How does compost work?

How Composting Works

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Photo courtesy Karim Nice
Home composting is an ideal way to reduce solid waste. See more pictures of green living.

Americans generate abo­ut 210 million tons (231 million short tons) of trash, or solid waste, each year. Most of this trash (57 percent) gets placed in municipal landfills. About 56 million tons (27 percent) is recovered through either recycling, in the case of glass, paper products, plastic or metals, or through composting, in the case of yard waste. Composting is a method for treating solid waste in which organic material is broken down by microorganisms in the presence of oxygen to a point where it can be safely stored, handled and applied to the environment. Composting is an essential part of reducing household wastes. It can be done inexpensively by every household and produces a product — finished compost or humus — that can benefit the environment as a natural fertilizer for gardening and farming.

Trash Audit
How much trash do you make in a year? What kind of things do you throw out? How much can be reduced by recycling or composting? To answer these questions, perform a trash audit.

In this article, we’ll look at what happens when solid waste is composted, how you can make your own compost, why you benefit from composting, and how you can make a benchtop compost column to study composting in a classroom laboratory or science-fair setting.

Turn Your Spoils into Soil…COMPOST !

What is Composting? Composting is a biological process during which naturally occurring microorganisms, bacteria and insects break down organic materials such as leaves, grass clippings and certain kitchen scraps into a soil-like product called compost. It is a form of recycling, a natural way of returning needed nutrients to the soil.

Why Compost? By composting kitchen scraps and yard trimmings at home, you can conserve valuable landfill space normally used to dispose of this material and help reduce air emissions from the incinerator plants that burn garbage. In fact, if you compost on a continual basis, the volume of garbage you generate can be reduced by as much as 25%! Composting is practical, convenient and can be easier and less expensive than bagging these wastes and taking them to the landfill or transfer station.

Benefits of Using Compost. By using compost you return organic matter and nutrients to the soil in a form readily useable to plants. Organic matter improves plant growth by helping to break heavy clay soils into a better texture, by adding water and nutrient-holding capacity to sandy soils, and by adding essential nutrients to any soil. Improving your soil is the first step toward improving the health of your plants. Healthy plants help clean our air and conserve our soil. If you have a garden, a lawn, shrubs, or even planter boxes, you have a use for compost.

How to Compost. Composting is easy. You can compost in your yard by saving yard trimmings (leaves, grass clippings, and garden debris) and certain kitchen and meal scraps by preparing them properly and placing them in a compost pile. Just follow these easy, basic guidelines:

Step 1. Choose the right materials. Anything that was once alive will compost, but not everything belongs in a compost pile. In general, do not compost foods containing animal fats (such as meat, bones, cheese, grease and oils); plants infected with disease, invasive weeds, weeds that have gone to seed, or dog and cat feces. Yard trimmings, like leaves, grass clippings, prunings, garden debris, and most kitchen scraps make excellent compost.

Do Compost Do Not Compost
  • Fruit and vegetable scraps
  • Egg, peanut and nut shells
  • Stalks, stems and vines
  • Coffee grounds and filters, tea bags
  • Bark
  • Wood ashes (in limited amounts)
  • Manure (horse, cow, chicken & rabbit)
  • Garden clippings
  • Leaves
  • Grass clippings
  • Apple cores and citrus rinds
  • Meat and fat
  • Fish
  • Poultry
  • Bones
  • Dairy products
  • Plastic or synthetic fibers
  • Diseased plants
  • vegetable oils
  • Dog and cat feces
  • Weeds which have gone to seed
  • Invasive weeds

Step 2. Select and prepare a site. First, choose a place in your yard or garden to start a compost pile. It doesn’t really matter if it is in the sun or shade, but a place that receives a little of both during the day would be ideal. What’s more important is that it is somewhere convenient to use. Then, decide how you wish to compost. There are many different ways to prepare a compost pile, and it’s really personal preference which one you choose. You can choose to:

Use no enclosure at all. Simply pile the materials up, keeping them in a fairly dense heap.

Build your own compost bin. Enclosed bins will typically have a neat appearance, help keep out pests, and hold in heat and moisture. You can assemble wooden stakes and chicken wire or hardware cloth into a simple round enclosure; construct a wooden bin out of salvaged lumber or old pallets; fashion a three-sided enclosure by placing cinder blocks on top of each other, leaving the front open; or even drill holes in the bottom and sides of a garbage can.

Purchase a compost bin. Order a pre-built compost bin from a garden center, mail order garden catalogue or home improvement/hardware store. Also, check with your local recycling coordinator or Public Works Department to see if they sponsor a bin distribution program.

Step 3. Prepare the compost materials and build a pile.

Prepare the materials. Begin by cutting or shredding the ingredients into small pieces. This will help them decompose faster. Although shredding leaves is not necessary, it will shorten the time it takes for them to compost. The same is true for kitchen scraps and garden waste.

Build the pile. Put a layer of course material, like wood chips, or small twigs on the bottom to facilitate drainage and aeration. Then add materials in layers 2-6 inches thick alternating between “greens” (food scraps, grass clippings, manure) and “browns” (leaves, straw, woody materials) to help balance the proportion of carbon and nitrogen. Water and mix well after every two layers. If you don’t have “greens” and “browns” available at the same time, build the entire pile out of “browns” and then add the “greens” as they become available. When adding food scraps, bury them completely in the center of the pile. Add a shovel full of garden soil periodically. Save a few bags of autumn leaves to use during the following spring and summer. Ideally, the pile should measure at least 3 feet high by 3 feet wide by 3 feet long.

Keep it moist. The pile should be kept moist, but not soggy, about the consistency of a wrung-out sponge. If it’s not moist, it won’t decompose.

Give it air. Oxygen is essential to the organisms breaking down the materials. Fluff the pile with a pitchfork or hoe every time you add material. If you can manage to do a more aggressive turning in the spring and fall (so that the pile is turned completely inside out and upside down), you can usually get finished compost in one year. Less frequent turning results in slower composting.

Observe your pile. As composting takes place, heat is generated. Don’t be surprised if you see steam rising from the pile, especially when it is turned. This means the conditions for decomposition are at their best. If your compost pile is properly prepared, contains no animal fats and is turned periodically, it will not attract pests or create odors.

Step 4. Test whether the compost is ready… Decomposition will be complete anywhere from two weeks to two years depending on the materials used, the size of the pile, and how often it is turned. Compost is ready when it has cooled, turned a rich brown color, and has decomposed into small soil-like particles.

Step 5. Use the compost. About one month before planting, apply 1-3 inches of the finished compost and work it into the top four inches of soil. Compost can also be used in the garden as a top dressing or mulch throughout the summer. Screened through a ½” sieve, compost can be used to create a potting soil by combining equal parts of compost, sand and loam. Large particles can be put back in the compost pile. Lawns can benefit from a ¼” application of compost which helps stimulate biological activity in the turf. If you have more compost than you can use, give it to a friend or neighbor!

Troubleshooting. In addition to the information in the table below, please refer to the Building Blocks Page for a more detailed list of principles to follow that make a good home compost pile.

Symptom Probable Cause Suggested Remedy
The pile has a bad odor Not enough air or too wet Turn pile thoroughly
The center of the pile is dry Not enough water Moisten materials while turning pile
The pile is damp and warm in the middle, but nowhere else Pile is too small Collect more material and mix old material into a new pile
The pile is sweet smelling, but still will not heat up Lack of nitrogen Mix in a nitrogen source such as fresh grass clippings, fresh manure, bloodmeal, or a commercial fertilizer high in nitrogen

Mulching. Before they decompose, chipped woody debris and leaves make excellent mulch or garden path material helping to keep the soil weed-free and moist. As they decompose, these same materials will enrich the soil. Simply place them up to 1″ deep beneath the plants, but not touching the stems. Grass clippings should be dried before using as mulch, or simply leave clippings on the lawn where they will return nutrients to the soil. Do not mulch with grass clippings that have been treated with herbicides, or it may harm your plants. Composting them first, however, will break down most commonly used lawn herbicides.

Composting with worms. Don’t have a yard? No problem, you can compost indoors using earthworms! For information on worm composting, (vermicomposting), please see our Composting and Organics Resource Page to consult one of the references devoted to this subject.

Video Download “Home Composting – Turning Your Spoils to Soil”

Full Color Brochure “Composting Has A-PEEL”

Resources. There is a myriad of published information on home composting, grass cycling, worm composting, and organics recycling. Please visit our Composting and Organics Resource Page for suggestions on books, videos and links to related websites. While you’re there, read about DEEP’s video entitled “Home Composting – Turning Your Spoils to Soil”, now also available for , or in VHS video from the DEEP Store.

Composting

Content Last Updated on August 6, 2019

Printable Version

Compost: Turn food waste into soil nutrients!

Compost is an important soil amendment made of decomposed plant matter including food scraps. You can make right it in your backyard!

With the right recipe, your compost heap will not omit bad odors, will lighten the load (and cost) of your trash, and will greatly reduce greenhouse gas emissions from landfills.

Adding compost to soil helps to restore the organic matter content allowing for greater moisture and nutrient retention and providing necessary food for essential microorganisms that live in healthy soil. Read The Living Soil – Microorganisms to learn more about soil microbes.

Composting Tips
Basic Steps for Composting
Troubleshooting the Compost Pile

Composting Tips

Develop your recipe
A compost pile built with the right recipe of ingredients is like an open invitation to all of the critters and microorganism to join your compost party. If your recipe is right, they’ll come and do the work for you, and rather quickly at that! A pile built with the wrong ingredients will result in no one coming to your compost party, or worse, will result in only stinky anaerobes or large mammals following the smell!

Ingredients
There are two categories of materials that make up the ingredient list: Carbon materials and nitrogen materials. All matter contains carbon and nitrogen but for simplicity think of it this way: green materials (mostly nitrogen) and brown materials (mostly carbon). A 3:1 ratio of carbon to nitrogen is ideal.

  • Green materials are those that are still living or wet. These are your nitrogen materials. This includes all kitchen scraps, vegetative garden debris, grey water, apples from the field, anything rotting (anything at all), raw manure, weeds, and more. (I prefer to only add young weeds, or those that have not gone to seed. I never add any invasive or aggressive weeds to the compost, especially not those which spread by root).
  • Brown materials are those that are dead or quite dry. These are your carbon materials. This includes, dried and crushed leaves, sawdust, hay, straw, cardboard, paper, stalks or thick stems (broccoli, corn, flower stalks that have dried…), and more.


Stockpile ingredients
Have ingredients on hand. Always keep a supply of dry carbon materials nearby to the compost, in a separate bin or in a lidded trash barrel.

Shape the pile
If your compost pile is not covered, shaping the pile can help to manage the moisture content. During dry times try for a flat or slightly concave shaped pile. This will collect the water needed to aid in decomposition. During wet times go for a hill-shape to allow excess water to run off.



Turning
You do not need to turn your compost until you turn it out of the bin to use in the garden. If the pile is built using the right recipe, all of the critters and worms will do it for you during the decomposition process. One exception is when the pile becomes to wet or anaerobic. Then, a good turn and maybe even a re-stack of the pile may be in order to give it some air. See the troubleshooting section below for more tips.

Timeline
If you are composting primarily kitchen scraps (and you eat veggies regularly) it should take about a year to fill a bin. Its okay to add to a bin for many years but if you’d like to harvest from your compost you should plan to cap it at some point and start a new one. A capped pile means you stop adding to it in order to allow the compost to ‘finish’.

How do I know when my compost is ready? You will see the contents shrink to 1/3 to 1/2 it’s size as the microorganisms do their work decomposing. When the pile is finished it will stop shrinking. At that point you can turn the pile out of the bin and find finished compost therein. Any remaining brown matter that has not decomposed can be added in to the new pile as brown matter, or use it as mulch on your garden.

Usually, a capped pile can sit undisturbed for 2-3 months during the warm season before turning the entire pile out of the bin.

Sifting compost If there are still remains of carbon material like hay or straw in your otherwise finished compost, sift out the carbon matter that has not yet decomposed. Use it again in the next pile, or use it as mulch in the garden.

Basic Steps for Composting

The recommended steps for building a healthy and active backyard compost pile below are intended for a 4’x 4’x 4’ bin (approximately). There are many easy-to-build design plans available online with a quick search for ‘compost bins’. You don’t need a bin but it will certainly help to contain the pile, to control moisture, and to keep critters out.

1) Start your pile from the bottom up. Start a pile by layering about 12 inches of brown, dry, carbon material spread evenly across the bottom of the pile, edge to edge in a criss-crossed pattern. Hollow stems such as those from cornstalks, garlic stalks, or from perennials such as phlox, sylphium, or jerusalem artichoke work very well. This bottom layer is providing a well-aerated structure for a year’s worth of compost to sit on top of.

2) Create a bio-filter to keep down smells. Layer several inches of hay, or any carbon material, around the perimeter of the pile (like a doughnut), leaving the center bare for your compost deposit (like a doughnut hole). This perimeter hay will act as an insulator and a bio-filter around the edges of the pile aiding to keep the smell down making it less attractive to rodents and other mammals, and more attractive to people! The center of the pile, the ‘doughnut hole’, is the high-activity spot in a compost heap.

3) Make a deposit. Add 6-12 inches of mixed food scraps (nitrogen materials) inside the ‘doughnut hole’ to be level with the perimeter of carbon materials. If the deposit you are adding is very wet, layer in a bit of carbon material to add air space.

4) Cover the pile. Build a top bio-filter by layering 4-6 inches of loose carbon material on top of the deposit, and edge to edge in the bin.

Repeat steps 1-4 until the bin is full. It’s that easy!

Managing the compost heap
A pile may not become very active until it has enough mass. You’ll know when that is by the heat being created in the center of the pile, usually after a few deposits or when the pile is about 2 feet high. At that point you should have an idea of the volume of food scraps produced each week and of how much time you can afford to manage your pile. Having a regular system in place will afford the best results. Here are two systems that work for most people.

Pay as you go composting: This style is for those who make small daily or weekly deposits. Dig into the center of the pile to create a new center hole with each deposit. Using a spade fork, pull the top layer of carbon materials to the sides of the bin revealing a partially decomposed center. Mix those contents in with the carbon matter around the perimeter. Make your deposit. Repeat steps 3-5 in this fashion.

Batch composting: This style is for those who produce a lot of veggie scraps and like to save em’ up for a monthly deposit. Follow the same instructions for ‘pay as you go’ but create a larger center hole for your deposit and have extra carbon materials at the ready to layer in, and to build up the bio-filter around the outer perimeter.

Capping the pile
When the bin is full, cap it with a final 4-6” layer of carbon materials. Leave this finished pile to cure for about 3 months or until it has stopped shrinking. Then, dig in and harvest! The best compost will be at the bottom. If anything is not decomposed, add it to your active pile. Start a new active pile in another bin.

Troubleshooting the Compost Pile

  • If your pile is too wet it will stink. Add brown material (carbon) to fix it. Dig into the center of the pile and assess the heat, and moisture content. If the entire pile is cold and very wet you may need to rebuild the pile from scratch to give it proper air circulation. Consider adding a cover to your bin to keep future rain out. Water is necessary for compost but too much will cool it and make it uninhabitable for microorganisms.
  • If your pile is too dry it will not be active. Add green materials (nitrogen) to fix it. Dig into the center of the pile and add your nitrogen materials, then cover with some of the carbon materials you just pulled to the side. You may need to water the pile if it has been a very dry period.
  • If the activity in your pile seems too slow, sprinkle a very thin layer of soil, or slightly more compost or manure edge to edge before you make a deposit. Healthy soil, compost, and manure are compost activators, containing potentially billions of microorganisms.
  • To jump start activity in an old pile, dig in to the center of the pile and add some raw manure and green matter. Cover it back up with carbon materials. You may need to rebuild the pile from scratch to give it proper air circulation.

Gardening Tips Gardening Soil Life compost food waste microorganisms soil health

How does organic waste convert to compost?

Composting is a way to treat solid waste so that microorganisms break down the organic material, helping along the natural process of decay until it can be safely handled, stored and applied to the environment.

The composting process requires organic waste, such as leaves, grass, fruit and vegetable scraps, soil (which contains microorganisms), water and oxygen. The microorganisms eat the organic waste, breaking it down into its simplest components. The humus (finished compost) they produce is rich with fiber and inorganic nutrients, such as phosphorus, potassium and nitrogen, and it makes a natural fertilizer that is beneficial to the environment. In order to produce this humus, the microorganisms need water, as do all living things, and oxygen for aerobic respiration. The microorganisms access this oxygen when you turn over the compost every day or two. In the respiration process they give off heat (temperatures of up to 150 degree Fahrenheit or 66 degrees Celsius) and carbon dioxide. If you regularly water and turn the compost in your compost bin or pile, the compost can completely decompose in just two to three weeks. Otherwise it can take months to decompose.

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Aside from regular turning and watering (so that the mixture is moist but not too wet), your compost needs enough soil (so it has enough microorganisms) and the right ratio of carbon to nitrogen (about 30:1). The smaller the pieces in your compost bin, the faster they will break down.

The foodweb, or organization of organisms, within your compost pile helps to increase the efficiency of the decomposing process. The foodweb includes fungi and bacteria that break down the organic matter in your trash; protozoa, nematoids (small worms) and mites that feed on the fungi and bacteria; and invertebrates, such as beetles, sowbugs and millipedes that feed on the protozoa, nematoids and mites.

How to Make Compost

Some common misconceptions of home composting are that it’s too complicated, it’ll smell funny, and it’s messy. These are all true if you compost the wrong way. Composting the right way is a very simple approach: Simply layer organic materials and a dash of soil to create a concoction that turns into humus (the best soil builder around!). You can then improve your flower garden with compost, top dress your lawn, feed your growing veggies, and more. With these simple steps on how to compost, you’ll have all of the bragging rights of a pro!

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Types of Composting

Before you start piling on, recognize that there are two types of composting: cold and hot. Cold composting is as simple as collecting yard waste or taking out the organic materials in your trash (such as fruit and vegetable peels, coffee grounds and filters, and eggshells) and then corralling them in a pile or bin. Over the course of a year or so, the material will decompose.

Hot composting is for the more serious gardener but a faster process—you’ll get compost in one to three months during warm weather. Four ingredients are required for fast-cooking hot compost: nitrogen, carbon, air, and water. Together, these items feed microorganisms, which speed up the process of decay. In spring or fall when garden waste is plentiful, you can mix one big batch of compost and then start a second one while the first “cooks.”

Related: How to Build a Compost Bin

Vermicompost is made via worm composting. When worms eat your food scraps, they release castings, which are rich in nitrogen. You can’t use just any old worms for this, however—you need redworms (also called “red wigglers”). Worms for composting can be purchased inexpensively online or at a garden supplier.

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What to Compost

Composting is a great way to use the things in your refrigerator that you didn’t get to, therefore eliminating waste. Keeping a container in your kitchen, like this chic white ceramic compost bucket from World Market, is an easy way to accumulate your composting materials. If you don’t want to buy one, you can make your own indoor or outdoor homemade compost bin. Collect these materials to start off your compost pile right:

  • Fruit scraps
  • Vegetable scraps
  • Coffee grounds
  • Eggshells
  • Grass and plant clippings
  • Dry leaves
  • Finely chopped wood and bark chips
  • Shredded newspaper
  • Straw
  • Sawdust from untreated wood

Editor’s Tip: Think twice before adding onions and garlic to your homemade compost pile. It is believed that these vegetables repel earthworms, which are a vital part of your garden.

What NOT to Compost

Not only will these items not work as well in your garden, but they can make your compost smell and attract animals and pests. Avoid these items for a successful compost pile:

  • Anything containing meat, oil, fat, or grease
  • Diseased plant materials
  • Sawdust or chips from pressure-treated wood
  • Dog or cat feces
  • Weeds that go to seed
  • Dairy products

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Step 1: Combine Green and Brown Materials

To make your own hot-compost heap, wait until you have enough materials to make a pile at least 3 feet deep. You are going to want to combine your wet, green items with your dry, brown items. “Brown” materials include dried plant materials; fallen leaves; shredded tree branches, cardboard, or newspaper; hay or straw; and wood shavings, which add carbon. “Green” materials include kitchen scraps and coffee grounds, animal manures (not from dogs or cats), and fresh plant and grass trimmings, which add nitrogen. For best results, start building your compost pile by mixing three parts brown with one part green materials. If your compost pile looks too wet and smells, add more brown items or aerate more often. If you see it looks extremely brown and dry, add green items and water to make it slightly moist.

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Step 2: Water Your Pile

Sprinkle water over the pile regularly so it has the consistency of a damp sponge. Don’t add too much water, otherwise, the microorganisms in your pile will become waterlogged and drown. If this happens, your pile will rot instead of compost. Monitor the temperature of your pile with a thermometer to be sure the materials are properly decomposing. Or, simply reach into the middle of the pile with your hand. Your compost pile should feel warm.

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Step 3: Stir Up Your Pile

During the growing season, you should provide the pile with oxygen by turning it once a week with a garden fork. The best time to turn the compost is when the center of the pile feels warm or when a thermometer reads between 130 and 150 degrees F. Stirring up the pile will help it cook faster and prevents material from becoming matted down and developing an odor. At this point, the layers have served their purpose of creating equal amounts of green and brown materials throughout the pile, so stir thoroughly.

Editor’s Tip: In addition to aerating regularly, chop and shred raw ingredients into smaller sizes to speed up the composting process.

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Step 4: Feed Your Garden

When the compost no longer gives off heat and becomes dry, brown, and crumbly, it’s fully cooked and ready to feed to the garden. Add about 4 to 6 inches of compost to your flower beds and into your pots at the beginning of each planting season.

Some gardeners make what’s known as compost tea with some of their finished compost. This involves allowing fully formed compost to “steep” in water for several days, then straining it to use as a homemade liquid fertilizer.

Every gardener is different, so it’s up to you to decide which composting method best fits your lifestyle. Fortunately, no matter which route you choose, compost is incredibly easy and environmentally friendly. Plus, it’s a treat for your garden. With just a few kitchen scraps and some patience, you’ll have the happiest garden possible.

Related: How to Make a Soil Sifter Box for Healthy Compost

  • By BH&G Garden Editors

Compost

Compost is a finely divided, loose material consisting of decomposed organic matter. It is primarily used as a plant nutrient and soil conditioner to stimulate crop growth. Although many people associate compost production with small garden compost piles that are tended with a shovel, most compost is produced in large municipal, industrial, or agricultural facilities using mechanized equipment.

Background

The expression “older than dirt” certainly applies to compost. Nature has been producing compost for millions of years as part of the cycle of life and death on Earth. The first human use of animal manure, a raw form of compost, was in about 3,000 B.C. in Egypt when it was spread directly on the fields as a fertilizer. Later, manure was mixed with dirty stable straw and other refuse and allowed to sit in piles until it was needed. Rain kept the piles wet and aided the decomposition process, producing a rich compost.

The Greeks and Romans knew the value of compost to boost crop production and even used the warmth of decomposing compost to produce summer vegetables in winter. Christian monasteries kept the art of composting alive in Europe after the fall of the Roman Empire, and by about 1200 compost was again being used by many farmers. Shakespeare mentions it in several of his plays written in the early 1600s.

In the United States, Presidents George Washington and Thomas Jefferson were prominent landowners during the late-1700s and early-1800s. When they were not involved with affairs of state, they both spent much of their time trying innovative farming practices, including experiments with various composting methods and materials. As years of successive crops depleted the nutrients in the soil on the East Coast, the practice of composting became widespread. This trend continued until the early 1900s when it was estimated that 90% of the fertilizer used in the United States came from compost.

That all changed in 1913, when a German company began producing synthetic nitrogen compounds, including fertilizers. These new chemical fertilizers could be produced less expensively than messy animal manure compost, and the farmyard compost pile quickly became a thing of the past. By 1950, it was estimated that only 1% of the fertilizer used in the United States was derived from compost.

One notable exception to this trend was the work started in 1942 by J.I. Rodale, a noted pioneer in the development of the organic method of farming. Rodale was one of the first to see the hazards of relying on synthetic fertilizers and the benefits of using compost derived from natural sources. Composting got a short-lived boost during the environmentally conscious era of the 1960s, but it wasn’t until the 1980s when it became a big business. This surge wasn’t the result of a renewed awareness of the positive aspects of compost, but rather a growing concern over the negative aspects of refuse. In short, in our efforts to get rid of our refuse, we were polluting our air, poisoning our rivers, and quite literally burying ourselves in it with our landfills.

In order to divert some of the municipal refuse away from landfills, several cities established recycling centers in the early 1970s where people could bring cans, bottles, and newspaper rather than throw them in the trash. This was followed by curbside recycling, where people could place these recyclable materials in separate containers for pickup in front of their houses. Finally, many cities added additional curbside containers for yard wastes to be composted. By 1992, almost 1,500 cities had yard waste composting facilities.

At the same time, tough new environmental laws mandated that industries could no longer simply dump their waste products onto the surrounding land or discharge them into nearby rivers. To meet these laws, many industries began their own recycling and composting programs. Environmental concerns also affected farmers, who were being blamed for the negative health effects that chemical fertilizers and pesticides had on humans and wildlife. As a result, many farmers decided to cut back or eliminate chemicals in favor of using compost.

Today, most compost is processed in large facilities designed to handle a specific type of raw material. Agricultural compost is usually produced and used on the same farm that generated the raw materials. Industrial compost may be bagged and sold to individual buyers, or the raw materials may be sold in bulk to other composting facilities. Municipal yard waste compost is usually produced in facilities operated by the city or the refuse collection company and is sold to local landscaping companies and garden centers.

Raw Materials

Technically, compost may be made from any organic material. That is, it may be made from any part of an organism, plant or animal, that contains carbon. Compost also requires a source of nitrogen, oxygen, and water, plus small amounts of a variety of elements usually found in organic material, including phosphorus, copper, potassium, calcium, and others.

In order for the organic materials to combine with the other materials and decompose into compost, several living organisms and microorganisms are needed. These include sowbugs, which help digest the materials and transport bacteria; earthworms, which aerate the materials with their tunnels; a variety of fungi, which help digest decay-resistant cellulose; mold-like bacteria called actinomycetes, which attack raw plant tissues; and many others.

The most common raw materials used to make compost are yard wastes such as grass clippings, leaves, weeds, and small prunings from shrubs and trees. Most home garden compost piles and municipal compost facilities use yard wastes exclusively because of the large volume of materials available.

Industrial compost facilities tend to use waste materials generated within a particular plant or region. For example, sugar beet pulp is mixed with other materials to make compost in an area where sugar refineries operate. Spent hops and grain from breweries also make excellent compost materials. Other materials include sawdust and wood chips from lumber mills, fish waste from canneries, and dried blood and pulverized animal bones from slaughterhouses.

Agricultural compost facilities use materials readily available on nearby farms. These include animal manure, used stable straw, spoiled fruits and vegetables, field refuse, vineyard and orchard prunings, rotted hay, and other agricultural waste products.

Some of the more unusual raw materials used to make compost include seaweed, chicken feathers, peanut shells, and hair clippings.

The Manufacturing
Process

The production of compost is both a mechanical and a biological process. The raw materials must first be separated, collected, and shredded by mechanical means before the biological decomposition process can begin. In some cases, the decomposition process itself is aided by mechanical agitation or aeration of the materials. After decomposition, the finished compost is mechanically screened and bagged for distribution.

There are several methods for producing compost on a large scale. The methane digester method places the raw materials in a large, sealed container to exclude oxygen. The resulting oxygen-starved decomposition not only produces compost, but also methane gas, which can be used for cooking or heating. The aerated pile method places the raw materials in piles or trenches containing perforated pipes that circulate air. The resulting oxygen-rich decomposition produces a great amount of heat, which kills most harmful bacteria. The windrow method places the raw materials in long piles, called windrows, where they are allowed to decompose naturally over a period of several weeks or months. It is the least expensive method of all. Here is a typical sequence of operations used to convert municipal yard wastes into compost using the windrow method.

Separating

  • 1 Yard wastes are deposited in separate containers by homeowners, and the containers are placed at the curb for pickup on the regular refuse collection day. Homeowners are instructed that only certain yard wastes are acceptable for collection. These include grass clippings, leaves, weeds, and small prunings from shrubs and trees. Short pieces of tree limbs up to about 6 in (15 cm) in diameter are also acceptable. Homeowners are also instructed that certain other yard wastes are not acceptable. These include rocks, sod, animal excrement, and excessive amounts of dirt. Palm fronds are prohibited because the frond spikes do not decompose and carry a poison. Food scraps, fruits, and vegetables are also prohibited because they can attract rodents, carry unwanted seeds, and contribute to odors.
  • 2 The yard wastes are collected by separate refuse trucks and are transported to the processing center where they are dumped in piles. The piles are visually inspected, and any oversized or unacceptable materials are manually removed.

Grinding

  • 3 A large, wheeled machine called a front loader picks up material from the piles and dumps it into a tub grinder. The tub grinder has a stationary vertical cylindrical outer shell with a rotating cylindrical inner shell. As the material passes between the two shells, it is ground into smaller pieces and thoroughly mixed. The ground material falls out the bottom and through a screen where the larger pieces are screened out. The remaining material is transported by a conveyor belt to a holding pile.
  • 4 The larger pieces are sold to landscaping companies for use as mulch or ground-cover without further processing. The rest is loaded into large dump trucks and transported to the composting area where it is dumped in long rows, called windrows. Each row is about 6-10 ft (2-3 m) high and several hundred feet (m) long with a triangular cross section. A flat space about 10 ft (3 m) wide is left between each row to allow vehicles to move along the length.

Composting

  • 5 The composting area may cover several acres (hectares). After a windrow is laid in place, the material is dampened by a tank truck that moves along the row spraying water. The water aids in the composting process and helps minimize wind-blown dust.
  • 6 Every few weeks, a special machine straddles each windrow and moves along its length to turn and agitate the material. This breaks down the material into even smaller pieces and exposes it to oxygen, which aids in the decomposition process. After the windrow is turned, it is sprayed with water again. This process continues for two or three months. In hot, dry weather, the windrows may have to be watered more often. During decomposition, the internal temperature of the pile may reach 130° F (54° C), which helps kill many of the weed seeds that might be present.

Curing

  • 7 The raw compost is scooped up with a front loader and moved to a large conical pile where it is allowed to finish the decomposition process over a period of several weeks. This process is called curing and it allows the carbon and nitrogen in the compost to adjust to their final levels.

Screening

  • 8 After the compost has cured, it is scooped up with a front loader and dumped into the hopper of a rotary screen. This device consists of a large cylindrical screen rotating on an axis that is slightly inclined above the horizontal. The openings in Diogram depicting the commercial processing of yard waste into compost. the screen are about 0.5 in (1 cm) in diameter. The compost is fed into the raised end of the rotating screen from the hopper by a conveyor belt. As the compost tumbles its way down the length of the rotating screen, the smaller material falls through the screen and is moved to a storage pile by a conveyor belt. The larger material that cannot pass through the screen falls out the lower end of the cylinder and is either returned to the compost piles for further decomposition or is sold as wood chips.

Distributing

  • 9 Much of the finished compost is loaded into large dump trucks and sold in bulk to landscaping companies, municipalities, nurseries, and other commercial customers. Some of it is sealed in 40 lb (18 kg) plastic bags for retail sale to homeowners. Using the windrow method, a typical suburban yard waste processing facility can produce as much as 100,000 tons (91,000 metric tons) of compost a year.

Quality Control

Composting companies regularly have their finished compost tested to ensure it is free of harmful materials and contains the proper amounts of plant nutrients. The tests measure the size of the particles, moisture level, mineral content, carbon-to-nitrogen ratio, acidity, nutrient content, weed seed germination rate, and many other factors. For example, waste particles should be between 0.5-2 in (1.2-5 cm) in diameter in order to encourage the flow of oxygen within the compost. Likewise, the level of moisture should be above 40% to facilitate the compost process. Moisture levels that dip below 40% slow the process and present the risk of spontaneous combustion. Also, the ideal ratio of carbon to nitrogen should average 30 parts carbon to one part nitrogen by weight. The ideal balance maintains a healthy microbial population that speeds decomposition and minimizes odor.

Harmful Materials

Compost made from yard wastes, such as leaves and grass clippings, rarely contains any harmful materials. Problems can occur, however, when compost is made from partially sorted municipal refuse, certain industrial wastes, or sewage sludge. In those cases, unacceptable levels of toxic metals, chemicals, or harmful bacteria may be present.

To protect the public, the federal Environmental Protection Agency (EPA) sets acceptable levels for thousands of materials that might be present in compost. Each state may have its own standards as well. For municipal refuse, source separation—that is, having homeowners sort their yard wastes into separate containers rather than throw them away with the rest of their trash—is felt to be one of the most effective way to produce clean, safe compost.

The Future

By separating home yard wastes and turning them into compost, it is estimated that municipalities can reduce the amount of trash going to landfills by about 20%. While that is a significant reduction, it is expected that even more trash will have to be diverted from landfills in the future. Materials such as soiled food packaging, disposable diaper padding, food scraps, natural fiber rags, pieces of wood, and other organic materials could all be composted. To do this, municipalities may have to establish municipal solid waste (MSW) treatment facilities to separate the compostible materials from the harmful materials, such as discarded batteries, motor oil, asbestos, and many household chemicals.

Eventually composting may also provide a means for handling and neutralizing even the harmful materials. For example, at several older military ammunition factories and storage facilities the surrounding soil is contaminated with the explosive material trinitrotoluene, also known as TNT. Researchers are using a specially formulated compost mix of vegetable wastes and buffalo manure to neutralize the soil through a simple biological composting process that converts the explosive organic components of TNT into less harmful compounds.

Where to Learn More

Books

Christopher, Tom and Marty Asher. Compost This Book! Sierra Club Books, 1994.

Hansen, Beth, editor. Easy Compost. Brooklyn Botanic Gardens, Inc., 1997.

Martin, Deborah L. and Grace Gershuny, ed. The Rodale Book of Composting. Rodale Press, Inc., 1992.

Periodicals

Raloff, Janet. “Cleaning Up Compost: Municipal waste managers see hot prospects in rot” Science News (January 23, 1993): 56-58.

Other

The Compost Resource Page. http://www.oldgrowth.org/compost/ (June 7, 1999).

Composting Council. May 1999. http://www.compostingcouncil.org/ (June 7,1999).

— Chris Cavette

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