Making and Using Composts
Composting: An Ancient Practice for Sustainable Agriculture
The natural decomposition of organic materials is a common occurrence in forests and fields everywhere. Composting is the technique and science of merging available organic wastes to decompose and form a consistent and stable end product. Composts serve as excellent organic soil amendments. Composting reduces volume, stabilizes soluble nutrients, and accelerates the creation of humus. A wide range of organic materials, such as manure, crop residues, grass clippings, leaves, sawdust, and numerous kitchen wastes, can be composted.
The microorganisms that contribute significantly to rapid composting thrive at high temperatures with abundant oxygen and moisture. These compost-adapted organisms span the entire spectrum of warm, or mesophilic (up to 110°F), and hot, or thermophilic (from 110° up to 130°F and even higher), conditions. Temperatures exceeding 160°F can occur in compost piles, aiding in the eradication of weed seeds and disease organisms, but this excessive heat usually decelerates the process, as it may cause extreme drying and initiate a die-off of all but the most heat-tolerant organisms. At temperatures below 110°F, the more abundant mesophilic organisms dominate and the composting rate slows down again, particularly as it falls towards ambient temperatures, a process referred to as “curing.” The composting process is hindered by anything that obstructs good aeration or the maintenance of sufficiently high temperatures and adequate moisture.
The composting of farm wastes and organic residues from outside the farm has become a common practice.
Composting: Techniques and Considerations for Sustainable Farming
Farmers near urban and suburban areas can generate some income by accepting and composting lawn and garden wastes. They may impose fees for accepting the wastes and for selling the resulting compost. Some farmers, particularly those without animals or perennial forage crops that contribute to increasing organic matter, may wish to use the compost as a source of organic matter for their own soils.
Creating Composts: Moisture
The moisture content of a compost pile is crucial. If the materials mat and rainwater cannot easily drain through the pile, it may not remain aerobic in a humid climate zone. Conversely, if composting is conducted inside a barn or under dry climatic conditions, the pile may not be moist enough to allow microorganisms to function effectively. 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’s advisable to moisten the pile initially if dry materials are used. However, if a high-nitrogen material like liquid manure is used, there will likely be enough moisture to initiate the composting process. The ideal moisture content of composting material is about 40% to 60%, roughly 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 desire.
A Sample Compost Recipe
Begin 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 achieve 60% moisture and a C:N of 30:1 is: 100 lbs of grass, 130 lbs of leaves, and 80 lbs of food scraps.
Types of Starting Materials
The combined organic materials used should have ample carbon and nitrogen available for the microorganisms to utilize. High-nitrogen materials, such as chicken manure, can be mixed with high-carbon materials like hay, straw, leaves, or sawdust. Compost piles are often constructed 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 readily if the average C:N ratio of the materials is about 25–40 parts carbon for every part nitrogen.
There are too many different types of materials that you might work with to give specific advice 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”. 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. 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
Optimizing Composting: Techniques and Considerations
A compost pile or windrow is a large, naturally convective structure—akin to numerous chimneys situated side by side. Oxygen permeates the pile as carbon dioxide, moisture, and heat ascend from it. The materials must be arranged in a manner that allows oxygen from the air to circulate freely. Conversely, it’s also crucial 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 required to ensure that sufficient air can penetrate the pile. Sawdust, dry leaves, hay, and wood shavings are commonly 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. 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
Turning the composting residues exposes all the materials to the high-temperature conditions at the center of the pile, and heat convection further exposes the upper reaches of the pile. 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. 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 accelerates 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 breathable 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 when turning in cold, windy weather if the pile is warm, for it may never reheat
The Curing Stage
After the high-temperature composting phase, the pile should be left to cure for approximately one to three months. This is typically done once the pile temperatures cool down to 105°F and high temperatures do not recur after the next turning. Curing is particularly necessary if the active (hot) process is brief or poorly managed. There is less need to turn the pile during curing because the phase of maximum decomposition has ended and there is significantly less need for rapid oxygen entry into the center of the pile 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 fully finish composting—determining when compost is finished can sometimes be challenging, but if it reheats, it is not finished—or is soaked by rain. Curing the pile further promotes 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. It’s important 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 believed 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.
Other Composting Techniques
High-temperature piles account for most composting in the U.S., but other methods are also used. Instead of making piles, small farmers in developing countries often dig pits for composting, especially in dry and hot climates. The pits can be covered with soil material to prevent animals from getting into them, and they retain moisture in the compost material better.
Vermicomposting involves the use of earthworms—typically red worms—to perform the decomposition process. The method is, in a way, still mostly bacteria-based, but the process occurs in the gut of the worm. The end product is worm casts, coated with mucus consisting of polysaccharides that make them into somewhat stable aggregates. The system requires bedding material—like newspaper strips, cardboard, hay, and similar materials—that mimics the decaying dried leaves that worms find in their natural habitat. The process is fast and efficient—worms can process half their weight in organic material in one day. The final product has an attractive feel and smell and is appealing to consumers.
Vermicomposting is often used to process kitchen scraps and can be done indoors in small bins. Recently, vermicomposting methods have been developed for large commercial operations. Two main approaches are used, using windrows or raised beds. With windrows, new materials are added on one side of the bed, and the other side is harvested for compost after about sixty days. With the raised-bed or container system—preferred for indoor operations in colder climates—the worms are fed at the top of the beds and the castings are removed at the bottom. Some vermicomposting operations are connected with livestock farms to process manure for export of excess nutrients off the farm as a value-added product.
Using Composts
Finished composts generally provide only low relative amounts of readily available nutrients. During composting, much of the nitrogen is converted into more stable organic forms, although potassium and phosphorus availability remains unchanged. However, it should be kept in mind that composts can vary significantly and some that have matured well may have high levels of nitrate. Even though most composts don’t supply a large amount of available nitrogen per ton
NUTRIGLOW
Specialization: Nutrition and Public Health Expert