Well, ‘compost’ is actually an umbrella term used to describe the process of recycling organic material.
Simply put, compost is actually what you get when organic material (like food scraps, leaves, and animal products) decomposes properly. It's a nutrient rich, dirt-like material that can be used to enrich soil and nourish growth.
While this process takes time in nature, we can help speed it up by creating the ideal environment.
By adding carbon (browns) and nitrogen (greens) to organic material, combined with lots of air and water, we create a nutrient rich substance that can be used as natural fertilizer to our soil - sustaining future growth.
Why Does Composting Matter?
Improves Soil Structure.
Natural proteins in compost help dirt bind together, which helps soil retain nutrients and moisture.
Did you know that soil can retain 16,000 gallons of water per acre for every 1% of organic material? That means that adding compost to soil can reduce the water needed for crops.*
It’s A Natural Fertilizer.
Compost introduces both microorganisms like bacteria and fungi; and nutrients like nitrogen, copper, phosphorus, and zinc, to soil, which makes it more fertile.
Compost makes soil healthier, causing plants to be more resistant to diseases and harmful insects.
For A Healthier Planet.
Throwing organic material into the garbage is harmful to the environment. Organics make up about 1/3 of our waste, removing organic material from our garbage reduces the amount of trash we send to landfills.
Plus, organics don't belong in the trash. Organic material decomposing in landfills releases harmful methane, a gas 20 times more effective at trapping heat in the atmosphere than carbon dioxide. Landfills make up 17% of the United States' methane emissions. By composting we can substantially reduce harmful emissions to the environment.*
*The United States Environmental Protection Agency
Landfills are a large producer of methane emissions in the US*
How Composting Works?
Four factors affect the speed of organic decomposition:
Composting is an aerobic process, which means that oxygen is needed for the organic material to decompose. Compost piles need to be turned regularly to create that airflow. People & pitchforks can move small compost piles, but bigger piles need better solutions, systems with air vents & pipes can increase oxygen through the pile, while small tumbler systems can also help to ease the turning process.
In your ventures with compost, you may learn about anaerobic digesters, which are machines created to help break down food without air. This is not considered composting because the end product & process is different, although the goal is still the same.
Compost piles should contain a mix of nitrogen-rich materials (greens) & carbon-rich materials (browns). Greens include any thing that used to be alive such as vegetable & fruit scraps, weeds, coffee grounds, eggshells, meat, dairy, & grass clippings. Browns are things like leaves, straw, mulch, wood products, sawdust, newspaper & shredded paper. A healthy compost pile is generally 50/50 greens to browns; but the ratio depends on what you put in, & your composting method of choice. Chopping your food scraps into smaller pieces will speed up the composting process. Larger pieces will still decompose, but take longer.
Always check the materials your compost program accepts. Most small compost piles don't allow meat, dairy, bones, or greasy items.
Moisture is important to a compost pile; ideally it should feel damp. Being about 50-60% water by weight, a lot of moisture comes from the food scraps. If your pile is dry, you can water the pile, but if you squeeze it and it drips, it's too saturated. You can regulate this with a tarp, enclosed container, or by adding more browns (like leaves, wood chips, or newspaper) to absorb moisture.
Organic matter decomposes faster in higher temperatures, meaning that an active compost pile is warm, even when it's cold out. Turning your compost will generate more heat, although some types of composting may require less heat and mixing than others.
Compost piles are usually hot, anywhere from 110 to 180 degrees Fahrenheit (most organisms cannot survive above 160 degrees). This temperature must be monitored, when it gets too cold (usually around 120 degrees), the pile should be turned. A hot pile is typically turned every 3 to 7 days, and takes 6 weeks to 3 months to decompose into compost. Hot piles also reach high enough temperatures to kill pathogens, weed seeds, & fly larvae.
Compost piles can be cold too, anywhere from 110 to 50 degrees Fahrenheit, this is considered “slow” composting and is lower maintenance. A cold pile can take 6 months to 3 years to finish composting and should not include weeds, meat or dairy. Since the pile doesn’t really need to reach a desired temperature to kill the pathogens in meat and dairy, you can add greens and browns as you collect them.
What To Include In Your Compost?
Every compost pile is different in terms of heat & process, which means that the items that go into each pile are also different.
Always check with the organization you're composting with before putting your items in the compost bin.
Fruit & Vegetable Scraps
Leaves & Dried Flowers
Wood Chips & Mulch
Egg & Nut Shells
Rice, Bread, & Wheats
Sometimes Include (depending on the pile):
Food Soiled Paper Products
Meat & Bones
Seafood & Shells
Fats, Oils, & Grease
Plants Infected with Disease or Insects
Ivy & Pernicious Weeds
Feces (from any animal)
Hazardous Chemicals & Cleaning Supplies
Plastics & Glass
What’s Happening Inside The Pile?
There are many different types of compost piles.
For hot composting, there are 3 main phases:
Mesophilic (moderate temperature), lasting for a couple of days
Temperature reaches up to 90 degrees Fahrenheit and beyond. There is rapid growth of Mesophilic bacteria and fungi, that breakdown soluble sugars and starches, causing the temperature to rise.
Thermophilic (high temperature), a few weeks
As temperatures increase well beyond 120 degrees F, Thermophilic bacteria, actinomycetes, and heat-tolerant fungi populate the pile. They break down proteins, fats, hemicellulose and cellulose. Eventually, the organisms begin to run out of food, and so the temperature of the compost pile drops.
Mesophilic Curing, several months
Temperatures drop back down below 140 degrees F, allowing Mesophilic actinomycetes, bacteria, and fungi to enter the pile. Over this several month period, the microorganisms break down lignin and other highly resistant compounds.
An Introduction To Ecosystems
Compost can be active in a variety of ecosystems, it happens in every environment.
What's Helping The Pile Decompose?
Inside a compost pile, microorganisms digest organic matter & turn it back into its inorganic nutrient form: carbon dioxide, water, & compound forms. This process is critical to composting because the microorganisms release nutrients in a form that plants can absorb & use.
Microorganisms, like bacteria, fungi, actinobacteria, earthworms, snails, & slugs, that feed on dead plants and animals to break up the organic matter.
These macrorganisms, like protozoa, mold mites, & nematodes, feed on primary consumers. They keep populations of the primary consumers in check, to aerate the pile as they move through it, & enrich the pile with their excretions.
Centipedes, ground beetles, & predatory mites feed on secondary consumers, keeping populations in check, they aerate the pile as they move through it, further enriching the material, aiding the decomposition process.
Inside a compost pile you will find a variety of microorganisms:
Bacteria make up 80-90% of the microorganisms found in compost. They’re the smallest of the microorganisms in the compost pile, but are the most nutritionally diverse, meaning they can break down the largest number of organic materials.
Actinomycetes break down cellulose, lignin, chitin & proteins, which are more complex, found in materials like tree bark, wood, & newspaper. Most bacteria & fungi cannot break these down, just as actinomycetes cannot compete for most of their simple carbohydrates. They are also responsible for the pile’s pleasant, earthy smell.
Fungi are found in the latter stages of composting, they move to the outside of the pile to avoid the heat. Fungi break down cellulose & tough debris that bacteria have a hard time processing, things that are dry, acidic, or high in nitrogen.
Protozoa and rotifers feed on bacteria & fungi in the compost pile, but only make up a small percentage of the pile. They play a very small role in composting.
Compost can get a bad reputation because people think it creates odor and attracts rats and insects. But when maintained properly, compost should not smell or attract pests.
Check this helpful table to find your compost pile’s problem and solution:
Bad Odor Like Rotten Eggs
Too much moisture, anaerobic conditions
Turn your materials frequently. Add dry brown material such as fallen leaves, woodchips, or straw.
Bad Odor Like Rotten Food
Exposed or inappropriate food scraps
Remove meat, dairy, or other inappropriate material. Cover food scraps with a thick layer of browns.
Bad Odor Like Ammonia
Too much nitrogen (greens), not enough carbon (browns)
Add browns to the pile. If needed, remove some greens, allow them to lie out, then reincorporate them into the pile in a few days.
Pile Not Breaking Down
Turn the pile more frequently to get oxygen to the decomposing organics.
Low Pile Temperature
Pile too small
Lack of nitrogen
Increase pile size by adding greens and browns.
Add water to pile, and turn the pile.
Turn pile or add aeration piping, depending on size.
Add more greens.
Increase pile size or insulate it with hay bales or Styrofoam.
High Pile Temperature
Pile too large
Reduce pile size.
Turn the pile more frequently.
Pile too dry or inadequately mixed. Contaminated or too much wood-based materials.
Make sure food scraps are covered in a layer of browns. If adding wood materials, make sure it is not contaminated with termites, carpenter ants or other insects. Add water to the pile while turning to deter bees and wasps.
The bin or pile isn’t adequately covered, food scraps are exposed, or the wrong food scraps are in the pile.
Make sure the compost pile is covered. It should have an outer layer of browns, and should be kept in an enclosed bin. Add hardware cloth and make sure the barrier reaches 6 to 8 inches into the ground (to prevent burrowing). If you are still having trouble, remove meat, dairy, and fatty foods from the pile.
Adapted from Easy Compost by The Brooklyn Botanical Garden and New York City Master Composter Manual by the NY Department of Sanitation.
When is Compost Finished?
The last stage of composting is called 'curing'. This is when the pile is set aside, not added to or turned, the temperature will lower to finish the process.
Curing can take anywhere from 1 month to 1 year. Many composters have their own preferences as to how long to cure a pile. Some will intentionally wait over a year to ensure higher quality compost, and others will use a shorter process because they need to make space for new piles.
There are a few ways to tell if your compost is ready to use:
It looks like dark, crumbly topsoil
It has a pleasant, earthy odor. It should not smell like ammonia
The original organic materials (with a few exceptions) should no longer be recognizable
The compost pile should have shrunk by half the size
The pile should have returned to air temperature, about 50 degrees F
While the majority of organic materials should not be recognizable in finished compost, it’s okay if there are a few stubborn materials, such as corncobs or wood chips that do not decompose. These materials should not be used in the finished compost, though – they should be filtered out by a process called ‘screening’.
Screening passes the finished compost through a filter. Objects that are larger than the filter can be added to a new compost pile. They can be beneficial to a new compost pile because they contain microorganisms that will help jump-start the composting process.
One simple way to test finished compost is to take a handful and put it in a sealed plastic bag. After 3 days, open the bag and smell. Does it smell sour? If so, the compost is not finished curing and still has microorganisms at work. If it smells pleasant and earthy, it’s ready to use.
There are also a number of composting tests using plant germination that you can use to see if your compost is safe to use with plants.
Beyond your own testing, compost can be approved by third party organizations. If you are selling your compost these tests may be required by the retailer.
A widely used and respected program for compost quality testing is The US Composting Council’s Seal of Testing Assurance program.
Below are factors that affect the quality of compost:
Time to Mature
Compost that has been allowed to cure, or mature, is healthier for soil than immature compost. Immature compost may contain plant inhibitors, such as bacteria that will compete with the plants for nitrogen in the soil. You will want to test the compost for maturity to make sure it is safe to use.
The materials added to the compost pile will affect the nutrients, soluble salts, and contaminants found in the soil. Compost made from food scraps is typically higher in nutrients, but also higher in soluble salts (which are harmful to plant growth).
Hot vs. Cold Composting
Cold compost is likely to have more nutrients than hot compost; however, hot compost is less likely to have pathogens and weed seeds.
How Is Finished Compost Used?
Compost is a fantastic soil amendment that helps improve soil structure, promoting air flow while retaining moisture. Adding compost as a soil amendment will depend on the type of plant bed or container.
For Annuals, Groundcovers, Shrubs or Trees:
Integrate compost into the top 3 to 5 inches of soil. While gardeners used to recommend adding compost to a tree or shrub hole, it is no longer recommended because it can impact root growth.
Add several inches of compost to the bed in the fall and till it in the spring. When you plant, add compost into the hole. You can also sprinkle a half-inch layer around the plant.
Mix 1 to 2 inches of compost into the top 6 to 8 inches of annual and perennial beds in the spring.
For Potted Plants:
Add 1 inch of compost twice a year. You can also create your own potting soil using 2 parts compost to 1 part sand or perlite.
For Lawn or Turf:
When planting a new lawn, add 1-2 inches of compost over the entire lawn area. Then incorporate it into the top 5-7 inches of soil for a mix of about 30% compost, 70% soil. On existing lawns, use compost in “bald spots” for replanting, or rake a ½ inch layer into the grass.
Mulch & Top Dressing
Mulch has many benefits to soil. It retains moisture, protects from erosion and compaction, deters weeds from growing, and keeps temperatures steady. Compost, when used as mulch, will provide all of these benefits – and it will add nutrients to the soil as it decomposes.
When using mulch, spread a layer of about 1-3 inches over the surface of the soil and use a rake to distribute it evenly. Lighter mulches made from leaves and plants will be less likely to weigh down the soil and can be added in thicker layers. Make sure not to add mulch in a “volcano” form around the tree trunk or plant stem. Leave a ring of mulch-free space around tree trunks to prevent fungal decay.
Compost tea is a liquid fertilizer that has extracted and concentrated the microbe population found in compost. To make it, compost is added to water, and then “food” and oxygen is added to stimulate bacteria and fungi growth. When compost tea is sprayed on plants, even in a small amount, those microorganisms help to fight off harmful bacteria and protect plants from disease.
To apply compost tea, spray the soil. Alternatively, spray plants every two weeks until the peak of the growing season.
Compost adds macronutrients, bacteria, and fungi to potting mixes that traditional commercial fertilizers do not contain.
To create a balanced potting soil mix, use:
- 1/3 high quality compost (screened & cured)
- 1/3 expanded coconut fiber (aka coir or coco peat)
- 1/6 perlite
- 1/6 vermiculite
Techniques & Technologies
Below are composting systems that you may want to consider for your site or in your home.
Contained Compost Solutions
A tumbler is a completely enclosed container, usually made out of plastic, with a mechanism for turning or spinning the container. Tumblers are great for containing odors and moisture, keeping out pests, and easy turning and maintenance. They have limited space, though, and can be expensive. A tumbler won’t necessarily speed up composting time, it will only make it cleaner and easier.
A modified trashcan is just what it sounds like – it’s a trashcan that has ventilation holes, so it can be used for composting instead of trash. It’s an easy, inexpensive DIY project. The trashcan will deter pests and can easily be rolled on its side for turning. The downside of a trashcan is that in can confuse users, causing contamination from non-compostable items.
Bottom-Access Plastic Bin
A bottom-access plastic bin has a plastic tray on the bottom that collects finished compost, so it’s easy to remove and use in the garden. While less attractive than slatted wooden bins, they are usually better in price, and helpful for keeping pests out and moisture in.
Slatted Wooden Bins
This three-bin system can be made of rot-resistant cedar, with removable front planks for easier turning.
There are many different building plans available for bin systems, however, in urban composting, it is important to include rat-proofing measures as part of your plan.
In-vessel composting is exactly what it sounds like - organic material is broken down inside a vessel or container. The temperature and air levels are controlled, and it usually has a mechanical apparatus for turning or agitating the material for proper aeration. They often have biofilters that provide negative pressure, meaning that air is sucked into a pipe and through a filter - this greatly reduces odors. In-vessel systems have the highest startup cost, but advantages in size, leachate control, vermin, and odor.
Compost Pile Systems
Windrows are long piles of organic material, typically 4-8 feet tall and 14-16 feet wide. They are organized in rows, which allows them to be manually turned into the next row or turned in place by a machine. Windrows are big enough to retain heat and moisture, but small enough to allow air to pass through. They are typically covered with a biofilter, such as finished compost or wood chips, to help trap odors. Turned windrows require a good amount of labor, since they must be turned frequently, especially in the early stages. They can be turned by manual labor, though many facilities will use equipment for this. This approach also requires a larger amount of space.
Aerated Static Piles (ASP)
ASP forces air into organic material via a ventilation system, which rapidly speeds up the composting process and greatly reduces labor. Usually, the ASP system is a series of tubes or pipes with holes, and air is pushed through the holes with a fan. The organic material can sit on top of the pipes, in a windrow or pile, or it can be kept in a closed container. The fan can be powered by a solar panel, or attached to an electrical source. Due to cost, ASP systems are usually utilized by larger-scale operations.
Underground Composting Systems
A century old style of composting, just bury your food scraps in the soil! Make sure to cover them with at least 8 inches of soil to prevent pests from detecting the food.
An in-soil digester is like a modified trash can, but it’s buried in the soil. The digester is buried about 2-3 feet in the ground with the top 1/3 to 1/2 of the can raised above the ground. To add food scraps, you can simply open the lid and add them, along with a layer of sawdust or dirt (to prevent odor). The water and leachate will leak into the soil via the holes in the can. The food scraps will compost over a year, and for an average sized family, the digester takes a few months to fill.
This is a more organized method of soil incorporation. Essentially, a gardener will bury food scraps in rows. Rotating each year, one row is for burying new organic material, one is for planting, and one is left as is.
A little more advanced than simply burying food waste, sheet composting is designed to save space in a garden. To start, build a lasagna of 1 inch newspaper, 4-6 inches of browns, 3-5 inches of greens, and 1 inch of peat moss or finished compost. Then, water the bed. You can stop there, or add another round of layers until you reach the top of the soil. The last layer should be finished compost. You can plant it immediately, or wait.
Worms & Fermentation
This is the process of using red worms to break down organic material into high value compost. A pound of worms, or about 1,000 worms, can eat a half a pound of compost a day. Vermicomposting operations can range from a small bin to large system that process thousands of tons per year. Worms are sensitive to temperature, and prefer an environment that is 55 to 77 degrees F. Therefore, it's ideal to keep worm bins indoors. The worms will eat almost any type of organic material other than compostable plastics.
Japanese for "fermented organic matter", Bokashi is the process of breaking down food scraps via anaerobic fermentation. Typically considered a type of composting, bokashi does not use oxygen, fermenting the organics instead.
Food scraps (including meat and dairy) are placed in an airtight container and sprinkled with a mixture of something carbon-based (such as bran or saw dust) and Effective Microorganisms (EM1). This mixture can be purchased or made at home. Once the food and mixture are added, the container is sealed to keep oxygen out.
Allow the mixture to ferment for a couple of weeks. When this process is complete, the fermented food scraps should be buried or added to a compost pile so that it can mature into compost.
Bokashi is easier than some other forms of composting because it is low maintenance, can be done indoors or outdoors, and does not produce foul odors. However, keep in mind that "finished" bokashi is not really compost. It needs to be buried or composted before it can act as a soil amendment.
Tools For Composting
Helpful for accessing the materials at the bottom of a compost pile.
A sharp, pointed edge that can break down large, thick materials, such as stalks and vines. You can also use it to turn the pile or harvest compost.
One of the best tools for hand-turning a compost pile.
Used as an airshaft that can push in and then pull out soil from the pile, which will allow air to enter. This will stimulate decomposition and reduce the amount of manual turning necessary. A typical aerating tool costs $20 to $60.
Pruning Shears and Loppers
As a low-tech alternative to a shredder or chipper, shears or loppers can be used to chop twigs, small branches and other tough materials into smaller pieces, which will break down faster.
Shredders and Chippers
A more expensive, high tech alternative to shears or loppers, shredders and chippers will break yard waste down into smaller pieces that will decompose much more quickly in a compost pile. They can cost anywhere from $150 to over $1000, so usually larger operations will invest in such equipment.
Watering Can and Hose
A pile breaks down the fastest when it is as moist as a wrung out sponge. In times of drought or hot weather, add water to the compost pile until it contains about 50-60% moisture by volume.
A screen will remove large clumps, debris, and stubborn materials that haven’t broken down. This is important if you’re using the compost as a component of potting soil or to top dress the lawn. You can make your own with hardware cloth, or you can buy a screener.
Cuts grass or leaves into small pieces, which can be left on the lawn, as mulch, or composted. These cost around $150 to $700.
A handheld tool that you can use to chop compost up into smaller pieces can be helpful in your garden ventures. Breaking down material into small pieces aids the decomposing process.
Less important in cold composting, a compost thermometer is helpful for monitoring the temperature of your pile. For a hot compost pile, it’s important to know when the temperature dips so you can turn the pile and rejuvenate thermophilic activity. For weed seeds and pathogen killing, the pile must be at least 131 degrees F for 72 hours, so a thermometer is key for monitoring this process. A compost thermometer typically costs between $25 and $250.
Anaerobic Digestion isn't exactly composting - it's a very different process for decomposing waste.
Anaerobic digestion breaks down organic matter without the use of oxygen, producing 3 outputs:
Biogas, similar to natural gas, mostly composed of carbon dioxide and methane, which can be captured and converted to fuel and energy.
A digestate mix, which can be a solid, liquid, or slurry of organic matter.
Water, which can be used after a purification process.
Anaerobic Digestion Occurs In 3 Phases:
Organic matter is decomposed into smaller, simpler molecules through bacterial hydrolysis.
Bacteria work to convert the simple molecules into amino acids.
The amino acids are converted to methane gas.
Anaerobic Digestion comes with benefits and baggage.
Below are some examples of each side of the conversation.
When landfilled, organic matter releases methane into the atmosphere, which causes climate change. Anaerobic digestion captures this methane and converts it to energy.
An aerobic digester facility can charge waste haulers a tipping fee and sell the energy generated from the methane. Some anaerobic digesters switch between electricity and fuel generation, depending on market prices.
In wastewater treatment facilities, fats, oils, and grease clog the water infrastructure. By diverting these substances to an anaerobic digester, it prevents sewer overflows, saving money and improving water quality.
Involves a large capital investment with a long payoff period. The prices of electricity and biofuel fluctuate, so profits will depend on the market.
The digestate may only be land applied in certain places because of laws and regulations. The benefits of adding digestate to the soil have also been questioned, because many of the nutrients have already been converted into energy – leaving the digestate potentially nutrient deprived. It may take additional transportation to move the digestate for land application, or additional effort to compost the digestate for land application.
Anaerobic Digestion & The Big Apple
NYC passed legislation mandating that commercial businesses who generate over 1 ton of waste per week cannot send their organic materials to landfill. The purpose is to reduce the amount of greenhouse gases, but also motivate new, local composting facilities and anaerobic digesters to enter the market.
NYC is particularly interested in anaerobic digesters because it would bring an additional renewable and localized energy source to the city. While there are no anaerobic digesters dedicated to food scraps currently in NYC, there are some projects that are piloting the technology.
Newtown Creek Digester Eggs
Located at NYC’s wastewater treatment plant in Greenpoint, Newtown is used to process 1.5 million gallons of sludge each day. Since the digesters have extra capacity, the Department of Sanitation hauls food scraps (turned into a slurry) to the facility for processing in the digesters. The digesters turn the organic material into methane gas, which is used to power the facility, and will soon be used to power some of the city’s homes through a partnership with National Grid.
Long Island Compost
Based in Westbury, NY Long Island Compost is building one of New York’s first anaerobic digesters specifically for food scraps. It will have capacity to accept 120,000 tons of food scraps per year. Long Island Compost has a commercial composting facility, but it does not accept meat or dairy, which limits its clientele, but with the digester they will be able to accept more compostable items.