Materials for Composting

What To Compost

The materials you put into your compost pile have a major impact on how well the composting process works and the quality of the final compost. The key to good composting is to have a variety of materials and a balanced carbon to nitrogen ratio. Variety increases the types of microorganisms at work in your pile and your chances of obtaining a nutrient rich compost. Some folks think they don’t have enough organic material to build and maintain a compost pile. In addition to the leaves and grass clippings that we usually think of composting, there are numerous other suitable organic materials. Most of these materials are easy to find at home. Occasionally, it may be helpful to find free or cheap local sources of organics to add to a pile.

In contrast to those who worry about having enough materials, some folks want to put almost any type of organic material into their pile. While anything organic will eventually decompose, it may not belong in a backyard composting pile. It is important to be aware of these materials and the reasons they should be avoided. New and potential composters often have questions about what materials can be composted. A list of some commonly available materials is included in Table 2. Compostable materials that need special handling are mentioned in Table 3. Materials that should be avoided are named in Table 4.

Commonly Used Compostable Materials

As you are collecting materials around your yard and home, it may not be easy to determine if materials are higher in carbon or nitrogen. Tables showing carbon to nitrogen ratios for particular materials are helpful, but they usually only show a limited number of materials. A simple method (also described earlier in Lesson 2) for differentiating between materials is to remember that fresh, juicy materials are usually higher in nitrogen. In addition, materials of animal origin (such as feathers, manure, blood meal) are typically higher in nitrogen. Drier, older, or woody vegetable and plant tissues are usually higher in carbon. The following table helps to illustrate this point. The presence of a carbon, nitrogen, or oxygen in the C/N column indicates whether a material’s effect on compost would be carbonaceous (C), nitrogenous (N), or other (O). Materials designated as other (O) do not affect the C:N ratio.

Before adding food scraps and lake weeds to your composting pile, check with your municipality to make sure that there are no restrictions on their use.

TABLE 2.Partial Listing of Compostable Materials

MATERIAL C/N MATERIAL C/N
Bedding,herbivorous C & N Hair N
Blood meal N Hay C
Bone meal N Lake weeds N
Coffee grounds N Leaves C
Crushed egg shells O,alkalizer Lint N
Feathers N Manure N
Fruit N Paper(non-recyclable) C
Fruit peels and rinds N Peanut shells C
Garden debris, dried C Straw C
Garden debris, fresh C & N Pumpkins N
Grass clippings, dried C Vegetable scraps N
Grass clippings, fresh N Tea grounds and leaves N

Compostable Materials That Require Special Handling

There are a number of compostable materials that require special handling before they are put into a backyard pile. Some of the materials listed below may require extra preparation or they may need to be added in layers or small quantities. Other materials listed may cause difficulties with the composting process or negatively affect the final product. The comments are intended to help you decide whether to include these particular materials in your own pile.

TABLE 3. Compostable Materials Requiring Special Handling

MATERIAL C/N COMMENT
Cardboard (non-recyclable) C Slow to decompose. Shred into small pieces. If desired, put in water and add a drop of detergent to further speed decomposition.
Corn cobs and stalks C Slow to decompose. Run through shredder or chop into very small pieces, mix with nitrogen rich material.
Diseased plants C Diseases may be hard to eliminate. Sun-bake plants in plastic bag until thoroughly dried, or leave in hot pile (131°-140°F) at least one week, or burn and put ashes in pile, or omit from pile.
Grass clippings with chemicals C Pesticides and herbicides are a concern, degradability ranges from one to twelve months. Leave grass clippings on the lawn (best) or add to pile if material composts for at least 12 months or wait 2-3 weeks before using clippings from lawn after chemicals applied. Do not use clippings as a garden mulch for at least 2-3 weeks (or after 2 mowings) after chemical application.
Hedge trimmings C or N Slow to decompose. Thin layers of hedge trimmings can be used occasionally for roughage; chop twigs and branches into small pieces.
Lime O,
Alkalizer
Changes pile chemistry, causes nitrogen loss, and too much lime hurts bacteria and other microorganisms. Omit from pile or use very sparingly in thin layers if pile is going anaerobic (do not mix with manure).
Nut shells
- walnut, pecan
C Slow to decompose. Pulverize with shredder.
Peat moss O, low in nutrients Highly moisture absorbent, slow to decompose. Mix thoroughly with other materials, add in small quantities. If possible, soak peat moss in warm water before adding to pile.
Pine Cones C Slow to decompose. Shred or chop into very small pieces.
Pine needles C Slow to decompose. Mix thoroughly with other materials, add in small quantities.
Rhubarb leaves N Contains oxalic acid which lowers pH and inhibits microbial activity. Add in very small quantities, mix thoroughly with other materials or omit from pile.
Sawdust C Slow to decompose, can negatively affect aeration. Work into pile in thin sprinklings, mix with nitrogen rich material.
Sod N Slow to decompose. Break into small clumps, mix thoroughly with other materials or cover top of the pile with roots up, grass down (better in fall), or compost separately with roots side up, water thoroughly, cover with a dark tarp.
Soil O,
Activator source
Can make finished compost heavy. Add small quantities in thin layers as soil activator or omit from pile (finished compost produces the same results and typically weighs less).
Walnut leaves C Contain juglone which can be toxic to plants. Add in small quantities, mix thoroughly; toxin will biodegrade in 30 to 40 days.
Weeds, pernicious C Rhizomatous root system hard to kill. Sun-bake in plastic bag until thoroughly dried or omit from pile.
Weeds, other N Weed seeds hard to kill. Best to use when green and no seed heads present or leave in hot pile (131-140°F) at least one week.
Wood ashes O,
Alkalizer, potash
Changes pile chemistry, can cause nutrient imbalance. Use very sparingly in thin layers; do not use on top of pile or omit from pile.
Wood chips C Slow to decompose. Shred or chop into very small pieces; mix with nitrogen rich material.

Organic Materials To Avoid

Someday when your compost pile has shrunk and looks disappointedly small, you may scour your yard and home for organics to add to it. Some of those materials do not belong in your backyard compost pile. Table 4 lists materials to avoid along with the reasons for omitting them.

TABLE 4.Materials To Avoid Putting In A Home Compost Pile

MATERIAL COMMENT
Bones Very slow to decompose; can attract pests.
Cat litter May contain pathogens harmful to humans; may also contain chemicals to perfume litter.
Charcoal and briquettes Contain sulfur oxides and other chemicals that are toxic to soil and plants.
Cooked food waste May contain fats which attract animals; slow to decompose.
Dairy products May smell, take a long time to decompose, and attract pests (butter, cheese, mayonnaise, salad dressing, milk, yogurt, sour cream).
Dishwater May contain grease, perfume, and sodium.
Fatty, oily, greasy foods Slow to decompose; will putrefy and smell bad; can attract pests.
Fish scraps Can attract pests; smells bad during decomposition.
Meat Can attract pests; smells bad during decomposition.
Paper, glossy colored May contain inks that could contribute toxins to the pile.
Peanut butter Can attract pests; slow to decompose.
Pet wastes, human excrement May contain pathogenic bacteria, viruses, and parasites that require prolonged high temperatures to be destroyed.
Sludge (biosolids) Requires special handling and high temperatures to kill disease organisms and get rid of toxic metals; do not use unless product is sold in compliance with government regulations.

Compost Additives

There are a wide array of compost inoculants, starters, and activators sold in stores and mail order catalogs. Fortunately, compost additives are not required for successful composting. In some situations, certain additives can be helpful.

Inoculants contain special cultures of dormant bacteria and fungi. The theory behind using them is that they are supposed to introduce microorganisms, hasten the breakdown of materials in a compost pile and produce a better product. They are rarely needed because leaves, kitchen scraps, finished compost, and other organic materials already contain ample bacteria that work readily on their own.

Commercial "starters" or accelerators are supposed to help the decomposition process by adding nitrogen, enzymes, and bacteria to a pile. Some people feel better putting these products in their piles, but independent tests conducted to date have not shown significant benefits. Tests conducted at universities and private research stations showed that the best compost additives are finished compost or topsoil from your yard. (Store bought soil is sometimes sterilized so it does not always add microorganisms.)

Activators contain a nitrogen source. Activators include organic types (manure, blood meal, finished compost, soil) and artificial types (chemically synthesized compounds such as commercial nitrogen fertilizers). While activators are not necessary for successful composting, they can sometimes help if a pile is made from materials low in nitrogen. Nitrogen is usually the limiting nutrient in a pile that doesn't heat up or decay quickly enough. Some purists do not recommend using commercial nitrogen fertilizers as an activator, but if you have some readily available, it may be helpful. Avoid using ammonium sulfate as it may be toxic to earthworms. Keep in mind that chemical fertilizers are not as effective as organic sources because they contain no protein (which microorganisms use). Organic sources are better sources of nitrogen if you need to add an activator.

TABLE 5. Amounts of Various Nitrogen Sources Needed To Apply 0.15 Pounds (2.4 oz) Nitrogen

NITROGEN SOURCE % NITROGEN OUNCES TO APPLY
Ammonium nitrate 33 7.0
Calcium nitrate 15 16.0
Urea 46 5.2
Dried Blood 12 20.0
Fish meal 10 24.0

Source: Dickson, et. al. 1991

If additional nitrogen is needed, apply approximately 0.15 pounds actual nitrogen per 3 bushels (3 3/4 cubic feet) of carbon rich materials such as leaves. Table 5 lists estimated amounts of various nitrogen sources to add. For example, 7 ounces (about 1 cup) of ammonium nitrate is equivalent to 0.15 pounds. Authors of The Rodale Book of Composting recommend adding 2 to 3 pounds of organic nitrogen supplement (blood meal, manure, bonemeal, alfalfa meal) per 100 pounds of low nitrogen materials (for example, straw or sawdust).