Table of Contents Acknowledgements What is Composting? Why Should I Make Compost? What Can I Compost? How Can I Use Compost? "Essentials" of Composting Building a Compost Pile? "Fast" Compost Recipe "Slow" Compost Recipe Recycling Yard Trimmings by Mulching and Soil Incorporation

What Is Composting?

Composting is controlling the natural decay of organic matter in a moist, aerobic (oxygen-demanding) environment.  Tiny organisms (mainly bacteria, fungi, and protozoa) break down garden and landscape trimmings to create a valuable product called compost--a dark, crumbly, earthy-smelling form of decomposed organic matter.  By composting, you control the natural decomposition process by providing the right conditions for composting critters to convert yard trimmings into a product that can be returned to your landscape and garden.

Why Should I Make Compost?

Composting is a practical and convenient way to handle yard trimmings.  It is easier and cheaper than bagging or removing them from your property.  The compost created from trimmings enriches your soil and plants.  If you have a garden, a lawn, trees, shrubs, or even houseplants, you have a use for compost.  Compost improves your soil.  When added to soil, compost breaks up heavy clay soils, helps sandy soils retain water and nutrients, and releases essential nutrients.  Compost also contains beneficial microscopic organisms that build up the soil and make nutrients available to plants.  Improving your soil is the first step towards growing healthy plants.

What Can I Compost?

Organic trimmings from your garden and landscape such as fallen leaves, grass clippings, flowers, and the remains of garden plants make excellent compost. Kitchen scraps, such as fruit and vegetable peels and trimmings, crushed eggshells, tea bags, coffee grounds and filters can also be composted.  Woody yard trimmings can be run through a shredder for composting, mulching, or for creating paths and walkways.  Organic materials that should not be added to your compost pile include meat, bones, and fatty foods (such as cheese, salad dressing, and leftover cooking oil), and pet litter.  Most pests, such as weeds, diseased or insect-infested plants, are destroyed in "fast" composts where the temperature in the center reaches 120 to 140 degrees F and the organic wastes are efficiently mixed into the center of the pile.

How Can I Use Compost?

Mix compost with soil to enrich the flower and vegetable garden, use it to improve the soil around trees and shrubs, or use it as a mulch.  Screen compost by separating the larger particles and any uncomposted materials from the finer ones and add it to the potting mix for houseplants (no more than one-quarter to one-third by volume of the potting mix should be compost) or use as a topdressing for lawns.  For potting mixes, compost should make up no more than one-fourth to one-third of the volume of the mix.  Compost "tea" can be made by soaking compost in a burlap or cheesecloth sack steeped in water.  The weak nutrient solution can be given to young plants.

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"ESSENTIALS" OF COMPOSTING

Biology

The compost pile is a hodgepodge of microscopic critters.  Bacteria, the most numerous and effective decomposers, are the first to break down plant tissues.  Fungi and protozoa soon join the bacteria. Somewhat later in the cycle, centipedes, millipedes, beetles, and earthworms do their parts to continue the decomposition process.

Materials

Many items in your refrigerator or growing in your yard are potential food for tiny decomposers.  Organic materials contain carbon and nitrogen:  nutrients that provide energy and growth to the microorganisms.  All organic materials have a ratio of carbon to nitrogen (C:N) in their tissues (see Table 1).  Leaves, straw, and sawdust are high in carbon, while grass clippings, manures, and vegetable scraps are higher in nitrogen.  It helps to think of these materials as greens and browns. Greens, such as grass clippings, are high in nitrogen. Browns such as leaves or sawdust, contain high amounts of carbon.  These C:N ratios are significant because the tiny decomposers need about 1 part of nitrogen for every 30 parts of carbon in the organic material.  If the ratio is greater than 30:1, nitrogen will be lacking and materials will decompose more slowly.  Be aware that anything organic will decay (as long as it is organic, the critters will eat it); however, it may take a long time to make compost when the C:N ratio is too high.  For example, a pile made solely of sawdust, will take years to decay.  Adding more greens, such as grass clippings or vegetable scraps, will speed up decay and produce compost in less time.  Experiment to find the right combination of materials for your compost pile.

Greens Material Ratio Pig manure 5-7:1 Poultry manure (fresh) 10:1 Alfalfa or sweet clover hay 12:1 Vegetable scraps 12-20:1 Poultry manure w/litter 13-18:1 Coffee grounds 20:1 Grass clippings 12-25:1 Cow manure 20:1 Horse manure 25:1 Horse manure with litter 30-60:1

Browns Material Ratio Leaves 30-80:1 Cornstalks 60:1 Straw 40-100:1 Bark 100-130:1 Paper 150-200:1 Sawdust 400:1 Wood chips 800:1

Surface Area

The more surface area the microorganisms have to work on, the faster the materials will decompose.  You can increase the surface area of your yard trimmings by chopping them up with a a shovel or running them through a shredding machine or lawnmower.

Volume

A large compost pile will insulate itself and hold in the heat created by the tiny organisms.  Piles smaller than 3' x 3' x 3' (27 cubic feet) have trouble holding this heat, while piles larger than 5' x 5' x 5' (125 cubic feet) prevent enough air from reaching the center of the pile to reach the microbes.  In addition, turning a large pile is a chore.  If your pile is large, you will have to turn it more often.  If the pile is small, you will get a good batch of compost during warm months.

Moisture & Aeration

The microbes in your compost pile need a certain amount of water and air to survive.  Microbes function best when the materials are about as moist as a wrung-out sponge and are provided with plenty of air. Too much moisture will force out the air and suffocate the microorganisms.  Too little moisture will slow down decay.  Whenever you add water, be sure to mix the material to distribute the moisture evenly.  Turning or aerating the materials in your pile supplies oxygen to the composting critters.  A lack of oxygen in a compost pile can lead to odor problems.  Ammonia and methane gases are produced when organic materials are allowed to undergo anaerobic (without oxygen) decay.  Anaerobic decomposition also leads to the production of chemical compounds that are toxic to plants.  Organic matter allowed to decompose anaerobically (for example, "composting" in closed garbage bags) should be exposed to air for several days to complete the composting process and to destroy any plant toxic compounds.

Temperature & Time

As a result of the decomposition process, a compost pile may heat up to 140 degrees F or higher.  The intensity depends on the amount of nitrogen in the materials.  The time required to produce compost depends on the kind and coarseness of the materials, volume of the pile, and availability of moisture and air. It can take a month, a year, or longer.

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Building A Compost Pile

There are no set rules when building a compost pile. It is important that you site your compost pile on a level surface.  This will reduce the chances for nutrients to runroff.  Pay attention to the "essentials" (see above) and use good judgment and common sense.  The following two recipes should help you create a "fast" or "slow" compost pile.

"Fast" Compost Recipe

A fast compost is labor intensive and requires a lot of turning.  This method can produce compost in a couple of months or less.  Consider using a turning unit that offers easy access to the organic materials. Units may be constructed of wood, wood and wire, or concrete blocks.

Ingredients

Greens and Browns (shredded)
Garden soil (optional)
Tarp or cover (optional)
Water
Pitchfork
Hotbed thermometer
Start your pile with a layer of browns.  Next, add a layer of greens. If the greens are not very fresh, sprinkle in some blood meal or cottonseed meal, poultry manure, or other nitrogen source. Sprinkle the materials with water if they are dry.  Mix the two layers together so the microorganisms can feed on them together.  Since microorganisms are present on all compost pile ingredients, it may not be necessary to add garden soil.  However, adding a layer of garden soil, old compost or manure to each brown-green layer will introduce more critters to speed up the process.

Continue adding and mixing layers of greens and browns until you fill the bin or run out of materials.  Make the top of the pile slanted to the center to catch rainfall.  At times you may want to cover your pile with a plastic covering or tarp to regulate the amount of moisture entering your pile.  The cover should not rest on the pile because it may cut off oxygen.  Periodically check the moisture content of your pile.  The compost should feel damp, and you should be able to squeeze out a drop or two of liquid.  Checking the temperature and using a calendar are two methods of monitoring your compost pile to help you judge when it should be turned.

1. Temperature.

Using a hotbed (or long-stemmed) thermometer, check the interior temperature of your pile at least 12 inches from the surface. It should peak between 90 and 140 degrees F. When the temperature begins to fall, or when it reaches 140 degrees F, turn the pile. Take materials from the outer edges and top of the pile and place them at the base and middle of the new pile; those from the middle should be on the outside edges and top of the new pile. (If you use a two- or three-bin unit, the yard wastes should be moved to the second bin.)  Continue monitoring the temperature in the pile.  The temperature will rise again as long as there is still undecomposed material that needs to be broken down.  Once your turning causes no rise in temperature, your compost is probably ready.  Compost will be dark, crumbly, and will no longer look like the original materials. (For three-bin units, move the compost to the third bin.)

2. Calendar.

Turn your compost pile every three to five days.  While turning can speed up the composting process, it also releases heat into the air.  Turn a pile less often during cold weather. Use the look and touch technique to see if your compost is ready:  if the material is dark, crumbly and unrecognizable from the original materials you added to the pile, you have compost.  If your pile does not heat up, you may need to add water or more nitrogen.  Refer to Table 2 to help you learn how to improve the efficiency of the composting process.  

Table 2.  Troubleshooting guide for efficient composting.

Symptoms	Problems	Solutions
Rotten odor	Not enough air; pile too wet	Turn pile; add coarse, dry materials (straw, corn stalks, etc.)
Ammonia odor	Too many greens (excessive nitrogen/lack of carbon)	Add browns (straw, paper, or sawdust)
Low pile temperature	Too small; not enough air or moisture; few greens; or cold weather	Make pile larger; add water while turning pile; mix in nitrogen sources (grass clippings, manure, or a synthetic fertilizer, such as 10-10-10); or insulate pile with a layer of straw or plastic.
High pile temperature	Too large; not enough air	Reduce pile size; turn pile
Pests--rats, raccoons, insects	Meat or fatty food scraps in pile	Remove meat and fatty foods from pile; cover with a layer of soil or sawdust; build an animal-proof compost bin.

"Slow" Compost Recipe

Slow composting is the least labor- and time-consuming way to compost; it is ideal for people who do not have a large amount of yard trimmings to compost all at once.  This method can take from six months to two years or longer to produce compost, so be patient.  The bins or containers can be made of old wooden pallets stood on their ends in a square or open square and nailed or tied together.  A chicken wire cage supported by three or four wooden stakes will also work well.

A standard-sized garbage can with eight or more slots in the sides of the can for ventilation and five in the bottom for drainage can also be used.  Elevate your bin a foot off the ground or start your pile with a three- to six-inch layer of small twigs or chopped corn stalks to improve air movement and drainage.  If you choose not to use a container, cover the heap with a layer of yard trimmings or soil to prevent moisture loss.

The ingredients are the same as those for a "fast" compost. Add greens and browns to your pile whenever they become available.  Turn the pile occasionally to mix the materials together to prevent the materials from clumping together and to avoid anaerobic decomposition.  You will know that your materials are decaying without oxygen by the foul odor:  a telltale sign for you to turn the pile. Look for ready-to-use compost near the bottom of the pile.

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Recycling Yard Trimmings by Mulching and Soil Incorporation

Yard trimmings such as leaves, grass clippings, and pine straw make excellent mulches for the landscape or vegetable garden.  Mulches provide the following benefits:

• conserve moisture
• help control weeks
• moderate soil temperatures by making the soil cooler in the summer and warmer in the winter
• reduce soil compaction, improve soil structure, and add nutrients
• prevent soil erosion

Shred large leaves and twigs with a lawnmower or shredder before using them.  A mechanical grinder or chipper is needed for large limbs and stumps.  Some local utilities, public works departments, and tree service companies will grind large materials for you.  Check with your local government for recommendations.  A two- to three-inch layer of mulch is adequate for woody plants.  Apply the mulch at least to the dripline of the tree, although the root system can extend two to three times the crown spread of the plant.  Keep mulch away from the main trunk of the plant to keep the bark dry.  When using grass clippings, use less than a two-inch layer to prevent matting.  Mulches made of chipped branches and trunks can also be used in animal pens, garden paths, and along fence rows to suppress weed growth.

SOIL INCORPORATION

Vegetable scraps, kitchen scraps (excluding meat, bones, and fatty foods), and other yard trimmings (especially diseased or insect-infested plants) can be spread in a layer in the garden and buried or tilled with a rotary tiller.  Cover organic materials with at least 8 to 12 inches of soil.  These organic materials will decompose, releasing nutrients and improving the structure of your soil.  Organic materials can also be buried with a posthole digger near the drip line of trees or shrubs and in small garden spaces.


Acknowledgements Special thanks to the Seattle Engineering Department and Seattle Tilth Association for granting Clemson Extension permission to reproduce portions of their material.

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The mission of Clemson University's Department of Horticulture is to promote personal and professional growth through the discovery, communication, and application of horticultural experiences, knowledge, and scholarship. Our work fosters environmental stewardship while improving economic wellbeing, health, and quality of life for all.