Building Soils for Better Crops, Third Edition

Making Composts



The male brush turkey of Australia gathers leaves, small branches, moss, and other litter and builds a mound about 3 feet high and 5 feet across. It then digs holes into the mound repeatedly and refills them—helping to fragment and mix the debris. Finally, the pile is covered with a layer of sticks and twigs. The female lays her eggs in a hole dug into the pile, which heats up to close to 100°F around the eggs while the outside can be around 65°F. The heat of the composting process frees the birds from having to sit on the eggs to incubate them.

—R.S. SEYMOUR (1991)


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% to 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.

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

Types of Starting Materials

The combined organic materials used should have lots of carbon and nitrogen available for the microorganisms to use. High-nitrogen materials, such as chicken manure, can be mixed with high-carbon materials like hay, straw, leaves, or sawdust. Compost piles are often built by alternating layers of these materials. Turning the pile mixes the materials. Manure mixed with sawdust or wood chips used for bedding can be composted as is. Composting occurs most easily if the average C:N ratio of the materials is about 25–40 parts carbon for every part nitrogen (see chapter 9 for a discussion of C:N ratios).

There are too many different types of materials that you might work with to give blanket recommendations about how much of each to mix to get the moisture content and the C:N into reasonable ranges so the process can get off to a good start. One example is given in the box “A Sample Compost Recipe.”


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 5.0% N)

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

—T. RICHARD (1996b)

Cornell University’s website for composting issues features formulas to help you estimate the proportions of the specific materials you might want to use in the compost pile. 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 (low in nitrogen). To balance your pile, you may need to add other materials or change the ratios used. The problems can be remedied by adding dry sawdust or wood chips in the first two cases or nitrogen fertilizer in the third. If a pile is too dry, you can add water with a hose or sprinkler system.

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. When residues contain high amounts of lignin, it means that the effective C:N can be quite a bit lower than indicated by using total carbon in the calculation (table 13.1). For some materials, there is little difference between the C:N calculated with total carbon and calculated with only biodegradable carbon.

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 result in compost containing chemicals that can harm crops.

Wood chips or bark is sometimes used as a bulking agent to provide a “skeleton” for good aeration. These materials may be recycled by shaking the finished compost out of the bulking material, which can then be used for a few more composting cycles.

Table 13.1: Total vs. Biodegradable Carbon and Estimated C:N Ratios
Material% CarbonC:N% CarbonC:N% Lignin% Cell Wall% Nitrogen
Wheat straw5188345823950.58
Poultry manure43104292384.51
Maple wood chips5051444513320.97
Source: T. Richard (1996a).


It is possible to compost dead farm animals, which are sometimes a nuisance to get rid of. Chickens and even dead cows have been successfully composted. Cam Tabb, a West Virginia beef and crop farmer, starts the process for large animals by laying the carcass, which has been in the open for one day, on a 3 to 4-foot bed of sawdust and horse manure—a good insulating material for the foundation. Then he covers it with 3 to 4 feet of sawdust and horse manure. He turns the pile after three or four months, although it can be left for months without turning (the Cornell Waste Management Institute recommends letting it sit for four to six months). After turning, he places more sawdust and horse manure on the surface to cover any exposed materials from the decomposing animal. Other materials with lots of available energy for organisms to use to help decomposition, such as corn silage, can also be used for the base or pile covering. The pile should be shaped as a pyramid so as to shed water, and when the animal is placed in the pile, there should be at least 2 feet of base material between the animal and the outside of the pile.

Pile Size

A compost pile or windrow (figure 13.1) is a large, natural convective structure—something like many chimneys all next to each other. Oxygen moves into the pile as carbon dioxide, moisture, and heat rise from 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 escape 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. Sawdust, dry leaves, hay, and wood shavings are frequently used as bulking agents. Tree branches need to be “chipped” and hay chopped so that these ingredients don’t mat and slow composting. Composting will take longer when large particles are used, especially those resistant to decay.

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 to 5 feet tall and about 6 to 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.

Turning the Pile

Figure 13.2 Compost pile dimensions and turning techniques.
Figure 13.2 Compost pile dimensions and turning techniques.

Turning the composting residues exposes all the materials to the 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. 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 a year or longer to complete, especially if it has settled down too densely. 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 remix and throw out piles.

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 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; break up clumps; and make the compost more uniform later in the process before use or marketing. Use caution turning in cold, windy weather if the pile is warm, for it may never reheat.

The Curing Stage

Figure 13.3. Turning a compost pile at a commercial facility.
Figure 13.3. Turning a compost pile at a commercial facility. Photo by Alison Jack.

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—or is soaked by rain.) 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 process 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—depleting all the available food sources—disease suppression qualities may decrease and eventually be lost.

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