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Extension > Garden > Yard and Garden > Soils > Composting and mulching guide > Requirements for efficient decomposition

Requirements for efficient decomposition

Decomposition of organic material in the compost pile depends on microbial activity. Any factor that slows or halts microbial growth will also impede composting. Efficient decomposition occurs when the following factors are used to fullest advantage.

Aeration: Microbes need oxygen for efficient decomposition of organic wastes. Some decomposition will occur in the absence of oxygen (anaerobic conditions); however, the process is slow and foul odors may develop. Because of the odor problem, composting without oxygen is not recommended in a residential setting unless the process is conducted in a fully closed system (see plastic bag method on page 5, Composting Structures). The oxygen content inside the pile should ideally range from 16.5% to 20.9%. The key to adequate oxygen is "free air space" within the pile. Initial free air space of 55% to 65% by volume has been proven to work well. Free air space can be measured with a five-gallon bucket and a one-gallon milk jug as shown below.

How to measure free air space in your compost pile: The five gallon bucket test

Materials needed:

  1. Check the volume of your five-gallon pail by filling the one-gallon jug and emptying it into the five-gallon pail five times. Mark the five-gallon "full line" on the pail.

    2. Fill the five-gallon pail one-third full with a typical mix of compost materials and drop the pail ten times from a height of six inches onto a cement floor or sidewalk (being careful to keep all the material in the pail).

    3. Add compost to fill the five-gallon pail two-thirds full and drop the pail ten times from a height of six inches onto a cement floor or sidewalk.

    4. Add compost to fill the five-gallon pail up to the "full line" and drop the pail ten times from a height of six inches onto a cement floor or sidewalk.

    5. Add compost to fill the five-gallon pail to the "full line."

    6. Now add and keep track of the amount water you can add to the five-gallon pail before it overflows.

    • If you can add 2.75 to 3.25 gallons of water to the five-gallon pail without it spilling over the top, you have adequate free air space. Your initial free air space is correct.
    • If you cannot add at least 2.75 gallons of water to the five-gallon pail without it spilling over the top, you have inadequate free air space. Add more bulking material like straw, coarse wood chips, or shredded bark.
    • If you can add more than 3.25 gallons of water to the five-gallon pail without it spilling over the top, you have too much free air space and you need to reduce the particle size. This can be done by grinding or shredding the materials or by adding finer materials to the mix.

    7. Make the needed corrections and retest until the test shows the correct initial free air space.

Moisture: Adequate moisture is essential for microbial activity. Dry yard waste will not decompose efficiently. If rainfall is limited you will need to water the pile periodically to maintain a steady decomposition rate. Add enough water to completely moisten the pile, but avoid overwatering. Excess water can lead to anaerobic conditions that slow down the degradation process and cause foul odors. Water the pile so that it is damp, but does not remain soggy. Approximately 50% to 55% moisture is a good starting point.

Particle size: Smaller particles have much more surface area that can be attacked by microbes. Organic material larger than 2 inches will be slow to compost. A shredder can be used before putting material in the pile, and is essential if brush or sticks are to be composted. A low-cost method of reducing the size of fallen tree leaves is to mow the lawn before raking. If the mower has an appropriate bag attachment, the shredded leaves can be collected directly. In addition to speeding up the composting process, shredding reduces the volume of the compost pile. The concern for maximum exposed surface area must always be balanced against the need for adequate free air space. Particles shredded too small will tend to pack and exclude oxygen. Initial free air space of 55% to 65% needs to be established. Free air space can be increased by adding larger pieces to the compost material such as wood chips or shredded bark. To reduce free air space, smaller pieces can be added to the mixture or the original material can be reduced in size by grinding or shredding.

Nutrients: Microbial activity is affected by the carbon-to-nitrogen (C/N) ratio of the organic waste. Because microbes need nitrogen for their own metabolism and growth, a shortage of nitrogen will slow down the composting process considerably. Material high in carbon relative to nitrogen, such as straw or sawdust, will decompose very slowly unless nitrogen fertilizer is added. Tree leaves are higher in nitrogen than straw or sawdust, but decomposition of leaves still benefits from an addition of nitrogen fertilizer. Grass clippings are generally high in nitrogen and enhance decomposition when mixed properly with leaves. Manure, cottonseed meal, or blood meal can be used as organic sources of nitrogen. Otherwise use a high nitrogen- containing fertilizer. You need an initial C/N ratio of about 30 parts carbon to 1 part of nitrogen. C/N ratios below 25:1 may give off ammonia odors and above 35:1 will take longer to compost. Other nutrients such as phosphorus and potassium are necessary, but are usually present in adequate amounts for decomposition.

Turning or mixing: Mixing the pile once or twice a month will blend the cold outer edges of the pile into the warmer, more active center of the pile and significantly hasten the composting process. A pile that is not mixed may take three to six times longer before it can be used. A well-mixed compost pile will also reach higher temperatures for longer time periods, which will help destroy more weed seeds and pathogens. Turning a compost pile adds oxygen to the pile, but this effect is often short-lived, lasting from fewer than 24 hours to fewer than 30 minutes. Turning a compost pile simply cannot overcome compost material that does not have enough free air space. To have adequate oxygen in the pile you must have adequate free air space in the material being composted.

During the initial stages of decomposition, organic acids are produced and the pH drops. Some sources suggest adding small amounts of lime to maintain and enhance microbial activity at this time. However, high rates of lime will convert ammonium-nitrogen to ammonia gas, which in turn will lead to the loss of nitrogen from the pile. Research has shown that the loss of nitrogen from the pile often offsets the benefits of lime1. In general, lime is not necessary for degradation of most yard wastes. The pH of finished compost is usually slightly alkaline without the addition of lime. In many areas, the water used to moisten the compost pile is alkaline and may also help to increase the pH of the compost. If large quantities of acid materials such as pine needles, spruce needles, or fruit wastes are composted, additional lime may be necessary.

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