Environmental Factors

Microbes are working on the surface of the decomposing materials and are easily subject to drying out, temperature fluctuations, and oxygen depletions.

The compost pile should be kept moist but not soaked. This environment provides comfortable and conducive conditions for microorganisms, mainly bacteria and fungi, to accomplish the decomposition process.

Moisture Content

Aerobic decomposition proceeds best between 40% and 70% moisture with good aeration. A high moisture content must be avoided because water displaces air from between the particles and gives rise to anaerobic conditions which limit composting organisms. However, too low a moisture content deprives the organisms of the water needed for their metabolism, and, consequently, also inhibits their activity.

Maximum moisture content for satisfactory aerobic composting will vary with the materials used. If the material contains considerable amounts of straw and strong fibrous material, the maximum moisture content can be much greater without destroying the structural qualities or causing the material to become soggy, compact, and unable to contain enough air in the interstices. But if the material has little structural strength when wet, or if it is granular, like ash and soil it may be difficult to maintain aerobic conditions at a moisture content above 70%.

chap2_7If the moisture content falls much below 40%, many of the organisms will cease to function. Moisture content above 70% may cause the pile to go anaerobic, thereby producing foul odors. Also, nutrients may be leached and decomposition will be very slow at the higher moisture content. When the composting materials are picked up in the hand and squeezed, just a few drop of water should come out. If excess water comes out, the pile is too wet. The pile should be turned to remove excess water, to loosen materials and to oxygenate the pile. Also, the addition of dry materials will soak up the excess moisture.

In hotter climates, covering a compost pile will help retain moisture. Also, covering compost piles in the summer may conserve moisture, while in very rainy winter areas covering the pile may help to keep it from becoming too soggy.

If the compost pile is not in a structure and is left freestanding, it will develop an “A” shape. When this occurs water will be shed just like old haystacks and the pile may become dry. An unstructured pile will need to be checked for dryness more often than a structured pile.


Microorganisms, which cause the generation of heat in a compost pile, are classified into two categories: mesophilic, those that live, function and reproduce in temperatures between 50° -113°F, and thermophilic, those that thrive between 113° – 158°F.

The term “critical mass” is often used in conjunction with heating of compost piles. This term has to do with the size of the compost pile. The minimum size for proper composting to occur is usually 1 cubic yard (3′ X 3′ X 3′). This size allows heating to occur;anything less in size will not have enough mass to generate internal temperatures high enough.

Proper temperature is a very important factor, particular in the aerobic composting process. In Georgia, the winter months temperatures will slow the process but most of the year temperatures will be sufficiently high enough for the intense microbial activity to take place.

Considerable amounts of heat are released by aerobic decomposition. Since the composting material has relatively good insulation properties, a sufficiently large composting mass will retain the heat of the biological reaction and high temperatures will develop.

High temperatures are essential for the destruction of pathogenic organisms and undesirable weed seeds. Decomposition also proceeds much more rapidly in the thermophilic temperature range. The optimum temperature range is 135°-175°F, with 150°F usually being the most satisfactory. Since only a few of the thermophilic organisms actively carry on decomposition above 170°F, it is undesirable to have temperatures above this for extended periods.

Although the eggs of parasites, cysts nematodes and flies are usually destroyed in a short time at temperatures above 135°F, these eggs and cysts have been found to survive in cooler parts of compost stacks for days though the temperature in the interior of the stack is above 135°F. Turning the pile exposes cooler materials to the interior heat of the pile. All the material should be subjected to a temperature of at least 150°F.

High temperatures vaporize ammonia, produced when the C:N ratio is low. Any small nitrogen loss due to high temperature is outweighed by the advantages of destroying pathogenic organisms and weed seeds, controlling flies, and providing better decomposition.

A drop in temperature in the compost pile before the material is stabilized indicates that the pile is becoming anaerobic and should be aerated (high temperatures do not persist whenthe pile becomes anaerobic). The temperature curve for different parts of the pile varies somewhat with the size of the pile, the ambient (surrounding) temperature, the moisture content, the degree of aeration, and the character of the composting material. Aerobic conditions, however, are important in maintaining high temperatures during decomposition. The size of the compost pile or windrow may be increased to provide higher temperatures in cold weather or decreased to keep the temperatures from becoming too high in warm weather. Experience will show that turning or aeration releases the heat of compost piles which have become so hot (170°-180°F) that bacterial activity is inhibited. When the material is actively decomposing, the temperature, which falls slightly during turning, will return to the previous level in two or three hours. However, it is usually not effective to reduce temperatures by watering the material because the mass may become waterlogged.

Variations in the moisture content between 30% and 70% have little effect on the maximum temperature in the interior of the pile. The initial temperature will rise a little more rapidly when the moisture content is 30% to 50% than when it is 70%. Studies, however, show an important and significant correlation between the moisture content and the temperature distribution within the pile. When the moisture content is high, the temperature near the surface will be higher, and the high temperature zone will extend nearer to the surface than when the moisture content is low. For example, in experiments at the University of California during mild weather when the air temperature fluctuated between 50° and 80°F, the zone of maximum temperature in a pile with a moisture content of 61% extended to within about one inch of the surface while the maximum temperature zone in a pile containing 40% moisture began 6 inches below the surface.

It is seen that the deeper pile caused higher temperatures and better temperature distribution and subjected a much higher proportion of the material to a high temperature at any one time. Hence, the actual mass of the material evolving heat is important in providing adequately high temperatures.

Shredding or pulverizing the material will also provide better temperature distribution and less heat loss. However, materials with a high C:N ratio or containing large amounts of ash or mineral matter will usually attain high temperatures more slowly in the compost pile.

Aeration and Oxygen

The compost pile must have a structure that allows for the movement of air. Sufficient air space within the organic material ensures an adequate oxygen supply, the removal of carbon dioxide and uniform moisture content throughout the organic material. Research indicates that 30%-35% of the initial volume of a pile should be made up of air spaces for optimum composting to occur. Bulking materials may need to be added to increase the porosity of fine-textured materials such as soil and livestock manure. Once air space has been provided, the finer the organic material is in structure, the more surface area will be immediately exposed to the microorganisms. The concern for maximum exposed surface area must always be balanced with the requirement for adequate free air space.

chap2_8Keep over 5% oxygen throughout the entirepile. Typical oxygen percents range from 6% -16% in the pile and 20% at the exposed portions of the pile. Failure to keep all parts of the compost pile above the 5% oxygen level will cause the pile to “go anaerobic”, with the accompanying odor problems. The more oxygen, the more quickly the composting will take place. 

Aeration is also useful in reducing a high initial moisture content in composting materials. Several different aeration techniques have been utilized with varying degrees of success. Turning the material is the most common method of aeration when composting is done in piles. Hand turning of the compost piles is most commonly used for small garden operations. The most important consideration in turning compost, apart from aeration, is to ensure that the material of the outside of the pile of units is turned into the center where it will be subject to high temperatures. If desired, piles can be combined when they are being turned, particularly if long composting periods are used.

The frequency of aeration or turning and the amount of aeration or the total number of turns are governed primarily by the moisture content and the type of material, the moisture content being the most important. A high moisture content reduces the pore space available for air as well as reducing the structural strength of the material. This permits greater compaction and less interstitial or air space in the pile. Materials with a high C:N ratio may not have to be aerated as often as material which decomposes more actively and rapidly.

Studies at the University of California indicate that turning at fairly frequent intervals during the first 10 to 15 days of composting achieved approximately the same degree of stabilization as making the same number of turns over a longer period. Greater aeration during the initial stages of decomposition intensifies the activity of the microorganisms, shortens the period of active decomposition, and, consequently, reduces the time needed for composting.

Experience soon enables the composter to estimate the need for adding water and the need for turning. A good rule of thumb is to turn the pile daily if foul odors of anaerobic and putrefactive conditions are evident when the pile is disturbed either by turning or by digging into it for inspection purposes. The pile should be turned daily until odors disappear. No matter how anaerobic a pile may become, it will recover under a schedule of daily turning which reduces the moisture and provides aeration. A temperature drop during the first 7 or 10 days of composting is a good indication that turning for aeration is necessary.

Daily turning apparently inhibits the development of molds and actinomycetes, characteristic of piles disturbed less often. In piles turned daily these organisms develop only sporadically. This effect of daily turning is probably due to the daily exposure of the microorganisms to inhibitory temperatures within the interior of the pile. Such exposure prevents their accumulation in the cooler outer shell.

In summary, the avoidance of anaerobic conditions, and the maintenance of hightemperatures, are the important criteria for the degree of aeration.