There are several factors, some of them interdependent, which are fundamental in planning a composting project or in analyzing composting operations. Some of the methods of composting may be used most economically under different conditions. Analyzing methods in the light of fundamental factors permits: (a) selecting established procedure best for the particular circumstances, (b) selecting different techniques from different established procedures, or (c) developing other methods to economically meet requirements of the individual situation.
When organic material decomposes with oxygen, the process is called "aerobic." When living organisms, which use oxygen, feed upon the organic matter, they develop cell protoplasm from the nitrogen, phosphorus, some of the carbon, and other required nutrients. Carbon serves as a source of energy for organisms and is burned up and respired as carbon dioxide (CO2
). Since carbon serves both as a source of energy and as an element in the cell protoplasm, much more carbon than nitrogen is needed. Generally about two-thirds carbon is respired as CO2
, while the other third is combined with nitrogen in the living cells. If the excess of carbon over nitrogen in organic materials being decomposed is too great, biological activity diminishes. Several cycles of organisms are required to burn most of the carbon. When organisms die, their stored nitrogen and carbon become available to other organisms. Nitrogen use from dead cells by other organisms forms new cell material and again requires burning excess carbon to CO2
. Thus, the amount of carbon is reduced and the limited amount of nitrogen is recycled. Finally, when the ratio of available carbon to available nitrogen is low enough, nitrogen is released as ammonia. Under favorable conditions, some ammonia may oxidize to nitrates. Phosphorus, potash, and various micro-nutrients are also essential for biological growth. These are normally present in more than adequate amounts in compostable materials.
The aerobic process is most common in nature, such as the forest floor, where droppings from trees and animals are converted into a relatively stable humus or soil manure. There is no accompanying smelling nuisance when adequate oxygen is present. A great deal of energy is released as heat in the oxidation of carbon to CO2
. For example, if a gram molecule of glucose is dissimulated under aerobic conditions, 484 to 674 kilogram calories (kcal) of heat may be released. If organic material is in a pile or is otherwise arranged to provide some insulation, temperatures during decomposition will rise to over 170º Fahrenheit. If the temperature exceeds 162º to 172º Fahrenheit, however, the bacterial activity is decreased and stabilization slows. When temperatures exceed about 120º Fahrenheit, thermophilic organisms, which grow and thrive in the temperature range 115º to 160º Fahrenheit, develop and replace the mesophilic bacteria in the decomposition material. Mesophilic organisms live in temperatures of 50º to 115º F. Only a few groups of thermophiles are active above 160º. Oxidation at thermophilic temperatures takes place more rapidly than at mesophilic temperature and, hence, a shorter time is required for stabilization.
High temperatures destroys pathogenic bacteria and protozoa (microscopic one celled animals), and weed seeds, which are detrimental to health and agriculture when the final compost is used on the land.
Aerobic oxidation does not stink. If odors are present, either the process is not entirely aerobic or there are materials present, arising from other sources than the oxidation, which have an odor. Aerobic decomposition or composting can be accomplished in pits, bins, stacks, or piles, if adequate oxygen is provided. Turning materials or other techniques for adding oxygen are necessary to maintain aerobic conditions.
Putrefactive breakdown of organic material takes place anaerobically. Organic compounds break down by the action of living anaerobic organisms. As in the aerobic process, the organisms use nitrogen, phosphorus, and other nutrients in developing cell protoplasm but reduce organic nitrogen to organic acids and ammonia. Carbon from organic compounds, which is not used in the cell protein, is liberated mainly in the reduced form of methane CH4
. A small portion of carbon may be respired as CO2
This process takes place in nature, such as decomposing organic mud at the bottom of marshes and in buried organic materials with no access to oxygen. Marsh gas, which rises, is largely CH4
. Intensive reduction of organic matter by putrefaction is usually accompanied by odors of hydrogen sulfide and of reduced organic compounds which contain sulfur, such as mercaptans (any sulfur-containing organic compound).
Since anaerobic destruction of organic matter is a reduction process, the final product, humus, is subject to some aerobic oxidation when put on the soil. This oxidation is minor, takes place rapidly, and is of no consequence in the utilization of the material on the soil.
There is enough heat energy liberated in the process to raise the temperature of the putrefying material. In the anaerobic dissimulation of the glucose molecule, only about 26 kcal of potential energy per gram-molecule of glucose are released compared to 484 to 674 kcal for aerobic decomposition. The energy of the carbon is in the CH4 released. If the CH4
is burned to CO2
, large amounts of heat are involved. In many instances, the energy of the CH4 from an aerobic destruction of organic matter is utilized in engines for power and burned for heat.
Since there is no significant release of heat to the mass in anaerobic composting, this could pose a problem for treatment of contaminated materials. High temperatures are needed to destroy pathogens and parasites. High temperatures do not play a part in the destruction of pathogenic organisms in anaerobic composting. The pathogenic organisms do disappear in the organic mass, because of the unfavorable environment and to biological antagonisms. The disappearance is slow and the material must be held for periods of six months to a year to ensure relatively complete destruction of Ascaris eggs. Ascaris are nematode worms that can infest the intestines. These are the most resistant of the fecal-borne disease parasites in wastes.
Anaerobic composting may be accomplished in large, well packed stacks or other composting systems containing 40% to 75% moisture, into which little oxygen can penetrate, or in composting systems containing 80% to 99% moisture so that the organic material is a suspension in the liquid. When materials are composted anaerobically in this way, not covered with water, the odor nuisance may be quite severe. However, if material is kept submerged, gases dissolve in the water and are usually released slowly into the atmosphere. If the water is replaced from time to time when removing some of the material, no serious nuisance is created.
While composting can be either aerobic or anaerobic, some bacteria can grow under either condition but may grow better under one condition. Compost piles under aerobic conditions attain a temperature of 140o to 160o F in one to five days depending upon the material and the condition of the composting operation. This temperature can also be maintained for several days before further aeration. The heat necessary to produce and maintain this temperature must come from aerobic decomposition, which requires oxygen. After a period of time, the material will become anaerobic unless it is aerated. There is probably a period between the times when the oxygen is depleted and anaerobic conditions become evident, during which the process is aerobic.
"Aerobic composting" requires a considerable amount of oxygen and produces none of the characteristic features of anaerobic putrefaction. In its modern sense, aerobic composting can be defined as a process in which, under suitable environmental conditions, aerobic organisms, principally thermophilic, utilize considerable amounts of oxygen in decomposing organic matter to a fairly stable humus
The term "anaerobic composting" is used to describe putrefactive breakdown of the organic matter by reduction in the absence of oxygen where end products such as CH4
S are released.
2. GRINDING OR SHREDDING THE MATERIAL
Shredding or grinding raw materials can produce several beneficial results, particularly when composting fibrous materials such as leaves, woody plants or corn stalks. Shredding exposes a greater surface area, which makes it more susceptible to bacterial invasion. A piece of wood, a pile of corn stalks or leaves packed together does not decompose quickly in a compost pile. Sufficient oxygen is not available in the center of such objects to permit aerobic and more rapid decomposition.
Shredding material makes it more homogenous, produces beneficial initial aeration, and provides a structure which makes material more responsive to moisture control and aeration as well as moving and handling. Shredded refuse heats more uniformly. It withstands excessive drying at the surface of the pile, is insulated against heat loss, and resists moisture penetration from rain better than does unshredded refuse. Fly control is easier when refuse is pulverized or shredded. Compost users find that shredded or ground material can be applied more readily and uniformly to the land.
The best size of particles for composting is less than 2 inches in the largest dimension, but larger particles can be composted satisfactorily. The particle size of material being composted is related to the finished product requirements and by economics. If the material is to be used on lawns or flower gardens, compost should be screened through a one-inch screen so it looks better and is easier to apply and work into the soil.
It may not be worth the added cost and labor to shred the material. Any particles that are too large can be forked or screened out and broken up when necessary. Individual farmers or gardeners are not necessarily particular about uniformity of compost structure when preparing the compost. Nor is uniformity as important for agriculture fields as for the hobby gardener.
Initial shredding of all material is not necessary in the composting operation. Often, the best practice is to limit initial shredding to only large pieces of organic materials. Some compost operators believe that permitting some larger irregular pieces to remain creates greater air spaces in the mass and hence more entrapped oxygen.
Vegetative and herbaceous matter should not be ground because it becomes soggy. The high moisture content of these materials makes it unsatisfactory for aerobic composting. The time of shredding or grinding is determined by the raw material to be composted, but it need not be a difficult operation. Regrinding can be done either after the compost is mature or ready for use, or near the end of the maturing process. Regrinding near the end of the period of active decomposition would serve as the last turn for aeration, and remaining stabilization would take place in large stockpiles.
Whether grinding or shredding should be practiced or not depends upon the nature of the raw material, the desired features of the final product, such as the appearance, size, quality, and the economic requirements of the operation. Shredding and grinding the materials will shorten the decomposition time.
3. CARBON-NITROGEN RELATIONSHIPS
The course of decomposition of organic matter is affected by the presence of carbon and nitrogen. The C:N ratio represents the relative proportion of the two elements. A material, for example, having 25 times as much carbon as nitrogen is said to have a C:N ratio of 25:1, or more simple a C:N ratio of 25. Actually, the ratio of available carbon to available nitrogen is the important relationship because there may be some carbon present in the form so resistant to biological attack that its presence is not significant.
Decomposition of organic matter is brought about by living organisms that use the carbon as a source of energy and nitrogen for building cell structure. More carbon than nitrogen is needed. If the excess of carbon is too great, decomposition decreases when the nitrogen is used up and some of the organisms die. Their stored nitrogen is then used by other organisms to form new cell material. In the process more carbon is burned. Thus the amount of carbon is reduced to a more suitable level while nitrogen is recycled. More time is required for the process, however, when the initial C:N ratio is much above 30.
In the soil, another factor enters into the nitrogen cycle series and occurs when carbon is in great excess - the presence of nitrogen in the soil in a form available to bacteria. In this case too great a C:N ratio will result in living microbial cells making maximum use of available carbon by drawing on any available soil nitrogen in the proper proportion. This is known as "robbing" the soil of nitrogen and has the effect of delaying the availability of nitrogen as a fertilizer for growing plants, until some later season when it is no longer being used in the life-cycles of soil bacteria. When the energy source, carbon, is less than that required for converting available nitrogen into protein, organisms make full use of the available carbon and get rid of the excess nitrogen as ammonia. This release of ammonia to the atmosphere produces a loss of nitrogen from the compost pile and should be kept to a minimum.
A C:N ratio of 20, where C and N are the available quantities, has been widely accepted as the upper limit at which there is no danger of robbing the soil of nitrogen. If a considerable amount of carbon is in the form of lignin or other resistant materials, the actual C:N ratio could be larger than 20. In view of the importance of preventing robbery of nitrogen from the soil and of conserving maximum nitrogen in the compost, the C:N ratio is a critical factor in composting.
Since living organisms use about 30 parts carbon for each part of nitrogen, an initial C:N (available quantity) ratio of 30 would seem most favorable for rapid composting and would provide some nitrogen in an immediately available form in the finished compost. Researchers report optimum values from 20 to 31. A majority of investigators believe that for C:N ratios above 30 there will be little loss of nitrogen. University of California studies on materials with a initial C:N ratio varying from 20 to 78 and nitrogen contents varying from 0.52% to 1.74% indicate that initial C:N ratio of 30 to 35 was optimum. These reported optimum C:N ratios may include some carbon which was not available. The composting time will increase considerably with increases in the C:N ratio above the range 30 to 40. If the unavailable carbon is small, the C:N ratio can be reduced by bacteria to as low a value as 10. Fourteen to 20 are common values depending upon the original material from which the humus was formed. These studies showed that composting a material with a higher C:N ratio would not be harmful to the soil, however, because the remaining carbon is so slowly available that nitrogen robbery would not be significant.
CARBON NITROGEN (C:N) RATIOS
Plant residues are made up largely of the following:
1. sugar, starch, simple proteins (decompose rapidly)
2. crude protein (decompose slowly)
3. hemicellulose (decompose slowly)
4. cellulose (decompose slowly)
5. lignin, fat, wax, etc. (decompose slowly)
Rate of decay and release of nutrients to the soil vary greatly. Likewise, demands of the living soil microorganisms vary as they "break down" plant residue. Sawdust (made primarily of lignin and cellulose) uses vast amounts of energy to maintain the lives of microorganisms digesting it. A major product of plant decay is nitrogen (N) while the undigested portion is primarily carbon (C).
The optimum ratio in soil organic matter is about 10 carbons to 1 nitrogen, or a C:N ratio of 10:1.
Following are some sample C:N ratios or organic matter:
Sandy loam (fine) 7:1
Food scraps 15:1
Alfalfa hay 18:1
Grass clippings 19:1
Rotted manure 20:1
Sandy loam (coarse) 25:1
Vegetable trimmings 25:1
Oak leaves 26:1
Leaves, varies from 35:1 to 85:1
Peat moss 58:1
Corn stalks 60:1
Pine needles 60:1 to 110:1
Farm manure 90:1
Alder sawdust 134:1
Sawdust weathered 3 years 142:1
Douglas fir bark 491:1
Sawdust weathered 2 months 625:1
4. BLENDING OR PROPORTIONING MATERIALS
Most home compost makers, or experienced compost operators, will judge by appearance what composition of the material will provide good compost. An experienced operator can generally do proportioning, when desirable, from visual estimates of the quantity and character of the materials. In large-scale municipal composting operations, however, there may be times when operators rely on laboratory analyses to determine how the various materials should be blended or proportioned for composting.
The C:N ratio and moisture content are the two factors to be considered in blending. There is no need for blending when the C:N ratio is between 25 and 50, although 30 to 40 is a better range. If materials containing much paper, straw, sawdust, or other substances rich in carbon are to be composted, the C:N ratio materials should be proportioned to provide a near optimum C:N ratio. Similarly, materials too dry for good composting and materials too wet to compost without odors should be blended in proper proportions. Where initial shredding is practiced, proportioning can usually be done at the shredder; otherwise, the materials are mixed and placed in stacks or pits together.
Some compost operators add soil to the organic materials with the idea of increasing the number of microorganisms and thus expediting composting. But organisms necessary for decomposition are indigenous to the organic materials, and those added in the soil will have no significant effect. Dry soil is also sometimes added to reduce the moisture content and to absorb ammonia in low C:N ratio materials. This procedure is fine if sufficient dry organic materials are not available, but the efficiency of nitrogen reclamation by the addition of soil is not great. The addition of cellulose organic matter to provide a C:N ratio above 30 is much more efficient. Soil may be added to compost if the materials have a high acidity content, to neutralize acid conditions. It may be added to improve the appearance of the finished compost, to give it a more granular texture, and to increase the ease of handling by giving the compost more body.
Adding soil to the compost pile might make the mass heavier to work with. Also there is a danger of adding more weight to the materials, and the weight could add to less air penetration.
Aerobic decomposition can proceed at any moisture content between 30% and 100% if adequate aeration can be provided. In practical aerobic composting, however, a high moisture content must be avoided because water displaces air from the interstices between the particles and gives rise to anaerobic conditions. On the other hand, too low a moisture content deprives the organisms of the water needed for their metabolism, and inhibits their activity.
Maximum moisture content for satisfactory aerobic composting varies with materials used. If it contains considerable amounts of straw and strong fibrous material, the maximum moisture content can be much larger without destroying structural qualities or causing material to become soggy, compact, and unable to contain enough air in the interstices. But if it contains considerable quantities of paper and garbage, which have little structural strength when wet, or if it is granular, like ash and soil, less water is better. It is difficult to maintaining aerobic conditions at moisture content around 70%.
In University of California studies, fibrous materials containing a considerable amount of straw were composted aerobically with moisture contents of 85% to 90%, but other composts containing much paper became anaerobic in one day when the moisture content was about 70%.
If anaerobic composting is practiced, the maximum moisture content is not as important, since oxygen maintenance is not a factor. The upper limit of moisture, which may be from 80% to over 90%, is the amount of which excessive drainage from the compost will be produced. If the composting procedure has initial aerobic conditions to produce high temperatures lasting a few days for the destruction of pathogenic organisms, followed by anaerobic composting, the maximum initial moisture content may be as high as 65% to 85%, depending on the character of the composting materials.
Proper temperature is an important factor, particular in the aerobic composting process. Considerable amounts of heat are released by aerobic decomposition. Since composting material has relatively good insulation properties, a sufficiently large composting mass will retain the heat of the exthermo-biological reaction and high temperatures will develop.
High temperatures are essential for destruction of pathogenic organisms and undesirable weed seeds. Decomposition also proceeds much more rapidly in the thermophilic temperature range. The optimum temperature range is 135º -160º Fahrenheit, around 150º Fahrenheit usually being the best. Since only a few of the thermophilic organisms actively carry on decomposition above 160º Fahrenheit, it is undesirable to have temperatures above this for extended periods.
Eggs of parasites, cysts and flies have survived in compost stacks for days when the temperature in the interior of the stack is above 135º Fahrenheit. Since a factor of safety is necessary, and since a higher temperature can be readily maintained during a large part of the active composting period, all the material should be subjected to a temperature of at least 150º Fahrenheit.
In some instances compost operators have avoided prolonged high temperatures because the nitrogen loss tends to be greater at high temperatures owing to the vaporization of ammonia, which takes place when the C:N ratio is low. But there are other ways of minimizing nitrogen loss than operating at a lower temperature. The advantages of destroying pathogenic organisms and weed seeds, controlling flies, and providing better decomposition outweigh any small nitrogen loss due to high temperatures.
A drop in temperature in the compost pile before material is stabilized indicates that the pile is becoming anaerobic and should be aerated. High temperatures do not persist when the 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. The provision of aerobic conditions, however, is the important factor 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 shows that turning to release the heat of compost piles, which have become so hot (170º-180º F.) that bacterial activity is inhibited, is not very effective. When the material is actively decomposing, the temperature, which falls slightly during turning, will return to the previous level in two or three hours. Also, it is impossible to bring about any significant drop in temperature by watering the material without waterlogging the mass.
Variations in the moisture content between 30% and 75% have little effect on the maximum temperature in the interior of the pile. The initial temperature rises a little more rapidly when the moisture content is 30% to 50% than when it is 70%. Studies, however, did show an important and significant correlation between the moisture content and the temperature distribution within the pile. When moisture content is high, temperatures 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 University of California during mild weather when the air temperature fluctuated between 50º and 80º Fahrenheit, 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.
Deeper piles caused higher temperatures and better temperature distribution, and subject more 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 also provides better temperature distribution and less heat loss.
Materials with a high C:N ratio or containing large amounts of ash or mineral matter usually attains high temperatures more slowly in the compost pile.
Aeration to maintain aerobic conditions in the compost pile is essential for high temperatures. When the compost pile becomes anaerobic, the temperature drops rapidly. Even small areas, which have become anaerobic, will often exhibit a lower temperature than the surrounding aerobic material.
Aeration is necessary for high temperature aerobic composting to obtain rapid odor-free decomposition. Aeration is also useful in reducing high initial moisture content in composting materials. Several different aeration techniques can be used with varying degrees of success. Turning the material is the most common method of aeration when composting is done in stacks. Hand turning of the compost piles or in units is most commonly used for small garden operations. Mechanical turning is most economical in large municipal or commercial operations. The most important consideration in turning compost, apart from aeration, is to ensure that material on the outside of the pile of units is turned into the center where it will be subject to high temperatures. In hand turning with forks, this can be easily accomplished. For piles or windrows on top of the ground, material from the outer layers can be placed on the inside of the new pile. Volume reduction during the stabilization period helps turning within the units. If desired, piles or windrows can be combined when turned, particularly if long composting periods are used.
Frequency of aeration or turning and amount of aeration or total number of turns are governed primarily by moisture content and type of material. Moisture is 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 void space for air in the pile. Materials with a high C:N ratio or containing large amounts of ash and other inert material may not have to be aerated as often as material which decomposes more actively and rapidly.
Studies at the University of California indicated 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 time and land area needed for composting.
Because availability of air is a function not only of the turning frequency but also of the moisture content and structure of the material, and because air requirement for the biological activity depends to some extent on the availability of nutrients in the waste (e.g., a very high C:N ratio material would not support as large a biological population), it is impossible to specify a minimum frequency of turning or number of turns for a variety of different conditions. Studies on composting of mixed refuse, (lawn and tree trimmings, and considerable quantities of paper and combustible rubbish) at the University of California indicated that the following schedule of turning is adequate to permit rapid decomposition.
If the initial moisture content is below 70%, the first turn should be made about the 3rd day. Thereafter, turn approximately as follows until the 10th or 12th day:
> Moisture 60%-70%: turn at 2 day intervals; approximate number of turns, 4 to 5
> Moisture 40%-60%: turn at 3-day intervals; approximate number of turns, 3 to 4
> Moisture below 40%: add water.
If material initially contains much more than 70% moisture, it should be turned every day until the moisture content is reduced to less than 70%. The above schedule should then be followed.
This turning schedule will permit rapid decomposition at thermophilic temperatures. Fewer turns would not produce as rapid composting but might be sufficient to prevent serious anaerobic conditions and odor.
When compost is stored before using, moving it into a stack can sometimes serve as the last turn. It should be noted that, while the above schedule was desirable for mixed refuse, less frequent turning might have been satisfactory under other conditions.
Experience soon enables operators to estimate turning and water needs. If foul odors of anaerobic and putrefactive conditions exist when the pile is disturbed either by turning or by digging into it for inspection purposes, turn the pile daily until septic odors disappear. No matter how anaerobic a pile may become, it will recover under a schedule of daily turning that reduces the moisture and provides aeration. Sometimes daily turning is necessary to controlling fly-breeding. A temperature drop during the first 7 or 10 days of composting is a good indication that turning for aeration is necessary.
Daily turning inhibits the profuse development of fungi and actinomycetes, characteristic of piles disturbed less often. In piles turned daily these organisms only develop sporadically, whereas in piles allowed to remain undisturbed for 2 or 3 days, they form a thick continuous layer, which reaches a maximum thickness in about 4 days.
In summary, avoiding anaerobic conditions, maintaining high temperatures, and controlling flies are the important criteria for degree of aeration.
8. ORGANISMS IN COMPOSTING
Compostable waste materials normally contain a large number of many different types of bacteria, fungi, molds, and other living organisms. It appears that more species of bacteria are involved in aerobic decomposition than in anaerobic putrefaction. Many of the same organisms are no doubt as active in anaerobic composting such as sludge digestion. However, since environmental condition of anaerobic compost stacks, particularly moisture and nutritional materials, differs greatly from that of sludge digestion tanks, the biological population would also be expected to differ.
Although many types of organisms are required to decompose different materials, the necessary variety is usually present and organisms thrive when environmental conditions are satisfactory. During decomposition marked changes take place in the nature and abundance of the biological population. Some of the many species will multiply rapidly at first but will dwindle as the environment changes and other organisms are able to thrive. Temperature and changes in the available food supply probably exert the greatest influence in determining the species of organisms comprising the population at any one time. Aerobic composting is a dynamic process in which the work is done by combined activities of a wide succession of mixed bacterial, actinomycetes, fungal, and other biological populations. Since each is suited to a particular environment of relatively limited duration and each is most active in decomposition of some particular type of organic matter, the activities of one group complement those of another. The mixed populations parallel the complex environments afforded by the heterogeneous nature of the compostable material. Except for short periods during turning, the temperature increases steadily in proportion to the amount of biological activity until equilibrium (state of balance) with heat losses is reached, or the material becomes well stabilized.
In aerobic composting bacteria, actinomycetes, and fungi are the most active. Mesophilic (low temperature) bacteria are characteristically predominant in the start of the process, soon giving way to thermophilic (high temperature) bacteria which inhabit all parts of the stack where the temperature is satisfactory, this is eventually, most of the stack. Thermophilic fungi usually appear after 5 to 10 days and actinomycetes become conspicuous in the final stages when short duration, rapid composting is practiced. Except in the final stages of the composting period, when the temperature drops, actinomycetes and fungi are confined to a sharply defined outer zone of the stack, 2 to 6 inches in thickness, beginning just under the outer surface. Some molds also grow in this outer zone. Unless very frequent turning is practiced, so that there is adequate time or conditions for growth, the population of fungi and actinomycetes is often great enough to impart a distinctly grayish white appearance to this outer zone. The sharply defined inner and outer limits of the shell (in which actinomycetes and fungi grow during the high temperature active-composting period) are due to the inability of these organisms to grow at the higher temperatures of the interior of the stack. The thermophilic actinomycetes and fungi have been found to grow in the temperature range between 120o and 150o Fahrenheit. Frequent turning -such as is sometimes necessary for fly control- inhibits their growth, since the cooler outer shell is turned into the interior before they can develop in large numbers. Various investigations show that many different types of thermophilic bacteria apparently play a major part in decomposing protein and other readily broken down organic matter. They appear to be solely responsible for the intense activity characteristic of the first few days, when temperatures reach 150o to 160o Fahrenheit. Major changes in the nature of the compost stack are taking place then: the stack is drastically shrinking and the appearance of the material is undergoing rapid change. They continue to predominate throughout the process in the interior of the piles, where temperatures are inhibitory to actinomycetes and fungi.
Fungi and actinomycetes play an important role in the decomposition of cellulose, lignin, and other more resistant materials, despite being confined primarily to the outer layers and becoming active only during the latter part of the composting period. These tough materials are attacked after more readily decomposed materials have been utilized. There are many bacteria that attack cellulose. However, in the parts of compost stacks populated chiefly by bacteria, paper hardly breaks down, whereas in the layers or areas inhabited by actinomycetes and fungi it becomes almost unrecognizable. Considerable cellulose and lignin decomposition by actinomycetes and fungi can occur near the end of the composting period or "curing" when the temperatures have begun to drop and the environment in a larger part of the pile is satisfactory for their growth. Hence, in the interest of their activity, turning should not be more frequent during curing than is necessary for providing aerobic conditions and controlling flies. Among the actinomycetes, streptomyces and micromonospora common in compost, micromonospora are the most prevalent. Compost fungi include termonmyces sp., Penicillium dupontii, and Aspergilus fumigatus.
Since the necessary organisms for composting are usually present and will carry on the process when the environment is suitable, an extensive knowledge of the characteristics of the various organisms is not necessary for operating a compost operation. A more detailed knowledge of the organisms, however, may lead to further improvement and economics in the process.
A compost pile is a zoo of critters! Here are just a few samples of what you will find if you look closely in your pile:
Actinomycetes: Primarily decomposers common in early stages of compost. They produce the grayish cobwebby growths throughout compost and give it an earthy smell, similar to a rotting log. They prefer woody material, and survive in a wide range of temperatures.
Fungi: They are also primary decomposers. Fungi send out thin mycelia fiber like roots, far from their spore forming reproductive structures. Mushrooms are most common. They're not as efficient as bacteria, since they can't live in the cold.
Nematodes or roundworms: They are the most abundant invertebrates in soil. Less than one millimeter in length, they prey on bacteria, protozoa, fungal spores and each other. Most nematodes in the soil are beneficial.
Fermentation mites or mold mites: These transparent bodied creatures feed primarily on yeast in fermenting masses or organic debris. They can develop into seething masses over a fermenting surface such as a winery, but are not pests in compost.
Springtail: Along with nematodes & mites, they share numerical dominance among soil invertebrates. They feed on fungi, nematodes and small bits of organic detritus. They help control fungi.
Wolf spiders: They build no webs, but run freely hunting prey. They prey on all sizes of arthropods, invertebrate animals with jointed legs and segmented bodies.
Centipede: They prey on almost any type of soil invertebrate near their size or larger.
Sow bugs: They feed on rotting woody material and leaf tissues.
Ground beetles: Most feed on other organisms but some feed on seeds and other vegetable matter.
9. USE OF INOCULA
Many composters have discussed the importance of special inocula (bacterial activators), supposedly containing several pure strains of laboratory organisms or other biological factors essential to decompose organic matter and fix nitrogen, They call them "enzymes," "hormones," "preserved living organisms," "activated factors," "biocatalyst," etc. In fact, several commercial composting processes are built around the use of some special inoculum, often known only to its discoverer and proponent, who claims it to be fundamental to the successful operation of the process. The need for inocula have always been debatable, and most composting studies have strongly indicated that they are unnecessary.
That inocula in composting organic waste containing refuse, manure, vegetable wastes etc. are not necessary or advantageous seems logical, since bacteria are always present in very large numbers in such material and can be eliminated only by drastic sterilization methods. In any case, the number of bacteria is rarely a limiting factor in composting because, provided that the environmental factors are appropriate, the indigenous bacteria, which are much better adapted when forms attenuated under laboratory conditions, multiply rapidly. Thus the rate of composting is governed simply by the environmental conditions.
The vast number of enzymes involved in decomposition, as well as the difficulty and expense involved in isolating and synthesizing them, would make composting with enzymes highly impractical even if satisfactory preparations were available. The addition of enzymes to raw compostable materials is unnecessary because bacteria synthesize efficiently and rapidly all the enzymes required. The term "hormones" is popularly used to designate the growth factors and vitamins needed by bacteria or other organisms. The organic constituents of mixed compostable materials usually contain all the growth factors needed for normal growth. Also, growth factors and vitamins can be produced by microorganisms and will undoubtedly be produced in sufficient quantities in a mixed microbial population to meet normal requirements.
The terms "biocatalyst" and "activated factors" are applied to various biological materials which are supposed to activate and accelerate decomposition and stabilization of organic material. Experimental investigations with sludge digestion and activated-sludge treatment of sewage indicated that biocatalyst did not affect either of these processes. In some cases the "activator" usually supplied some material which was lacking in the compost. For example, straw or paper, which does not contain the necessary biological nutrients, is not decomposed readily alone, but if nitrogen and phosphorus are added, the straw and paper will serve as the source of carbon for decomposition. The use of horse manure, compost material, normal soils, and special commercially prepared bacterial cultures in the composting of mixed garbage and refuse was investigated. Similar materials were composted with and without these different inocula, and it was found that, although rich in bacteria, none of the inocula accelerated the composting process or improved the final product. There was no significant difference in the temperature curves or in the chemical analyses of the material at different intervals during the composting period. The failure of the inocula to alter the composting cycle is due to the adequacy of the indigenous microbial population and to the nature of the process itself.
There have been some interesting recent research studies suggesting that special preparations made from specific plant and other substances and used in producing compost on Biodynamic farms can make a difference in the composting process. In these studies, Biodynamic treated composts maintained an average 3.4 degree C higher temperature throughout the 8 week active composting period, and reached maturity faster than the control compost.
On the whole, though, when the environment is appropriate, the varied indigenous (originating in a particular region) biological population will multiply rapidly and composting happens. Microbial inoculation would be useful only if the biological population in any emerging environment were unable to develop sufficiently rapidly, or take full advantage of the capacity of the environment to support the increase in numbers. In such a case, a time lag would occur which could be overcome by supplementing the initial population indigenous to the refuse. However, no such time lag has been observed in these experiments or in composting the usual materials which contain a large indigenous bacterial population. Some of the different groups of organisms in the mixed microbial population apparently remain inactive until the environment is satisfactory for their growth, and then emerge and perform the role in the succession of steps in the stabilization process. Since the process is dynamic and any individual group of organisms can survive a rather wide environmental range, one population may begin to emerge when another is still flourishing and yet another is disappearing. Hence, when any group of bacteria is capable of multiplying at a rate equal to that of its developing environment, any addition of similar organisms as an inoculum would be superfluous.
The best inoculum a home composter could use, would be a shovelful of their own already finished compost, which would contain the microorganisms present for their specific feedstock.
Successful compost operations that don't use special inocula in the Netherlands, New Zealand, South Africa, India, China, the USA, and a great many other places, provide convincing evidence that inocula and other additions are not essential in the composting of waste materials.
10. PLACEMENT OF MATERIALS FOR COMPOSTING
Various arrangements such as bins, barrels, pits and windrows have been used or suggested for composting organic matter. Open piles, windrows, or bins are by far the most widely used methods for aerobic decomposition and maturing of organic refuse. The exact use and arrangement of these systems depends on the local requirements of materials, labor, cost of systems, climatic conditions such as temperature, rainfall, and wind.
To aerobically maintain the composting process by frequent turning for aeration, windrows, piles and bins above the surface of the ground appear to be more efficient than pits. On the other hand, if the decomposition is to be entirely anaerobic or aerobic only during a short initial period, pits 3 to 4 feet deep and varying in length and width in accordance with the daily quantity of raw material should be used.
WINDROWS, PILES AND BINS
The material in aerobic composting piles should be loosely stacked to allow as much space for air in the interstices as possible. The windrows or piles may be of any convenient length, but the height of the pile is somewhat critical. If piled too high, material will be compressed by its own weight, thus reducing pore space which results in increased turning labor (costs) or longer composting time as anaerobic conditions develop. In some instances, the maximum practical height may be governed by the equipment used for stacking the refuse, or by the tendency of the pile to become excessively hot. Large piles in warm weather may reach temperatures excessively high for bacterial life.
Piles that are too low lose heat rapidly. Optimum temperatures for destruction of pathogenic organisms and decomposition by thermophiles (high temperature microorganisms) are not obtained. Also, if the piles are too small, the loss of moisture may be excessive, especially near the edges, and decomposition slows.
Five to six feet is about the maximum height for any refuse, and 3 feet is the minimum for most shredded fresh municipal refuse. The height can be greater in cold weather than in warm weather.
Thoroughly mixing compost materials in bins, windrows or piles provides quickest and most complete decomposition. The pile may normally be started directly on the ground. However, to provide aeration to the bottom of the pile and improve drainage, dig a trench across the base of the area and cover with stiff wire mesh (hardware cloth) before adding material.
Daily quantities of materials available for home gardeners will often be too small to permit the satisfactory use of windrows. In this case circular or rectangular piles approximately 6 feet in diameter and 3 to 5 feet high, with a rounded top for running off of the rain water, would be best.
For shallow pits, either the walls and bottom of the pit are lined with brick or masonry or the natural earth is tamped and packed. The material is stacked to a height of 1 foot or more above the ground, making a total of 3 to 4 feet. The material can be turned in the pit as often as necessary to provide the high temperatures and aerobic conditions as required. When pits are used, a smaller stack surface is exposed to the air, and the walls and bottom of the pit provide some insulation against heat and moisture loss.
Any type of pit should be lined and is usually provided with a chimney and trenches, or a porous bottom, for aeration and drainage of liquid seepage from the pile. The same shape trenches without aeration and drainage channels and without masonry lining may be used. But unless pits are lined, the walls are apt to crumble and the shape of the pit becomes irregular. When hand labor is used, turning the material in a pit may be about the same as in a stack on the ground surface.
We suggest composting in pits approximately 3 feet deep by a system of providing aerobic conditions and high temperatures for the first few days and then anaerobic conditions for 4 to 6 months. The material is mixed in the pit. There is sufficient oxygen in the initial stack for a high temperature to be produced by aerobic organisms during the first few days. Apparently, the high temperature is retained for some two weeks, owing to the insulating properties of the stack, even though anaerobic conditions exist after the first few days. The material is left to compost in the pit with no turning for about three months under conditions that are primarily anaerobic.
The initial pH of garbage, refuse, manure, and other compostable material is likely between 5.0 and 7.0 unless the waste contains ash or other highly alkaline materials. If the material has undergone putrefaction before being received for composting, the pH will be near the lower value. When the initial pH is between 6.0 and 7.0, the pH of the composting material will usually drop a little during the first two or three days of aerobic composting, owing to the formation of some acid. If the pH is 5.0 or 5.5, there will be little change during this period.
After two to four days the pH usually begins to rise and will level off at between 8.0 and 9.0 towards the end of the process. The control of the pH in composting is seldom a problem requiring attention if the material is kept aerobic, but large amounts of organic acids are often produced during anaerobic decomposition on a batch basis. Ash, carbonates, lime or other alkaline substance will act as a buffer and keep the pH from becoming too low. However, the addition of alkaline material is rarely necessary in aerobic decomposition and, in fact, may do more harm than good because the loss of nitrogen by the evolution of ammonia as a gas will be greater at the higher pH. Since the optimum pH for most organisms is around 6.5 to 7.5, it would probably be beneficial if the pH could be maintained in that range. However, since composting is necessarily a batch-process operation, minor changes in the pH must be expected.
Apparently, initial pH values of 5.0 to 6.0 do not seriously retard initial biological activity since active decomposition and high temperatures develop rapidly after material is placed in the stack. Temperatures do appear to increase a little more rapidly when the pH is in the range around 7.0 and above. The usual waste materials available for composting present no problem of pH control.
12. CLIMATIC CONDITIONS
Climatic conditions, particularly temperature, wind, and rainfall influences the composting operation. The effect of atmospheric temperatures, particularly the lowest temperature at which composting might be satisfactorily done, is not known. A slightly larger compost pile in winter weather will reduce the heat loss per unit volume.
Organic refuse has excellent insulation properties. As has been previous shown, a steep temperature gradient exits at the outer surface of compost stacks. The difference in temperature may be several degrees Fahrenheit per inch of material. It seems reasonable to believe that composting can be satisfactorily conducted at severe freezing temperatures, providing snow conditions do not interfere with turning and the snow becomes mixed with the compost. It is probable that turning would not have to be done quite as often as in warm weather, because there would be a longer temperature recovery period after each turn when the colder exterior of the pile was turned into the interior.
Strong winds markedly lower temperatures on the windward side of the compost pile. Two factors play an important role in temperature lowering by winds: (a) the coarseness of the material, which affects the porosity of the pile and the evaporation, and (b) the moisture content. Unshredded or coarsely shredded material has a greater porosity and permits greater penetration of wind into the pile. Consequently, more evaporation takes place, and when the material becomes too dry, bacterial activity is inhibited. Shredding or grinding to produce a maximum particle size of about 2 inches provides a more homogeneous mass that is not as easily penetrated by winds. Thoroughly wetting the exterior of the pile, particularly on the windward side, will reduce wind penetration and permit the interior high-temperature zone to extend nearer to the surface of the pile. In an area of strong prevailing winds, a windbreak could be built to protect compost piles. This should seldom be necessary, however, since increasing the size of and wetting the pile will control temperatures, and all material will be exposed to high temperatures by turning. Wind cooling and drying of compost piles is of little significance when piles or bins are used, since the material is protected on all sides except the top, which wetting will protect.
Rain usually does not seriously affect composting if the piles are finished with a rounded top so that the rainwater can run off and if the compost piles or bins are adequately drained so that water does not stand around the piles and penetrate the bottoms. Heavy rains accompanied by high winds will penetrate a pile of coarsely shredded material as much as 12 to 15 inches on the windward side, but the resulting effect on large piles can be readily overcome by subsequent turning.
Turning should not be done in the rain, because the material may become waterlogged. If the material cannot be turned on regular schedule owing to rain, it is better to let it become deficient in air for a short time than to have the material soaked. Rainy weather can present more of a problem when composting is done in pits or bins. The top of the pit should be rounded to turn the water, which will, however, seep along the edges to the bottom. The bottom should therefore be adequately drained to remove the water and to allow a minimum of penetration into the compost. In rainy areas, pits should be lined with concrete, brick, or masonry, and provided with tile drains. Or roofs could be built over the bins or pits to protect them from rain.
During rainy weather, shredding or grinding, and the segregation of the materials should be done under cover. Facilities for storing the incoming materials for a short time should be provided, so that stacking or piling does not have to be done during rain.
Composting can be done satisfactorily in relatively cold climates or in areas of considerable rainfall with a minimum of roofed buildings. Heavy snowfall will greatly hinder continuous composting operations and removal of snow from the composting piles or bins will usually be required. Material will not become anaerobic or create an odor nuisance during really cold weather. Hence, if an ample composting area is available, the material can be allowed to stand for long periods without turning until the weather is favorable.
13. DESTRUCTION OF PATHOGENIC ORGANISMS
Destruction of pathogenic organisms is a most important aspect-and a problem- of compost containing sewage, or other highly contaminated materials. Experiments have shown that aerobic composting at high temperatures is effective in destroying pathogenic organisms. The absence of health hazards is characteristic of well-managed composting operations in many parts of the world. This is significant evidence of the effectiveness of thermophilic composting.
An analysis of the typical temperature and of thermal death points of a number of pathogenic microorganisms, parasites, and parasite ova, indicates the unlikelihood of survival of some of the common disease-bearing organisms. The highest thermal death points are appreciably lower than the maximum temperatures found inside the composting pile or bin. The magnitude and duration of the high temperatures, as well as the antibiosis which is characteristic of a mixed population of microorganisms, provide a sound basis for believing that no pathogens, parasites, or parasite ova survive the aerobic composting process.
The high temperature zone usually extends only to within 4 to 8 inches of the surface. Therefore, turning is necessary, quite apart from its function in aerating the mass, for ensuring pathogen and parasite destruction, particularly if a composting period under six months is used. The compost temperature curves and thermal-death-point values may indicate that one turning will be sufficient eliminate the pathogens and parasites if all of the surface material is completely turned to the inside, thus exposing any organisms present to lethal internal temperatures. But, although this may be true in many cases, as a safety factor, and to guard against failure to turn all of the material to the inside, at least two turns are required, and at least three for maximum assurance of complete destruction. Three turns would also be the normal practice for aeration purposes when rapid composting is done in stacks or piles on the ground surface.
In some composting operations the material is turned only once or not at all. A thermophilic temperature is developed after the initial aerobic stacking. This is considered to be sufficient to destroy pathogens and parasites. It is doubtful this practice is sufficiently safe when contaminated material is composted, since some pathogen and parasites may escape destruction in the cooler side and top layers.
Anaerobic composting in the mesophilic temperature range does not affect good destruction of parasites in an anaerobic environment. The biological antagonisms will eventually eliminate them, but this will generally take at least 6 months.
Unless six months or more can be allowed to elapse before the compost is used, anaerobic composting should be preceded by aerobic conditions and thermophilic temperatures for at least a week with at least one turning, in order to ensure the desired destruction of pathogens.
General cleanliness and systematic attention to the details of operation around the compost site, is necessary and particularly important when contaminated material is used.
14. COMPOSTERS AND CHEMICALS
Is it safe to use compost from yard wastes that have come in contact with pesticides, or other toxic chemicals? The major route of breakdown of pesticides is through microbial degradation, which is the process of decomposition. Any pesticide a homeowner can buy without a license will be broken down in the compost pile before the end of the process. The one exception to this is clopyralid, which is contained in certain Dow products. Confront is the product that homeowners might use. This is a long lasting herbicide, and vegetation that has been treated with this should NOT be composted, since the resulting compost can cause serious injury to sensitive crops.
Some typical home yard chemicals, and their reaction to composting:
Slug bait: Most commercial slug baits contain metaldehyde which, when exposed to water, quickly breaks down to a harmless alcohol. (Fresh metaldehyde is toxic to slugs, snails, birds, cats, dogs, raccoons, rabbits, and humans).
Herbicides: Some herbicides become harmless in a very short time in the soil and compost piles (such as Diquat, Paraquat). Others (such as 2,4-D and propanil) break down in compost piles, but only after thorough composting. Still others (such as arsenic, borate, picloram, simazine, sodium chlorate) are extremely long-lived and will probably survive most composting processes. Do not use organic matter in your compost pile if it was treated with long-lived herbicides, such as CONFRONT.
Insecticides: All contemporary insecticides will break down during the decomposition process. Old chlorinated hydrocarbon insecticides such as DDT (which has been banned for a long time) may survive.
Fungicides: Vegetation that has been just sprayed with a fungicide may suppress the development of decomposing fungi if it is added to the compost pile. A few weeks will degrade the fungicide enough so that it will not effect the decomposition process. Currently, one turf fungicide, PMA, contains mercury. This may only be used by commercial pesticide operators. This should not be used.
Composting bins are often made of pressure treated wood to prevent rot from destroying the bin. Contrary to intuitive expectations, it's safer to use wood treated with arsenic than wood treated with either creosote or pentachlorophenol. Several studies found no evidence that arsenic migrates from treated wood into garden plants growing in planter boxes of arsenically-treated wood. It seems reasonable to assume that arsenic would not migrate into compost either.
15. FLY and OTHER PEST CONTROL
One of the most important problems in composting is controlling flies. Garbage, animal manure, tomato and several other food-processing wastes, are excellent media for breeding and development of large fly populations. If adequate control measures are not practiced, particularly when composting manure, the compost systems will be infested with extremely large numbers of flies, and create a health hazard.
Fly-breeding can be satisfactorily controlled in composting operations during the fly season, with little more effort than is normally necessary for good sanitary composting. Added manure and fresh food scraps in the composting systems should be kept covered.
Fly larvae in composting material may originate from eggs laid in the material at the place of collection or from eggs laid during the handling of the material at the compost site. If the latter were the main source, fly control would be no problem. However, much of the material is infested with eggs and larvae in various stages of development, sometimes even at the pupal stage, before arriving at the compost site. Therefore, material must be prepared immediately for composting and placed in compost systems where high temperatures and environmental conditions are unsatisfactory for continued emergence of flies.
The predominant species of flies encountered in composting will vary with the area and with the type of material. The variety of materials available for composting offers satisfactory breeding conditions for many different species, but generally speaking, the compost operator does not have to interest himself/herself in the particular species, since the most satisfactory control measures in composting apply equally well to different species.
The life cycle of the ordinary housefly, " musca domestica," is usually from about 7 to 14 days when conditions are favorable. The time of the different stages varies with temperature and other conditions, but on average it may be considered as follows: egg, 1 to 2 days; larva 3 to 5 days; pupa, 3 to 5 days; emergence of young fly, 7 to 10 days; and egg laying by new fly, 10 to 14 days. Fly control measures must interrupt this cycle and prevent the adult flies from emerging.
Some procedures, particularly grinding, turning, and systematic cleanliness, which are useful in providing compost of good quality and in destroying parasites and pathogens, are also effective for controlling flies. Initial shredding or grinding to produce material more readily attacked by bacteria also destroys a large number of the larvae and pupae in the raw material. Also, the texture of material shredded to a maximum size of 2 inches is not as suitable for fly breeding.
Studies at the University of California on mixed garbage and refuse demonstrated that after raw material containing considerable numbers of eggs and larvae had been ground and placed on the pile, no fly breeding took place using normal composting procedures of turning every 2 to 3 days. Apparently, the destruction of the larvae by grinding, mixing, and the structural changes caused by grinding, results in garbage that is no longer attractive to flies. Heat quickly generated in compost piles effectively stops flies breeding in refuse containing a considerable proportion of garbage. However, this is not the case, for compost materials containing large amounts of animal manure, food scraps and other fresh and decaying fruits.
When materials attractive to flies and containing large numbers of larvae and pupae are composted, some of the larvae will move to other cooler layers and continue their life cycle. The most effective method of destroying these larvae is frequent turning. Turning compost stacks at daily interval, when the raw material contains many larvae and pupae and when fly breeding conditions are favorable, and at a maximum interval of 3 or 4 days when fly breeding conditions are not especially favorable, provides good fly control.
16. RECLAMATION OF NITROGEN AND OTHER NUTRIENTS
The two most important purposes for composting organic wastes are (a) reclamation or conservation of the nutrient and fertilizer values of waste, and (b) sanitary treatment and disposal to prevent the spread of disease. Of the major nutrients-nitrogen, phosphorus, and potash- the nitrogen conservation is the most important in most areas of the world because so often the shortage of nitrogen limits the amount of food that is produced. Nitrogen is also more difficult to conserve than phosphorus and potash, as are the micronutrients, which, owing to the chemical condition in which they are present, are lost only to leaching. Nitrogen may be lost by leaching, but the major loss comes from escape of ammonia or other volatile nitrogenous gases from compost material to the atmosphere.
There has been much research and writing on conserving nitrogen and other nutrients, particularly with respect to microbiology of the soil. Limited experience exists on nitrogen conservation and reclamation in the composting process. Results of investigations and studies on nitrogen utilization in the basic biological processes provide fundamental information on the control of nitrogen loss in composting.
Nitrogen loss as ammonia in aerobic composting is affected by the C:N ratio, pH, moisture content, aeration, temperature, and the form of nitrogen compounds at the start of the composting materials.
Since organisms use about 30 parts of carbon for each part of nitrogen, a C:N ratio in the raw compostable material of around 30:1 is best for good composting, would also seem satisfactory for tying up or binding nitrogen in biological cell material and thus preventing its escape.
Various research workers reported optimum ratios of C:N to avoid nitrogen loss under different conditions of from 26 to as high as 38. A ratio of available carbon to available nitrogen of about 30 or more permits minimum loss of nitrogen, but the ratio of carbon to nitrogen measured chemically is often not the ratio of available carbon to available nitrogen. Since most refuse contains considerable amounts of cellulose and lignins, which are resistant to biological decomposition, and since most of the nitrogen is usually in a readily available form, an actual C:N ratio of considerably over 30 may be necessary to provide maximum conservation of nitrogen. Also, studies indicated that nitrogen conservation decreased rapidly as the C:N ratio increased from 40 to 50. This rapid decrease is not entirely consistent with the fundamental aspects of bacterial decomposition. Above a C:N ratio of 50, nitrogen conservation remained uniform at about 70% of the optimum. Basically there should be little drop in nitrogen conservation below the maximum when the initial C:N ratio is above the ratio utilized by the organisms. When carbon is higher than the ideal C:N ratio, organisms will require all the nitrogen for decomposition of the carbonaceous materials. University of California studies found Nitrogen losses of around 50% when the C:N ratio was in the range 20 to 25. From about 30 upward, nitrogen losses were very small.
NITROGEN CONSERVATION IN RELATION TO C:N RATIO
||Initial C:N ratio
||Final percentage of nitrogen
||Nitrogen conservation %
This table shows a few examples of nitrogen conservation for different C:N ratios. It was found that in manure composts nitrogen was conserved only when the C:N ratio was adequate and when immediate decomposition set in, resulting in transformation of soluble forms of nitrogen into insoluble forms. Whenever decomposition was delayed, owing to too low or too high a temperature, losses of volatile forms of nitrogen occurred. From 85% to 90%, and possibly 95%, of the nitrogen in the raw materials can be conserved if the C:N ratio is high and other avenues for nitrogen loss are controlled.
There are three phases in the relation of nitrogen supply and conservation to available carbon in biological decomposition:
(a) When more nitrogen is available than necessary for organisms to use carbon, large quantities of ammonia and volatile forms of nitrogen are given off and lost;
(b) When the requisite amount of nitrogen to carbon for bacterial utilization is present, decomposition proceeds without appreciable loss of nitrogen;
(c) When nitrogen is low in relation to carbon, some of the organisms will die and their nitrogen will be recycled. Small additional amounts of nitrogen may be picked up by nitrogen fixation when conditions are satisfactory.
In all three phases there is a tendency to reach the same final amount of nitrogen, that which the bacteria can hold when the compost is in a stabilized condition. In the first phase nitrogen is lost; in the second, it is stabilized and conserved; and in the third, it is recycled, conserved, and sometimes accumulated. This illustrates that composting operations can be operated to conserve most nitrogen.
Ammonia escapes as ammonia hydroxide as the pH rises above 7.0. In the later stages of composting the pH may rise to between 8.0 and 9.0. At this time there should not be an excessive amount of nitrogen present as ammonia. Materials that contain large amounts of ash will have a high pH and may be expected to lose more nitrogen.
Some compost operators add lime to improve composting. This should be done only under rare circumstances, such as when raw material has a high acidity due to acid wastes or contains materials which give rise to highly acid conditions during composting. When the pH remains above 4.0 to 4.5, lime should not be added. The pH will be increased by biological action and nitrogen conserved.
The moisture content of compost affects nitrogen conservation less than the C:N ratio and the pH. Ammonia escape is greater when the moisture content is low. The water serves as a solvent and diluent for the ammonia, thereby reducing vapor pressure and volatilization. A moisture content range of 50% to 70%, satisfactory for other aspects of composting, will assist in conserving nitrogen.
Aeration and turning adversely affect nitrogen conservation.
If ammonia is present, it will escape more easily when material is disturbed and exposed to the atmosphere. However, if the initial C:N ratio is high enough, nitrogen losses during turning will be small. Since some ammonia may be present during the dynamic transitional phases of active decomposition, turn only as often as necessary to maintain aerobic conditions and control flies.
High temperatures increase volatilization and escape of ammonia. Since high temperatures are fundamental in aerobic composting and destruction of pathogen, not much can be done about controlling temperatures other than to avoid temperatures above 160o Fahrenheit, which retard bacterial activity and permit ammonia accumulation. Since the greatest ammonia loss occurs during early stages of active decomposition, only little conservation of nitrogen will be gained by reducing temperatures after the two turns or after the first 6 to 8 days of active decomposition.
The nitrogen initially present in the material may affect nitrogen conservation. If large amounts of ammonia are present in raw materials, some of this ammonia may be volatilized and lost before the organisms have had sufficient time to utilize and stabilize it, even though the C:N ratio is satisfactory for nitrogen conservation. This can be an important factor since much of the nitrogen loss occurs during the first few days of composting.
Some materials, such as cellulose and porous fibrous matter, have the capacity to absorb or hold moisture and volatile substances, thereby reducing the tendency to escape. Materials of this type play a part in reducing nitrogen loss from compost, which contain accumulated ammonia. Materials containing considerable quantities of horse or cow manure seemed to exhibit less nitrogen loss at low C:N ratio than other materials, and should be considered to be nitrogen carriers. This could have been due to the form of nitrogen, to the absorptive of nitrogen holding capacity, or to some other characteristic of the manures.
Also, addition of soil to compost with a high ammonia content absorbed some of the nitrogen.
Loss of nitrogen by leaching may occur in rainy weather or if the composting material has too high initial moisture content and excess liquid drains away. Loss by leaching depends on the amount of soluble nitrogen in the compost and on the amount of rainfall. Arranging compost piles so that water can't enter may minimize leaching.
The greatest nitrogen conservation may be accomplished by anaerobic digestion in water when liquids as well as the solids are conserved. In such cases, while nitrogen fixation would not be expected, there should be almost no nitrogen loss, since ammonia in low concentration in the liquid would not escape.
Conservation of phosphorus and potash in composting is not difficult since about the only loss occurs through leaching during rainy weather
17. TIME REQUIRED
The time required for satisfactory stabilization depends primarily upon:
a. initial C:N ratio;
b. particle size;
c. maintenance of aerobic decomposition; and
d. moisture content.
If moisture content is in the optimum range, compost is kept aerobic, and particles of material are of such size as to be readily attacked by the organisms present - all factors that can be controlled in the composting operation - the C:N ratio determines time required for stabilization. Low C:N ratio materials are decomposed in the shortest time because the amount of carbon to be oxidized to reach a stabilized condition is small. Also, in low C:N ratio compost, a larger part of the carbon is usually in a more readily available form, while in higher C:N ratio materials more of the carbon is usually in the form of cellulose and lignin which are resistant to attack. The changing biological population in the changing environment attacks cellulose and lignin last. When the available C:N ratio is above 30, additional time is required for recycling nitrogen.
If material is not kept aerobic so that high temperatures can be maintained during the active decomposition period, or if the particle size is so large that the bacteria cannot readily attack the material, or that the interior of the particle becomes anaerobic, longer composting periods are required.
Under aerobic conditions at high temperatures and when the initial C:N ratio is in the optimum range or below, the material takes on the appearance and odor of humus after 2 to 5 days of active decomposition. However, active decomposition is not complete at this stage, and the C:N ratio may not have been lowered to the level desired for fertilizer.
The actual composting time is not particularly important, provided that it is sufficient for destruction of pathogens and parasites, and for nitrogen conservation.
So long as satisfactory compost can be produced, turning time of composting, storage, and other factors should be adjusted for home composting and for nitrogen conservation.
18. TESTING AND JUDGING THE CONDITION OF COMPOST
There are many tests and checks by which various aspects of the composting process and the condition of compost may be judged. From the point of view of the overall operation and the final product there are three groups of tests:
a. test of the sanitary quality of the operation and of the finish product, i.e., pathogen and parasite destruction and absence of flies and odors;
b. test of fertilizer or agricultural or horticultural value, i.e., the amount of nitrogen, phosphorus, potash, and other nutrients, nutrient conservation, the C:N ratio, and compost value ; and
c. economic test, i.e., whether the total cost of producing the compost is less than its value as fertilizer plus the cost of disposal by other means, such as incineration or land fill.
The gardener, small farmer and other small compost operator usually will not be concerned with detailed tests other than those to confirm that the material is safe from a health standpoint. This will be judged from its temperature, and its satisfactory appearance as a soil additive.
Health organizations and laboratories can make tests for organisms of public health significance when necessary. Chemical tests for nitrogen in its different forms, phosphorus, potash and the organic character of the material can be made by standard techniques and are useful in analyzing the finished product and to determine the effect of different composting procedures. For routine day-to-day operations, temperature, appearance of material, odors, and the presence of flies are important tests. Cleanliness and the absence of flies at the site, as well as the absence of large numbers of larvae in the piles, are criteria of sanitary quality of the compost operation. Temperature is the best single indicator of the progress of aerobic composting and also the basis for determining whether pathogen, parasites, and weed seeds are being destroyed.
The temperature of compost can be checked by:
a. digging in the pile and feeling the temperature of the material;
b. feeling the temperature of a rod after insertion into the material; or
c. using a thermometer.
Digging into the pile will give an approximate idea of the temperature. The material should feel very hot to the hand and be too high to permit holding the hand in the pile for very long. Steam should emerge from the pile when opened. A metal or wooden rod inserted 2 feet into the pile for a period of 5-10 minutes for metal and 10-15 minutes for wood should be quite hot to the touch, in fact, too hot to hold.
These temperature-testing techniques are satisfactory for the smaller compost operations. Long stem metal thermometers are available for temperature testing.
When aerobic composting progresses in a typical manner, there will be a rapid rise in temperature to 140o-170o F. in the first three days. In small pits or piles, a pause in the temperature rise often occurs somewhere between about 85o and 135o F., during the transition from mesophilic (lower) to thermophilic (higher) decomposition. After the initial temperature rise, a high temperature is maintained for several days during the active decomposition period, provided that aerobic condition are maintained. Then a slow decline of temperature starts as the rate of heat generation falls below the rate of heat radiation of the material. During this period the rate of bacterial activity is dropping faster than the temperature indicates, owing to the insulating qualities of the composted material.
The failure of a compost pile to attain a high temperature in a period of 3-6 days indicates that the pile may be too small to retain the heat, that it may be too wet or dry, or that it has insufficient organic material and nutrients for rapid decomposition.
The temperature alone, however, cannot determine conditions within the composting mass. A temperature drop may result from the development of environmental conditions unfavorable to aerobic thermophiles, either through excessive heat, onset of anaerobic conditions, or lack of sufficient moisture. In rare instances, not usually encountered in composting municipal wastes, when some acid material has been added a low pH might also cause a lowering of the temperature.
Compost may be considered finished when it can be stored in large piles indefinitely without becoming anaerobic or generating appreciable heat. It can be safely spread because of its low C:N ratio or the poor availability of its carbon. The material, however, is still slowly active and will "ripen" somewhat in the large stacks. At this time it should be grayish-black or brownish-black in color, depending on what color of materials were used. However, color alone is not a good criterion of finished compost because the appearance of rich soil humus develops in a good compost long before the temperature decline signals the decrease in microbial activity.
Characteristic changes in odor during the period of composting help define stable compost. The material should be odorless, or have a slightly earthy odor or the musty odor of molds and fungi.
These approximate physical tests are adequate for most small compost operations.
Laboratory analyses for nitrogen, phosphorus, and potash are more precise and require more elaborate equipment, but are relatively simple chemical determinations to make. The determination of the C:N ratio, which is so important in regard to nitrogen conservation and for estimating the quality of the finished compost, is more of a problem, because the quantitative analyses of carbon is difficult, time consuming, and expensive.
If compost is modified by adding ammonium sulfate, phosphates, or other nutrients for special fertilizer purposes, percentages of these nutrients on a dry basis must be determined, so that users can compare them with other fertilizers.
19. COMPOST QUALITY
The nutrient value of composts varies widely, depending upon the nature of the material being composted. If initial material contains grass clippings, weeds, or manure, it will be richer in nitrogen and other nutrients than if it contains mainly straw, litter, dirt or corn stalks.
The following analyses shows the ranges of values, on a dry basis, in which the chemical characteristics of most finished composts generally lie. These ranges vary because different initial materials will yield final composts of widely varying chemical characteristics.
Substance Percentage by weight:
Organic matter................................... 25.0-50.0
Nitrogen (as N).................................. 0.4- 3.5
Phosphorus (as P2O5)....................... 0.3- 3.5
Potassium (as K2O)........................... 0.5- 1.8
Calcium (as CaO).............................. 1.5- 7.0
Composts also contain a great variety of micronutrients. Since organic materials for composting contain products of agriculture or horticulture, it is logical to expect these nutrients to be present in the compost. Experiments indicate that compost manures have beneficial effects greater than those to be expected from nitrogen, phosphorus, potash, and humus content alone.
20. ECONOMIC ASPECTS
Economic returns from utilizing organic manure and wastes on the land are generally known to all farmers and experienced gardeners. They realize that yields and the maintenance of soil fertility depend upon reclamation of these organic materials. Composting organic matter to make them safe for use on agricultural lands and gardens is economically sound, and a way to cut down on the volume of waste materials at the landfills or incinerators. Keeping the organic wastes out of the solid waste stream holds down the cost for the community in disposal cost.
Compost contains valuable nutrients that could replace and/or supplement use of commercial fertilizers by homeowners. Use of chemical fertilizers can be cut down to a minimum. Excessive usage of commercial fertilizers by homeowners can contaminate surface and groundwater with nitrates. Excess nitrates in ground and surface water can lead to human health hazards.
Municipalities that own wastes after collecting, and are responsible for sanitary disposal, are usually not directly concerned with their utilization in agriculture/horticulture. Municipality interest is primarily in the sanitary disposal of the waste materials.
Salvaging urban wastes for agricultural use offers an opportunity for closer cooperation between urban and rural elements in improving the total economy of an area. It is impossible to evaluate such cooperation in monetary terms; however, it has been demonstrated many times in various areas of the world that developments in one segment of a community can benefit another and be profitable for both.
Economic reclamation of municipal organic wastes depend upon low cost production which permits distribution of large quantities of composted organic materials at a sufficiently low price to make its use attractive to agriculture and horticulture operations.
Many commercial compost plant operators have found a profitable market among truck gardeners, nurseries and landscaping operation. There is a need for good humus in our fast growing community. Many new homes and commercially buildings have topsoil brought in, which is usually stripped, from good agricultural land. The humus from composting organic wastes could be used as a substitute for topsoil now used by landscape contractors and homeowners
21. USE OF COMPOST
Compost is ready for use when the temperature in the pile drops to the temperature of the surrounding air. Other signs are:
- It smells earthy-not sour, putrid or like ammonia
- It no longer heats up after turned or watered
- It looks like dark soil
- It's crumbly, and doesn't have identifiable food items, leaves or grass.
The pH is usually around 7.5, and it will have a C:N ratio ranging from 10:1 to 20:1.
Planting in compost before it is finished could damage plants. Undecayed carbon materials as wood chips or leaves uses nitrogen from the soil to continue decomposing, robbing it from the plants you grow. Undecayed nitrogen materials can harbor pests and diseases. Immature compost can introduce weed seeds and root-damaging organic acids.
Compost can be used in many ways in the garden. Coarse, semi-decayed woody material is suitable as mulch to put on top of the soil around the plants. It can be used as mulch around trees and shrubs, to keep the moisture in, to prevent weeds from growing around trees and shrubs. The decayed material is good for digging into the soil together with commercial fertilizers at preparation time. It can be used for installing new lawns. A fine-screened layer can be used for a top dressing on established lawns. It can be used in the planting areas of landscapes. It should be used extensively in vegetable gardens to improve the organic matter content in the soil. It can be used for houseplants, for starting seeds in planting beds or flats, or made into a compost tea for watering plants.
Using compost as mulch, in the soil or as potting media is beneficial in many ways. Compost contains a full spectrum of essential plant nutrients. You can test the nutrient levels in your compost and soil to find out what other supplements it may need for specific plants.
- Compost contains macro and micronutrients often absent in synthetic fertilizers.
- Compost releases nutrients slowly-over months or years, unlike synthetic fertilizers.
- Compost enriched soil retains fertilizers better. Less fertilizer runs off to pollute waterways.
- Compost buffers the soil, neutralizing both acid & alkaline soils, bringing pH levels to the optimum range for nutrient availability to plants.
Compost helps bind clusters of soil particles, called aggregates, which provide good soil structure. Such soil is full of tiny air channels & pores that hold air, moisture and nutrients.
- Compost helps sandy soil retain water and nutrients.
- Compost loosens tightly bound particles in clay or silt soil so roots can spread, water drain & air penetrate.
- Compost alters soil structure, making it less likely to erode, and prevents soil spattering on plants-spreading disease.
- Compost can hold nutrients tight enough to prevent them from washing out, but loosely enough so plants can take them up as needed.
- Compost makes any soil easier to work.
Compost brings and feeds diverse life in the soil. These bacteria, fungi, insects, worms and more support healthy plant growth.
- Compost bacteria break down organics into plant available nutrients. Some bacteria convert nitrogen from the air into a plant available nutrient.
- Compost enriched soil have lots of beneficial insects, worms and other organisms that burrow through soil keeping it well aerated.
- Compost may suppress diseases and harmful pests that could overrun poor, lifeless soil.
Healthy soil is an important factor in protecting our waters. Compost increases soil's ability to retain water & decreases runoff. Runoff pollutes water by carrying soil, fertilizers and pesticides to nearby streams.
- Compost encourages healthy root systems, which decrease runoff
- Compost can reduce or eliminate use of synthetic fertilizers
- Compost can reduce chemical pesticides since it contains beneficial microorganisms that may protect plants from diseases and pests.
- Only a 5% increase in organic material quadruples soils water holding capacity.
When that first batch of finished compost is ready to spread, congratulate yourself for your efforts because you are ecological minded, and know that organic waste materials should be recycled into the soil instead of being put in a garbage can. By recycling the organic materials, valuable nutrients and organic matter are recycled. You have helped alleviate the solid waste problem!
Yard waste disposal used to be a backburner issue, because refuse disposal was inexpensive and landfill capacity, before the eighties, was not as scarce as it is now.
Yard waste collection and reuse varies widely in strategies and success. Like other aspects of waste management, yard waste recycling may be closer to an art than a science. Development in a given state, county, or community will probably be based strongly on the local situation. The experience and preference of the designer of a given program will dictate what is done at the local level.
Yard waste composting has been successfully demonstrated in many states, including Washington, (Seattle Tilth Association) California, Minnesota, Nebraska, New York, and New Jersey. It has been shown in these areas to be economically competitive with other waste management methods. In addition, compost is generally seen as an environmentally beneficial product.
With the continued depletion of available landfill space and anticipated high collection and disposal fees needed to cover the cost of the refuse disposal facilities being built today, the separation of leaves, grass clippings, brush, and other yard debris from refuse will become increasingly attractive.
Remember: twenty to thirty percent of materials in the solid waste stream are compostable organic matter!
While this tutorial has been updated in places, the ordinal text was found on the Washington State University's web site. Unfortunately, that portion of their site has been discontinued, or at least we can not locate it anymore, so we do not know who actually producted the orginal text. Hence, we are unable to provide any further credit for what we think is a nice little Composting Fundamentals Tutorial.