. Actinomycetes not only reduce lignin and other resistant materials, but also in the garden they work many feet below the soil surface to make food for deeper-reaching plants. They secrete digestive enzymes that help decompose cellulose, protein and starch as well.
Invertebrates, creatures that have no spinal columns, break down organic materials in a compost pile physically, while the micro-organisms break it down chemically. The various insects, larvae and worms that inhabit the pile contribute to the decomposition process in several ways. As they feed on the raw materials in the pile they break them into smaller pieces, making it easier for the smaller bacteria and fungi to process them. As they wiggle and burrow around in the pile, invertebrates also transport the tiny microorganisms from one site to another, helping to distribute them throughout the pile. Finally, as they digest the material that they eat, these larger residents of the compost pile process organic materials into compost, their castings representing the byproducts of decomposition within their bodies.
Invertebrates in the Compost Pile
The Actual Decomposition Process
Now that we have an understanding of the variables in the composting process, let's take a look at the process itself. Natural decomposition of organic materials takes place in roughly 5 steps: oxidation, reduction, degradation, conversion, and maturation. Whether a pile is built and forgotten for a year or turned every 5 days, the materials in the pile will go through these five steps.
Oxidation Phase -- Chemical oxidation from exposure to air and water happens before material is collected and while it is sitting waiting until compost pile is built. This is a minor issue in learning about composting.
Reduction Phase -- Large scale reduction of particle size by insects, worms, and the human who is building the pile facilitates decomposition. Chopping or shredding the material before it goes into the pile is going to speed things up. Mesophilic bacteria are already beginning their reduction activities during this stage whether you chop things or not. If decomposition becomes advanced enough for the pile to begin to heat up, the invertebrate worms, springtails, and mites will move to the outside of the pile or die.
Degradation phase -- Now true decomposition moves into full swing. During this phase the thermophilic bacteria replace the mesophilic bacteria. The actinomycetes, and fungi, spread throughout the pile. At its simplest level the degradation phase involves microorganisms consuming protein and carbohydrates found in the waste material. As they multiply and grow, they create energy in the form of heat, water, and carbon dioxide (CO2). These microorganisms multiply and are consumed by other microorganisms whose burgeoning population is encouraged by the environmental conditions within the compost pile. The temperature in the pile rises. In this phase anaerobic microorganisms, bacteria that exist without oxygen or air, are more common in the passive pile than in the active pile. The organic materials are slowly broken down by this process into materials not readily reduced by this group of microorganisms.
Conversion Phase -- During this phase the temperature drops in the pile and other bacteria and/or fungi take over to complete the decomposition. At this stage, the compost is considered to be "fresh" or "raw". Decomposition will continue if given time, but the compost can be used at this point. If this fresh compost is put into the soil, it will cause some of the nitrogen in the soil that is normally used by the plants, to be consumed by this final decomposition activity of the compost. Thus, fresh compost is not considered as valuable as what is called "aged" or "mature" compost in which the decomposition process has all but ceased.
Maturation Phase -- Here the compost ages and changes from "fresh compost" to "aged compost". During this phase the bacterial activity subsides, the pile cools down and earthworms, springtails and mites gradually return. The longer the compost sits the less nitrogen will remain in the final product. This point will be discussed later when we look at making "active" compost.
What is Final Product?
What is left after the chemical and physical breakdown of the organic materials is complete? Compost is made up largely of microbial cells, microbial skeletons, partially decomposed particles of organic matter (cellulose and lignin) and inorganic particles including glass, sand, rock, and other mineral elements. That is much the same as the content of what we call "humus". In the real world, the actual composition of compost is likely to vary with almost every compost pile. The final product is directly related to the nature of the initial ingredients, and no two home compost piles are likely to be exactly the same.
Therefore, the chemical composition of a mature compost depends essentially upon the raw material from which it is made. For example, an analysis of compost made from municipal waste compared to compost made of farmyard manure shows quite a different chemical composition.
The nitrogen content of a particular batch of compost is a function of both the amount of N existing initially in the compost pile's ingredients and to the procedure that was used to produce the final compost product. Compost from the active method usually has slightly more nitrogen than does compost produced by the passive method, but the differences are not significant from the homeowner's point of view. As you will see in later discussion, the content of phosphorus and potassium can be manipulated by adding rock powders to the pile when it is built. Adding limestone to a pile will influence the amount of calcium in the final product, but then it causes the loss of more nitrogen; there are always trade-offs.
The point here is that most composts will contain varying amounts of carbon, nitrogen, phosphorus, potassium, calcium and many other micronutrients valued by plants. It is not possible, at this time, for the average homeowner to control the composting process in enough detail to be able to have any control over the content of the final product. The presence of precise percentages of each element is not as important as having some kind of mature compost. Regardless of its chemical composition, compost is an outstanding soil amendment.
pH of Compost - Most compost made with a variety of common backyard organic materials will have a pH that is close to neutral, in a range from 6.8 to 7.2. There is considerable disagreement in the scientific community about how the pH of the final product is related to the pH of the materials used at the start of the process. Some reports indicate that compost made exclusively of very acidic oak leaves will have an acidic pH after
es have produced a neutral compost using such acidic ingredients. There is even no agreement about why some composts can be acidic 5.5 to 6.5 or alkaline 7.5 t. 8.5. More research needs to be done on this question.
Research done in Connecticut suggests that the whole argument about the pH of compost may be not be critical in most cases. Some years ago, researchers of the University of Connecticut discovered a property in the northwestern part of the state that was a natural laboratory. A man, who loved rhododendrons, had bought this land 20 years previously which, unbeknownst to him, had very alkaline soil. He proceeded to plant dozens of rhododendron shrubs, which need acid soil to thrive, in this very alkaline soil. For 20 years, this man routinely put a one inch layer of compost around each of those rhododendron bushes. The researchers believe that the compost was, for the most part, neutral in pH. The man used no other fertilizers or chemicals in caring for those plants. When the researchers investigated the soil around the rhododendron plants, they were amazed. The soil around the rootball of each plant had the level of acidity preferred by rhododendrons. The soil a distance from the plants was still alkaline. While the researchers have no idea how it happened, it is apparent that the plants created their own acidic conditions. It is assumed they could do that because of the availability of the one inch of compost each year.
Most gardening books maintain that the best soil for growing good vegetable crops should have a pH of from 6.0 to 6.8. Researchers at the Rodale Research Farm in Kutztown, Pennsylvania have found that when their very alkaline soil, over 8.0, has sufficient organic or humus content, fine crops of vegetables can be grown year after year. Obviously, more research is necessary before we can fully understand the relationship of the pH of organic materials to the pH of compost made from those materials.
In the meantime, if you use a variety of materials to make compost, you should not have to worry about the pH of the final product. If it is not close to having a neutral pH, it will likely have a pH reflecting the acidity or alkalinity of the materials you used which we assume are from local sources. The compost then will reflect the pH of soils in your area. If the plants in your area, in general, are doing well, then the compost produced from the waste from those plants will be good for amending that local soil. Until more research is done, you can accept the general fact that people who use lots of homemade compost always have good soil and healthy plants.
Fatty Acids in Compost - Fatty acids are used in certain insecticides to control many small, soft bodied insects such as aphids, mites, and white flies. In certain cases, depending on the materials used to make compost, the final compost product may have some of these same fatty acids which will help control parasitic nematodes in the soil to which the compost is added. Researchers have found that rye and timothy grasses are particularly good for releasing fatty acids in compost. While more research needs to be done, it is believed that fatty acids in compost are one of the reasons that gardens with soils amended with lots of compost tend to have fewer insect problems than gardens in soils with no compost amendments.
Humic Acid in Compost -- Humic acid, another by-product of compost, is a valuable ingredient for helping make heavy clay soils more friable and easy to manage. Humic acid also plays a special role in soils that may contain too much aluminum. It binds with the aluminum to render it relatively inactive.
Volume Reduction of Compost -- When you finish the composting process you will see that there has been a significant reduction in the volume of the final product compared to the volume of the pile that you started with. In most cases, you can assume that the final compost material will be roughly half to one quarter the volume and about half to one third the weight of the original mass of organic matter used to build the pile. A pile starting at 4 x 4 x 4 feet (64 cubic feet) might end up being about 1-2 x 4 x 4 feet (32 cubic feet).
It is possible to make compost with almost any kind of organic material, including straw, hay, wood chips, sawdust, and non-meat food wastes from restaurants. However, home compost systems usually use the yard waste materials normally produced by caring for of the average home landscape--leaves, grass clippings, weeds, plant trimmings, sod, and non-meat kitchen waste. No matter what material is used in composting, it will contribute some basic nutrients to the composting equation in the form of nitrogen (N), phosphorous (P), and potassium (K), the three primary plant nutrients. Much of the nitrogen is going to be consumed by the decomposition process, but much of the phosphorous and potassium will end up in some form in the final product, the finished compost.
[Photo - Piles of the most common home composting materials including grass clippings, chopped leaves, and kitchen garbage]
The NPK ratings of composting materials are useful only to illustrate for you the relative value of each of the materials that you may use in your particular pile. Home composters do not need to worry about mixing various materials to achieve any kind of NPK balance in the final product. Such fine tuning is not possible in any kind of precise manner. Monitoring nutrient balances in compost is best left to commercial composting systems, and often, even at that level, the compost is usually made of whatever material happens to be available. As each composting material is discussed below, the approximate NPK value of that material will be noted, just for comparison to other materials. Remember, materials high in nitrogen will need to mixed with a higher proportion of materials high in carbon and vice versa to maintain an optimum carbon/nitrogen ratio for efficient decomposition in the pile.
Common Composting Materials
In this section we'll review the relative merits of those materials most commonly available for composting over most of North America. Then some other materials that may be available in certain parts of the country will be reviewed.
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List of Common Household Materials That Can be Composted
Grass, leaves, weeds, vegetable peelings, fruit rinds, coffee grounds, chunks of turf, vines, grass clippings, tea bags, paper towels, egg shells, corn husks, peanut shells, corn cobs, twigs, shredded branches, bark, pine cones.
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Grass clippings make up the largest portion of the average volume of yard waste produced by American homes. As we pointed out in Chapter ###, it is best, in most cases, to leave grass clippings right on the lawn as it is mowed so they can feed and revitalize the grass plants directly. In those instances when you do feel you must collect your grass clippings, make use of them as a thin mulch around plants in the yard. In the end, however, there may be situations where it is expedient to put grass clippings into the compost pile. Grass clippings are good for composting ONLY if they are mixed with leaves or some other carbon or dry material such as straw, hay, sawdust or even Canadian sphagnum peat moss purchased from the local garden center. If grass clippings alone are piled in a compost bin, the pile will rapidly become anaerobic (without oxygen) and begin to smell terribly.
[Photo - A line of bags of grass clippings sitting on the curb waiting for municipal trash pickup]
NPK Percentage Value of Grass Clippings In Compost
Nitrogen Phosphorous Potassium
Green clippings 0.66 0.19 0.71
Dried Clippings 1.20 0.40 1.55
Some home composters take advantage of their neighbors' bad habits and collect bags of grass clippings from around the neighborhood to add valuable nitrogen to the compost pile. In those cases where large amounts of grass clippings are available for composting, take time to dry them first. Spread the freshly cut grass over a paved driveway or similar surface to bake in the sun for at least a day. When it dries and begins to turn pale and strawlike, the grass can be dumped in a compost bin without danger of its putrefying and smelling bad.
Another concern with grass clippings is whether there are any residues of herbicides or other pesticides on the clippings when they are introduced into the compost bin. The best way to be sure the clippings are safe, if they are not from your lawn, is to accept only clippings that are collected after a fairly heavy rain. A steady rain will wash any dangerous pesticide residues from the blades and down into the soil.
For homeowners with deciduous trees, leaves represent a large percentage of the total yard waste produced by that landscape. As was noted in Chapter ##, chopped leaves make a marvelous mulch and we recommend that you use as many of the leaves accumulated in the fall for that purpose as is practical. Then, after all the mulching has been done, the remaining chopped leaves will make a excellent carbon rich material for the compost pile or bin.
[Photo - Someone raking leaves in the fall]
NPK Percentage Value of Leaves In Compost
Nitrogen Phosphorous Potassium pH
Red Maple 0.52 0.09 0.04 4.7
Sugar Maple 0.67 0.11 0.75 4.3
American Beech 0.67 0.10 0.65 5.1
White Ash 0.63 0.15 0.54 6.8
White Oak 0.65 0.13 0.52 4.4
East. Hemlock 1.05