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Composting

Derived from Backyard Composting Guide H-110. By George W. Dickerson. New Mexico State University © Cooperative Extension Service.

What is Compost?

  1. Compost is a dark, loose, partially decomposed, amorphous form of organic matter that reveals no hint of its origin.
  2. The actions of various microorganisms and invertebrates convert raw organic matter in a compost pile into finished compost.
  3. Maximum decomposition occurs when these organisms are exposed to an appropriate balance of oxygen, moisture and nutrients.

The Value of Compost

  1. Compost contains a broad base of both macro and micro nutrients. Most of these nutrients are bound in an organic form and are made available to plants slowly throughout the growing season.
  2. The major benefit of compost lies in its soil conditioning qualities.
  3. Compost improves the water-holding capacity of sandy soils and their ability to retain nutrients and release them to plant roots.
  4. In heavier clay soils, compost improves both water drainage and the ability of plant roots to penetrate the soil.
  5. Conditioning soil surface layers with compost also will improve water penetration and decrease soil erosion and soil crusting.
  6. Compost contributes organic acids that weather minerals of the soil, making nutrients available.
  7. Compost also improves the buffering capacity of the soil, helping stabilize soil pH levels.
  8. Compost attracts earthworms that aerate soil and make more nutrients available from deeper in the soil.
  9. Compost will also cool the soil and buffer soil water fluctuations.
  10. Yard and food waste makes up 30–40 percent of the solid waste stream. Backyard composting is one of the easiest ways to process yard wastes at the source.

Compost Use

  1. Compost is most frequently applied to garden soils in the spring before planting. Compost should be liberally applied and turned under to a depth of 4-6 in. Handfuls of compost also can be incorporated in planting holes for transplants.
  2. Compost makes an excellent top dressing for lawns, trees, shrubs and perennial flowers.
  3. Compost mulches around vegetables and other plants help reduce water evaporation from the soil and cut down on annual weed growth, as well as serving as a slow-release source of nutrients.

Meet the Composters

  1. Aerobic decomposition is the preferred composting technique in most backyard composting piles, because it smells better and is more efficient.
  2. Aerobic composting generates heat up to 140°F or higher, which is hot enough to kill most plant pathogens and weed seed in the pile when properly managed.
  3. Composting organisms include bacteria and invertebrates.
  4. Aerobic bacteria are the most important in terms of initiating decomposition and heat generation. Bacteria are one-celled colorless organisms that cannot make their own food through photosynthesis. They reproduce by splitting, producing billions of offspring over a relatively short time, although the life span of any particular generation may be only 20–30 minutes. As a group, they can eat almost any type of organic matter, although specific bacterial populations will differ from pile to pile depending upon the makeup of the pile and the decomposition stage.
  5. Populations of bacteria can be characterized by their favored temperature. Optimum temperature for psychrophilic bacteria is 55°F. The most efficient decomposers, the mesophilic bacteria, are most active when the temperature of the pile is at 70–100°F. Thermophilic bacteria (113°F to 155°F) are important for killing weed seeds and plant disease organisms. Thermophilic bacteria will decompose material rapidly, peaking in 2–3 days unless fed new material by turning the pile.
  6. Other important microorganisms include actinomycetes and fungi. Similar to bacteria and fungi, actinomycetes give the compost pile its faintly earthy odor. At the end of the composting process, they may appear as a blue-gray to light green powdery or cobweb-like layer in the outer 4–6 in. of compost. Fungi generally intermingle with the actinomycetes.
  7. When the inner pile starts to heat up, most invertebrates are killed or migrate to cooler areas of the pile. In the cooler areas, nematodes prey upon bacteria, protozoa and fungal spores. Larger mites and springtails also feed on fungi.
  8. The life cycle within the pile continues to become more complex as predaceous mites and pseudoscorpions feed on other mites as well as nematodes. Complex invertebrates like centipedes and ground beetles feed on lower life forms, and decaying plant life in the pile attracts sowbugs, snails, slugs and earthworms.

Composting Secrets

Table 1. Selected carbon:nitrogen ratios (by weight).
Material Low/High C:N ratio
Grass clippings Low 14:1
Food scraps Low 15:1
Vegetable wastes Low 16:1
Coffee grounds Low 20:1
Cow manure Low 18:1
Horse manure Low 25:1
Poultry manure Low 15:1
Leaves High 30–80:1
Straw High 40–100:1
Paper High 170–200:1
Sawdust High 200–500:1
  1. A compost pile that heats up properly and decomposes rapidly depends on a number of environmental factors within the pile.
  2. One of the most critical factors is the balance of nitrogen and carbon within the plant and animal wastes added to the pile, commonly expressed as “C:N” or “C/N” ratio.
  3. Microorganisms require carbon for energy, while they need nitrogen for protein synthesis.
  4. Decomposition within the compost pile is optimal when the C:N ratio is about 30:1.
  5. Excess nitrogen in the pile results in the production of ammonia when there is not enough carbon in the pile for microorganisms to synthesize new cellular material.
  6. When there is not enough nitrogen to produce proteins for microorganism growth, the composting process slows.
  7. Most dry or woody materials are high in carbon, while green wastes or livestock manures are relatively low (table 1). They must be balanced to produce the ideal of 30:1
  8. Decomposition within the pile can be aided by increasing the surface area of organic material added to the pile. Microbial activity occurs mostly on the surface of waste.
  9. One of the most efficient ways to increase the surface area of organic waste is to run it through a shredder or chipper or run it over with a rotary mower.
  10. Aeration can be accomplished by turning the compost pile periodically. Much of the air available to microorganisms comes from air trapped in it when the pile is turned.
  11. Aeration also depends on the amount of water in the pile and the size of spaces between particles in the pile. These spaces can hold both water and air. As water contact increases, possibilities for air contact decrease. Mixing coarse dry materials like autumn leaves with wet green materials like grass trimmings will increase the airflow through the pile.
  12. A garden fork or pitchfork is one of the best tools to turn or fluff up a pile. Compost piles can also be constructed so that they “breathe” on one or more sides.
  13. Moisture content of the pile should range between 40 and 60 percent. A higher percentage will increase anaerobic decomposition, while a lower moisture content will slow down the composting process as microorganisms die or become dormant.
  14. Squeeze compost in your hand to judge moisture content. If the material feels like a damp sponge and yields only a few drops when squeezed tightly, its moisture content is sufficient.
  15. Piles that are too wet should be turned to increase air content; piles that are too dry should be turned and sprinkled with a hose.

Building A Compost Pile

Table 2. Wastes to include / exclude from the compost pile.
Include Exclude
Weeds (annual) Weeds (perennials)
Sawdust or Paper Diseased plants
Grass clippings, leaves or straw Cat, dog or pig manure
Vegetable and Fruit wastes Meat
Tree and brush trimmings (shredded) Milk and cheese products
Egg shells or Coffee grounds Fat, oils and lards
Livestock manure (except pig)  
  1. Locate compost piles in an inconspicuous but convenient location close to a water source. Locating the pile in a shaded area will keep it from drying out.
  2. Minimum dimensions for a pile are 3 x 3 x 3 ft, as smaller piles will not heat up or decompose.
  3. The pile should be large enough to hold the heat that it generates, yet small enough to allow air movement into the center of the pile. Piles taller than 5 ft are difficult to turn and may restrict air flow due to weight.
  4. Two or more compost piles or bins are convenient when turning the material, while multiple bins initiated at different times provide for compost at different stages of decomposition.
  5. The most popular composting technique involves alternating layers of organic materials, soil, and fertilizer or manure. Layers can be laid down in either specially constructed bins or piles.
  6. Structures to hold the compost can be made of wire fencing, cinder blocks, wooden slats or pallets, or a rotating barrel.
  7. A 6–8 in. layer of coarse organic material, such as shredded corn cobs or sawdust, is put down first to provide aeration from the bottom. This material will also help absorb leachate that trickles down through the pile.
  8. Sprinkle a thin layer of a complete fertilizer (1 cup of 12-12-12 or equivalent per 25 square ft of surface area) over the layer of organic material. Fertilizers keep P and K in balance with the N and provide nitrogen necessary to reduce the high C:N ratios associated with coarse materials like corncobs and sawdust.
  9. A 1–2 in. layer of poultry, rabbit or livestock manure can be substituted for the fertilizer. Horse manure mixed with bedding materials is an excellent source of nitrogen and microorganisms that start the composting process. A layer of soil will also introduce microorganisms into the pile.
  10. Wet grass clippings also can be mixed with drier, coarser materials for a source of nitrogen. Be careful not to layer wet grass clippings in the pile as they tend to mat, and matted wet clippings become sticky masses of slime through anaerobic decomposition, restricting air and water movement in the pile.
  11. Continue to layer organic materials, fertilizer or manure and soil until the pile is 3–5 ft high. Be sure to moisten each layer with water as it’s laid down. This is particularly important for drier, coarser materials.
  12. Almost all plant materials can be used in a compost pile.
  13. Whenever possible, select plant materials for composting that will balance high C:N ratio materials with low C:N ratio materials. Compost piles made of mostly high C:N ratio materials may require extra nitrogen fertilizer or livestock manure to reduce the average C:N ratio for the pile.
 

Maintaining and Troubleshooting A Compost Pile

  1. Proper, uniform moisture levels for optimum microbial activity can best be maintained by turning the pile. Soggy piles will revert to anaerobic decomposition and need to be fluffed up. Aerobic microbial activity slows in dry piles; thus, when turning dry piles, moisten the compost.
  2. Generally, the more compost is turned the faster it decomposes. Frequently turned piles can yield finished compost in as little as 14 days. Piles turned less frequently may take a year. The time required to develop finished compost also depends on the size of the pile and time of year or air temperature.
  3. When turning, place compost on the outside edges of the pile or bin in the middle of the new pile so that it can heat up in the composting process. The finished compost pile should be half the size of the original pile.
  4. The compost pile may fail to heat up and decompose properly for a number of reasons. Consult table 3 for common symptoms, problems and solutions.
Table 3. Troubleshooting composting problems.
Symptom Problem Solution
Rotten egg smell Insufficient air; excess moisture Turn pile and incorporate coarse organic matter (sawdust, leaves)
Ammonia smell Organic material too high in nitrogen Add coarse organic material (sawdust, leaves)
Pile does not heat up Pile too small Add more organic matter
Pile does not heat up Insufficient moisture Turn pile and add water
Pile does not heat up Lack of nitrogen Incorporate manure, fertilizer or low C:N ratio plant material (lawn clippings)
Pile does not heat up Poor aeration Turn pile
Pile does not heat up Cold weather Increase pile size or insulate with straw