Jones Valley Urban Farm : Compost

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Composting Fundamentals

Good composting is a matter of providing the proper environmental conditions for microbial life. Compost is made by billions of microbes (fungi, bacteria, etc.) that digest the yard and kitchen wastes (food) you provide for them. If the pile is cool enough, worms, insects, and their relatives will help out the microbes. All of these will slowly make compost out of your yard and kitchen wastes under any conditions. However, like people, these living things need air, water, and food. If you maintain your pile to provide for their needs, they'll happily turn your yard and kitchen wastes into compost much more quickly. Keep in mind the following basic ideas while building your compost piles:

AIR

Composting microbes are aerobic -- they can't do their work well unless they are provided with air. Without air, anaerobic (non-air needing) microbes take over the pile. They do cause slow decomposition, but tend to smell like putrefying garbage! For this reason, it's important to make sure that there are plenty of air passageways into your compost pile. Some compost ingredients, such as green grass clippings or wet leaves, mat down very easily into slimy layers that air cannot get through. Other ingredients, such as straw, don't mat down easily and are very helpful in allowing air into the center of a pile. To make sure that you have adequate aeration for your pile and its microbes, thoroughly break up or mix in any ingredients that might mat down and exclude air. You can also turn the pile to get air into it, which means completely breaking it apart with a spade or garden fork and then piling it back together in a more 'fluffed-up' condition.

WATER

Ideally, your pile should be as moist as a wrung-out sponge to fit the needs of compost microbes. At this moisture level, there is a thin film of water coating every particle in the pile, making it very easy for microbes to live and disperse themselves throughout the pile. If your pile is drier than this, it won't be very good microbial habitat, and composting will be slowed significantly. If your pile is a great deal wetter, the sodden ingredients will be so heavy that they will tend to mat down and exclude air from the pile, again slowing the composting process (and perhaps creating anaerobic odor problems). If you are using dry ingredients, such as autumn leaves or straw, you'll need to moisten them as you add them to the pile. Kitchen fruit and vegetable wastes generally have plenty of moisture, as do fresh green grass clippings and garden thinnings. Watch out for far-too-soggy piles in wet climates (a tarp may help to keep rain off during wet weather). In dry climates, it may be necessary to water your pile occasionally to maintain proper moisture.

FOOD

In broad terms, there are two major kinds of food that composting microbes need.

'Browns' are dry and dead plant materials such as straw, dry brown weeds, autumn leaves, and wood chips or sawdust. These materials are mostly made of chemicals that are just long chains of sugar molecules linked together. As such, these items are a source of energy for the compost microbes. Because they tend to be dry, browns often need to be moistened before they are put into a compost system.

'Greens' are fresh (and often green) plant materials such as green weeds from the garden, kitchen fruit and vegetable scraps, green leaves, coffee grounds and tea bags, fresh horse manure, etc. Compared to browns, greens have more nitrogen in them. Nitrogen is a critical element in amino acids and proteins, and can be thought of as a protein source for the billions of multiplying microbes.

A good mix of browns and greens is the best nutritional balance for the microbes. This mix also helps out with the aeration and amount of water in the pile. Browns, for instance, tend to be bulky and promote good aeration. Greens, on the other hand, are typically high in moisture, and balance out the dry nature of the browns. If you'd like specific information on different materials, check the 'What to Compost' section.

OTHER THINGS TO CONSIDER

If you live in a cold climate, your compost pile will probably go dormant in the winter. No problem -- it'll start back up again when the springtime thaw comes.

A common misunderstanding about compost piles is that they must be hot to be successful. This just isn't true. If you have good aeration and moisture, and the proper ingredient mix, your pile will decompose just fine at temperatures of 50 degrees Farenheit or above.

Hotter piles will decompose a bit faster, however. One way to understand why this is so is to realize that the heat in a hot pile is the result of the collective body heat of billions of microbes that are busy digesting the ingredients in the pile. Generally speaking, a hotter pile means more microbes or conditions that allow the microbes to have faster metabolisms, and therefore a faster composting process. If you'd like to keep your pile as warm as possible, consider the following:

For a pile to get hot and stay hot for a long period of time, the typical minimum size for the pile is one cubic meter (a cube one meter, or about three feet, on a side). A pile this size has plenty of mass in which those billions of heat-generating microbes can live, yet is also large enough that the center of the pile is well-insulated by the material surrounding it. Smaller piles just cannot insulate themselves well enough to remain hot for long, if at all. You can also provide additional insulation to a pile by stacking bales of hay or straw, or bags of dry autumn leaves, around your bin system. Some people even used stacked hay bales to make bin systems (this kind of bin will slowly compost itself, of course).

Composting Questions and Answers:

When is my compost finished?

Finished compost is dark in color and has an earthy smell (like the smell of soil). Usually, it's difficult to recognize any of the original ingredients, although bits of hard-to-decompose materials (such as straw) sometimes can be seen.

There is no single point at which compost is finished -- it's a bit more subjective than that. For many outdoor garden applications, for instance, it can be fine to use compost that still has a few recognizable bits of leaves or straw -- it will finish rotting in the soil. If you plan to use compost in seed-starting mixes, though, you're best off having a well-finished compost, because seedling roots may be attacked by decomposer microbes if the roots contact unfinished compost.

How can I use my finished compost?

To tell you the truth, well-finished compost looks so fine that I'm tempted to eat the stuff sometimes. However, there are several more common ways that compost can be used, on gardens, lawns, landscapes, and houseplants:

COMPOST AS SOIL AMENDMENT: Many people put compost into their garden soil by digging it in prior to spring planting. The image shows a potato harvest by apprentices at the UCSC Farm and Garden in 1990. Due to the use of copious amounts of compost, the potato beds yielded about one pound of potatoes per square foot, or about 1000 pounds total from these four beds, each 80 feet long.

Others actually do their composting in the soil, by burying kitchen wastes and other materials in trenches in the garden. Compost can also be used as a 'top dressing' on the soil during the growing season -- in this case it is added in around the bases of plants, where irrigation and soil animals will slowly incorporate it into the soil. On lawns, many people sprinkle/broadcast sifted compost as a top dressing in the spring -- I have been doing this on a 'problem area' of a lawn for several years, in an attempt to improve the soil there for better grass growth. It is also fine to top-dress houseplants occasionally with small handfuls of finished compost.

COMPOST AS MULCH: Compost tea is made by combining equal parts of compost and water and letting it sit for a while. The liquid can help to provide a 'quick boost' to ailing houseplants or young seedlings and transplants (I recommend diluting it quite a bit for use on seedlings). Stu Campbell, in Let it Rot, says that the same compost can be used to make several batches of tea (2). When you're finished making compost tea, use the mucky dregs as a mulch in the garden or landscape.

How does compost improve the soil?

Compost does several things to benefit the soil that synthetic fertilizers cannot do. First, it adds organic matter, which improves the way water interacts with the soil. In sandy soils, compost acts as a sponge to help retain water in the soil that would otherwise drain down below the reach of plant roots (in this way, it protects plants against drought). In clay soils, compost helps to add porosity (tiny holes and passageways) to the soil, making it drain more quickly so that it doesn't stay waterlogged and doesn't dry out into a bricklike substance. Compost also inoculates the soil with vast numbers of beneficial microbes (bacteria, fungi, etc.) and the habitat that the microbes need to live. These microbes are able to extract nutrients from the mineral part of the soil and eventually pass the nutrients on to plants.

A great variety of things can be composted at home, saving them from a one-way trip to the landfill, and turning them into a valuable soil amendment for home use. This list describes some of the items you may want to add to your home compost pile. You may want to read about how to compost to learn about the difference between 'brown' and 'green' ingredients and the roles each plays in the composting process. Also, see the list of what NOT to compost.

These items can be put in your compost:

GRASS/LAWN CLIPPINGS

Actually, it's usually easier to leave grass clippings in the lawn, where they will decompose and benefit the soil directly. However, they can be composted, too. Be cautious to add grass clippings in very thin layers, or thoroughly mix them in with other compost ingredients, as they otherwise tend to become slimy and matted down, excluding air from the pile. Fresh grass clippings are high in nitrogen, making them a 'green' compost ingredient.

HAY

Farmers are often very happy to get rid of spoiled hay bales that have been out in the rain, and will give them away or sell them at a low price. Grass hay will probably contain a lot of seed, which can resprout in your garden. Alfalfa hay will compost very readily. The greener the hay, the more nitrogen it contains. Be sure that any hay you plan to compost is well-moistened prior to addition to the pile.

GRASS/LAWN CLIPPINGS

KITCHEN WASTES

Fruit and vegetable peels/rinds, tea bags, coffee grounds, eggshells, and similar materials are great stuff to compost. They tend to be high in nitrogen (this puts them in the 'greens' category), and are usually quite soft and moist. As such, kitchen wastes need to be mixed in with drier/bulkier materials to allow complete air penetration. Many people compost their kitchen wastes in enclosed worm bins or bury them 8" deep in the soil, to keep from attracting pests to an outdoor compost pile (check with your local government to see if it has regulations about this -- some forbid open piles containing food wastes because of the pest issue). Avoid composting meat scraps, fatty food wastes, milk products, and bones -- these materials are very attractive to pests.

LEAVES

If you live in an area where autumn leaves are still thrown away as garbage, cash in on the bounty each year by acquiring your neighbors' leaves! Generally, leaves are an excellent compost ingredient. They can mat down and exclude air, though, so be sure that any clumps are thoroughly broken up, or that the leaves are only used in very thin layers. Ash and poplar/cottonwood leaves can raise soil pH if used in compost -- this may not be beneficial if your soil is already alkaline, as many soils are in the West (especially in semiarid and arid climates). Dead, dry leaves are in the 'browns' category, while living green leaves contain abundant nitrogen and are considered 'greens'.

STRAW

Dry straw is a good material for helping to keep a compost pile aerated, because it tends to create lots of passageways for air to get into the pile. Be sure to wet the straw, as it is very slow to decompose otherwise. Straw is definitely a 'brown' and also requires mixture with 'greens' to break down quickly. Many stables use straw as a bedding material for horses -- straw that has undergone this treatment is mixed in with horse manure and breaks down more quickly.

WEEDS AND OTHER GARDEN WASTES

Many types of weeds and old garden plants can be composted. Avoid weeds that have begun to go to seed, as seeds may survive all but the hottest compost piles. Some types of weeds are 'pernicious weeds' and will resprout in the compost pile -- avoid using these unless they are thoroughly dead. Green weeds are (you guessed it) a 'green', while dead brown weeds are a 'brown'.

WOOD CHIPS AND SAWDUST

Wood products belong in the 'browns' category, because they are fairly low in nitrogen. Some sawdusts, especially from broadleaved/deciduous tress, will break down quickly in an active compost pile. Others, especially from coniferous trees, will take longer to decay. Stir sawdust thoroughly into the pile or use very thin layers. Coarse wood chips will very slowly decay, and are probably better used as mulch unless you have lots of time to wait. Be sure not to compost chips or sawdust from any sort of chemically-treated wood -- you could be adding toxics like arsenic to your pile if you do.

Whether because of toxins, plant or human diseases, or weed troubles, there are some things that shouldn't be put into compost piles.

Avoid composting the following materials:

CHEMICALLY-TREATED WOOD PRODUCTS

Sawdust is often available from constructions sites, friends, or your own building projects. If you are considering composting sawdust, be sure of the origin of the sawdust. Sawdust from chemically-treated wood products can be bad stuff to compost. For example, take pressure-treated wood (sometimes called CCA), which usually has a greenish tint to it (I have also seen it in other colors). It contains arsenic, a highly toxic element, as well as chromium and copper. There is evidence to suggest that arsenic is leached into the soil from these products when they are used to make compost bins or raised beds, so composting the sawdust would certainly be a mistake. You may wish to read the 'Letters' section of Organic Gardening, April 1994 and July/August 1992, for more information. Avoid other chemically-treated wood products and sawdust as well, such as wood treated with creosote or 'penta' preservative.

DISEASED PLANTS

Many plant disease organisms are killed by consistent hot composting, but it's difficult to make sure that every speck of the diseased material gets fully composted. It's best not to compost diseased plant material at all, to avoid reinfecting next year's garden.

HUMAN WASTES

Human feces can contain disease organisms that will make people very sick. Composting human feces safely requires that the compost pile reach high (thermophilic) temperatures over a period of time. It isn't necessarily that difficult to reach these temperatures in a home compost pile, but the potential health costs of improper composting are high. Composting of human feces should not be attempted, except by experienced 'hot pile' composters who are well informed of the temperatures and times required to kill pathogens, and who are willing to take 100% responsibility for the process and product. If you would like to learn more about composting humanure, I recommend The Humanure Handbook, listed in the resources section of the Rot Web.

MEAT, BONES, AND FATTY FOOD WASTES

These materials are very attractive to pests (in an urban setting, this could mean rats...). In addition, fatty food wastes can be very slow to break down, because the fat can exclude the air that composting microbes need to do their work.

PERNICIOUS WEEDS

Morning glory/bindweed, sheep sorrel, ivy, several kinds of grasses, and some other plants can resprout from their roots and/or stems in the compost pile. Just when you thought you had them all chopped up, you'd actually helped them to multiply! Don't compost these weeds unless they are completely dead and dry (you may want to leave them in a sunny place for a couple of weeks before composting). Remember also that composting weeds that have gone to seed will create weeds in next year's garden, unless a very hot pile temperature can be maintained to kill the seeds.

PET WASTES

Dog and cat feces may carry diseases that can infect humans. It is best NEVER to use them in compost piles. Some people do bury them 8" deep in the soil, but ONLY in areas where food crops are never grown.