13.2: Making Composts

A Sample Recipe For Backyard Composting

Start with the following:

• grass clippings (77% moisture, 45% C and 2.4% N)
• leaves (35% moisture, 50% C and 0.75% N)
• food scraps (80% moisture, 42% C and 5% N)

The ratio of the materials needed to get 60% moisture and a C:N of 30:1 is 100 pounds of grass, 130 pounds of leaves and 80 pounds of food scraps.

—T. Richard (1996b)

Common and Uncommon Feedstock

Composting of wastes and organic residues, both on and off farms, has become a more common practice. Farmers, municipalities and community composters accept many organic residuals, and tipping fees are often charged to offset the cost of managing this waste.

The list of source materials is endless and includes anything, plant or animal, that was alive and is now dead and needs to be managed. Some examples include crop residuals; food processing residuals; livestock carcasses; pet, zoo and human manure; chipped trees; mixed leaf and yard residuals; road kill; egg shells; glucose solutions; brewery waste; paper from document destruction; bakery excess; floral and cut flower production waste; coffee/tea grounds; off-spec human food; residuals from fish canneries and slaughterhouses; poultry feathers; livestock wool; butcher waste; fish from fish kills; aquatic weeds; biochar; whey and other milk products; fats/oils/greases; bagasse (the pulpy residue left from crushing and extracting liquid from sugar cane); drywall; and untreated small pieces of wood.

Feedstock materials cannot just be thrown together randomly; they require a recipe that allows for the appropriate physical conditions (e.g., allowing air flow and the right texture for handling) and lots of carbon and nitrogen available for the microorganisms to feed on. Compost piles are often built by alternating layers of these materials. Turning the pile mixes the materials. Composting occurs most easily if high-nitrogen materials are mixed with high-carbon materials, with the average C:N ratio of the materials being about 25–40 parts carbon for every part nitrogen (see Chapter 9 for a discussion of C:N ratios). Therefore, manure mixed with straw, wood chips or bark can be composted as is, because it has the right C:N balance. Wood chips or bark also provide the coarse structural matrix (skeleton) needed for airflow and handling, and may be recycled by shaking the finished compost out of the bulking material and then used for the next composting cycle. Manure and sawdust would also provide a good C:N mix but the texture of sawdust is too fine to allow for effective air flow.

It’s important to avoid using certain materials such as coal ash and especially wood chips from pressure-treated lumber. And it’s a good idea to go easy using manure from pets or large quantities of fats, oils or waxes. These types of materials may be difficult to compost or may result in compost containing chemicals that can harm crops or humans. There are too many different combinations of materials to give blanket recommendations about how much of each to mix to get the moisture content and the C:N ratio into reasonable ranges for a good start on the process. One example is given in the box “A Sample Recipe for Backyard Composting.” There are formulas to help you estimate the proportions of the specific materials you might want to use in the compost pile (see Cornell University’s http://compost.css.cornell.edu). Sometimes it will work out that the pile may be too wet, too low in C:N (that means too high in nitrogen), or too high in C:N (too low in nitrogen). To balance your pile, you may need to add other materials or change the ratios used. Adding dry sawdust or wood chips will remedy the first two problems, and adding nitrogen fertilizer will remedy the third. If a pile is too dry, you can add water with a hose or sprinkler system.

Figure 13.1. On-farm composting facility, in which tarps are used to control moisture and temperature. The piles in the background are curing.

One thing to keep in mind is that not all carbon is equally available for microorganisms. Lignin is not easily decomposed. (We mentioned this when discussing soil organisms in Chapter 4 and again in Chapter 9 when we talked about the different effects that various residues have when applied to soil.) Although some lignin is decomposed during composting, probably depending on factors such as the type of lignin and the moisture content, high amounts of carbon present as lignin may indicate that not all of the carbon will be available for rapid composting. This means that the effective C:N can be quite a bit lower than expected based on total carbon (Table 13.1). For some materials, there is little difference between the C:N calculated with total carbon and calculated with only biodegradable carbon.

Pile Location and Size

Composting sites should be appropriately situated. They need to be readily accessible by equipment and because they will have some natural leakage (especially in humid climates), they need to be kept away from watercourses, sinkholes, flood plains, seasonal seeps, wells and other poorly drained areas. Also, depending on the feedstock, composting may be associated with undesirable odors, so it is best to be away from residential areas. Backyard composting can be done in piles or vessels and is best done in a safe location away from children and pets.

A compost pile or windrow (Figure 13.1) is a large, natural convective structure, something like a set of chimneys next to each other. Oxygen moves into the pile while carbon dioxide, moisture and heat rise out of it. The materials need to fit together in a way that allows oxygen from the air to flow in freely. On the other hand, it is also important that not too much heat escapes from the center of the pile. If small sizes of organic materials are used, a “bulking agent” may be needed to make sure that enough air can enter the pile. Dry leaves, wood shavings/chips and chopped hay or straw are frequently used as bulking agents, which need to be appropriately cut to size to prevent matting and slow composting. Composting will take longer when large particles are used, especially those resistant to decay like large wood chips, while overly fine particles like sawdust decompose well but cause the pile to become too dense for air flow.

Composting Dead Animals

The compost pile should be prepared with a base layer of organic absorbent materials, typically 2 inches or less in size with some sizable 4- to 6-inch chunks included. The pile needs to be large enough to retain much of the heat that develops during composting, but not so large and compacted that air can’t easily flow in from the outside. Compost piles should be 3–5 feet tall and about 6–10 feet across the base after the ingredients have settled (see Figure 13.2). (You might want it on the wide side in the winter, to help maintain warm temperatures, while gardeners can make compost in a 3-foot-tall by 3-foot-wide pile in the summer.) Easily condensed material should initially be piled higher than 5 feet. It is possible to have long windrows of composting materials, as long as they are not too tall or wide. —Bonhotal et al. (2008)

Moisture

The amount of moisture in a compost pile is important. If the materials mat and rainwater can’t drain easily through the pile, it may not stay aerobic in a humid climatic zone. On the other hand, if composting is done inside a barn or under dry climatic conditions, the pile may not be moist enough to allow microorganisms to do their job. Moisture is lost during the active phase of composting, so it may be necessary to add water to a pile. In fact, even in a humid region, it is a good idea to moisten the pile at first, if dry materials are used. However, if something like liquid manure is used to provide a high-nitrogen material, sufficient moisture will most likely be present to start the composting process. The ideal moisture content of composting material is about 40–60%, or about as damp as a wrung-out sponge. If the pile is too dry, 35% or less, ammonia is lost as a gas and beneficial organisms don’t repopulate the compost after the temperature moderates. Very dry, dusty composts become populated by molds instead of the beneficial organisms we want.

Table 13.1 Total Versus Biodegradable Carbon and Estimated C:N Ratios

Material	% Carbon	C:N	% Carbon	C:N %	Lignin %	Cell Wall %	Nitrogen
	(Total)		(Biodegradable)
Newsprint	39	115	18	54	21	97	0.34
Wheat straw	51	88	34	58	23	95	0.58
Poultry manure	43	10	42	9	2	38	4.51
Maple wood chips	50	51	44	45	13	32	0.97
Source: T. Richard (1996a)

Monitoring and Turning the Pile

Turning the composting residues exposes all the materials to high-temperature conditions at the center of the pile, and heat convection further exposes upper reaches of the pile (Figure 13.3). Materials at the lower sides of the pile often barely compost. Turning the pile rearranges all the materials and creates a new center. Equipment is now available to quickly turn long compost windrows at large-scale composting facilities (Figure 13.3). Tractor-powered compost turners designed for composting on farms are also available, and some farmers use manure spreaders to build piles. Monitoring of the pile is done primarily by checking temperatures. Internal compost temperatures affect the rate of decomposition as well as the reduction of pathogenic bacteria, fungi and weed seeds. The most efficient temperature range for composting is generally between 104°F and 140°F (40°C and 60°C), however, piles can reach temperatures as high as 170°F (77°C). Spontaneous combustion can be a problem. On the other hand, if temperatures get too high, this can indiscriminately kill beneficial as well as pathogenic organisms, causing temperatures to drop.

Compost temperatures depend on how much of the heat produced by the microorganisms is lost through aeration or surface cooling. During periods of extremely cold weather, piles may need to be larger than usual to minimize surface cooling. As decomposition slows, temperatures will gradually drop and remain within a few degrees of ambient air temperature. Thermometers with long probes and data loggers are available to help monitor the process. Measuring oxygen will also indicate how well the process is progressing. With static piles it is important to keep oxygen levels high by using bulky carbon sources. Ideally, oxygen levels should be kept at 5–14%. If piles are gently turned every time the interior reaches and stabilizes for a few days at about 140°F, it is possible to complete the composting process within months, all other factors of moisture and aeration being optimal. On the other hand, if you turn the pile only occasionally, it may take longer to complete, especially if it has become compacted.

Minimum Turning Technique

Farm-quality composts can be produced by turning the pile only once or twice. You need to carefully construct the pile: build it up to reasonable dimensions, use and thoroughly mix materials that give good porosity, and make sure the pile stays moist. A pile that is uniformly heating is getting sufficient air to decompose and therefore may not need turning. As the heat declines, the pile may be getting too dense or not getting sufficient air, and it may need to be turned. A good example of this is composting animal mortalities in wood chips where the pile heats and organic materials degrade without turning.

Figure 13.2. Compost pile dimensions and turning techniques. Illustration by Vic Kulihin.

Although turning compost frequently speeds up the process, too much turning may dry out the pile and cause more nitrogen and organic matter loss. If the pile is too dry, you might consider turning it on a rainy day to help moisten it. If the pile is very wet, you might want to turn it on a sunny day, or cover it with moisture-protective material like chopped straw (like a thatched roof) or compost fleece, a type of breathing cover that is now widely available. Very frequent turning may not be advantageous because it can cause the physical breakdown of important structural materials that aid natural aeration. The right amount of turning depends on a variety of factors, such as aeration, moisture and temperature. Turn your compost pile to avoid cold, wet centers; to break up clumps; and to make the compost more uniform later in the process before use or marketing. Use caution when turning in cold, windy weather if the pile is warm, for it may not reheat.

Finally, piles should not be actively turned in all cases. When composting livestock or roadkill carcasses, the animals are placed in the middle of the pile above a base layer (and lanced to avoid bloating), covered with another 2 feet of organic materials and then allowed to sit for 4–6 months without turning to allow the carcass to fully degrade (see case study at end of chapter).

Controlling Pathogens

Pathogens are a large concern with composts, especially when they involve excrements and carcasses. Different methods of composting will result in varying levels of pathogen reduction. Turned piles will continue to move material into the center of the pile so that all material is exposed to thermophilic temperatures. Different regulators have different time-temperature requirements to meet certain needs. For example, the United States Environmental Protection Agency lists processes to further reduce pathogens, which requires temperature between 131°F and 170°F. To comply with the standard, composting operations that utilize an in-vessel or static aerated pile system must maintain a temperature within that range for a minimum of three days. Composting operations that utilize a windrow composting system must maintain a temperature within that range for a minimum of 15 days, during which time the materials must be turned five times. This protocol is set up to ensure that pathogen levels are low at the time of compost application. It may take longer to kill pathogens in passively aerated windrows than in-vessel or turned piles. Composts from feedstocks with potentially dangerous pathogens will be safer than the original source materials, but caution should still be exercised. It should not be topdressed onto crops that are directly used for human consumption, and composters and applicators need to take precautions for their own health, like wearing masks and protective clothing.

Figure 13.3.(Left) Turning a compost windrow at a commercial facility.Photo by Alison Jack.

Figure 13.4.(Right) An example of a belowground composting pit, often used by small farmers in tropical countries and when the soil is well-drained.

The Curing Stage

Following high-temperature composting, the pile should be left to cure for about one to three months. Usually, this is done once pile temperatures cool to 105°F and high temperatures don’t recur following turning. Curing is especially needed if the active (hot) process is short or poorly managed. There is a reduced need to turn the pile during curing because the phase of maximum decomposition is over and there is significantly less need for rapid oxygen entry into the pile’s center when the decomposition rate is slow. However, the pile may still need turning during the curing stage if it is very large or didn’t really finish composting. Determining when compost is finished is sometimes difficult, but if it reheats, it is not finished. (The Solvita^® test measures carbon dioxide losses from compost as a way to determine compost maturity.) Curing the pile furthers aerobic decomposition of resistant chemicals and larger particles. Common beneficial soil organisms populate the pile during curing, the pH becomes closer to neutral, ammonium is converted to nitrate, and soluble salts are leached out if the pile is outside and sufficient precipitation occurs. Be sure to maintain water content at the moisture-holding capacity (around 50% or less during curing) to ensure that active populations of beneficial organisms develop. It is thought that the processes that occur during the early curing stage give compost some of its disease-suppressing qualities. On the other hand, beneficial organisms require sources of food to sustain them. Thus, if composts are allowed to cure for too long, which can deplete all the available food sources, disease suppression qualities may decrease and eventually be lost.