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On-Farm Compost Production:
Learn to Develop a Healthy Herd of Microbes
This article was first printed in the July - August 2003 issue of the Organic Broadcaster, published by the Midwest Organic and Sustainable Education Service.
Producing and using compost is a great place to start and to continue learning about soil fertility and plant health management. Compost is a key building block of organic farming. Producing high quality compost is not difficult, but it does require a certain degree of commitment and preparation.
Composting plant residuals, animal manure and bedding, or hay harvested specifically for making compost can provide a stable, weed- and pathogen-free source of organic matter and nutrients. In addition to being a source of available and stable nutrients, compost is a source of bacterial and fungal diversity that is often lacking in soils that have been used for conventional, chemical agriculture. Compost can be used as a potting media component for container-grown transplants. We are also learning more about how to use water extracts or teas of compost to manage foliar and soil-borne fungal plant pathogens.
We can also
think about what we feed a farm animal and how that influences the
manure. We can feed grain, which is high in protein and therefore
nitrogen. We can feed alfalfa or legume hay that will also have
protein and nitrogen, but not as much as grain. Or we can feed grass
hay, which will have less protein and nitrogen than grain or alfalfa.
Feeding an animal more protein might increase the growth, but it
will definitely increase the nitrogen in the manure.
Certain microbes, particularly bacteria, use more nitrogen, so if there is more nitrogen in the feed, there will be more bacteria. If there is less nitrogen, other microbes, particularly fungi, will take over feeding on the organic matter. In either case, eventually there is a veritable graveyard of dead microbes and microbe manure. When the microbes die, the nutrients become available for other microbes or for plants. When all the food is consumed, the microbe manure improves or becomes more stable with aging, just as with any other manure source.
Examples of materials available for composting include legume and or grass hay, leaves, farm or garden plant residues; vegetable or fruit processing residue, chopped corn (silage), soybean stems, animal manure, straw or wood shaving bedding and liquid or tankage materials. Mature compost or soil may be added as an additional microbe source.
A safe rule of thumb for starters is to use three times the amount of carbon materials than nitrogen materials (based on weight), and adjust the mix based on whether the pile heats up or not. Tables or book values of carbon and nitrogen content are available, and a few are listed in Table 1 on page eleven. If there is excess nitrogen at the start, it will probably be lost in leaching water, or as gaseous nitrogen--ammonia (NH3) if the pH is high, or nitrogen gas (N2) if oxygen is low.
In each case we can "fix" or adjust the recipe if one of the basics in missing. With too much nitrogen, a high carbon material such as leaves, straw, or sawdust, can be added. If there is not adequate nitrogen, an organic nitrogen fertilizer source can be added. Alfalfa hay is a good source of nitrogen that also brings a balanced addition of other essential nutrients. Animal manure or food wastes are also higher in nitrogen.
One of our better batches of compost to date at MSU was a mixture of 20% by volume swine manure and wood shavings, 20% dairy manure and horse (hay) bedding mixture, 20% peat, 10% aged leaves, 10% chopped corn silage, 10% alfalfa hay and 10% grass hay. While I don't have data or evidence, my first composting teacher was very big on materials and microbial diversity. He really liked variety in the pile.
Water and air
should also be considered as essential materials. Without adequate
moisture, the microbial activity will be limited Without oxygen,
microbial activity will continue, but the type of microbes and the
end products are generally not considered desirable. A general recommendation
is to avoid materials that have been piled and rotted without adequate
oxygen. These materials will usually have a foul odor or stench
of decay. The bacteria that form when materials decay without adequate
oxygen usually creates products in the compost that are not desirable.
The farmer needs to know some basic characteristics of the materials, primarily the amount of carbon and nitrogen (C:N ratio), moisture content, and physical structure, in order to come up with a good compost mix. Table I lists the C:N ratio, moisture content, and bulk density of a variety of farm-generated organic materials. The first six materials listed are typically below the desired optimal C:N ratio and are considered nitrogen feedstocks. The remaining five materials are well above the desired C:N Ratio and are considered carbon feedstocks. Materials like "vegetable produce" itself will have a wide range of variability in terms of the type of vegetables, but vegetable matter in general should be treated as a nitrogen feedstock. High nitrogen feedstocks, especially wet manure and vegetable matter, should immediately be incorporated into a compost mix. If these materials are stockpiled for several days, microbial activity could deplete the available oxygen and create anaerobic conditions. This would create a host of problems including odor, diminished quality to the compost, and attracting pests such as flies and rodents.
With packed bedding, some farmers have adjusted how much straw bedding is continually added to stalls so that when it is removed from the stalls, typically every six to eight weeks, it is already at an ideal C:N ratio and moisture content for composting. The packed bedding cleaned from stalls at the Michigan State University Dairy Teaching and Research Center is typically above the optimal C:N ratio. Manure collected from the tie and free stall dairy units is added to bring the nitrogen levels up.
It is better to be higher than the optimal C:N ratio than lower. The higher the carbon levels in the compost mix, the more nitrogen conserved and the less likelihood for the process to go anaerobic, thus preventing the occurrence of offensive odors. The drawback is increasing carbon also increases the amount of time to produce finished compost, and this in turn increases the space required.
Do not exceed the optimal moisture content! Excessive moisture will lead to anaerobic conditions and the production of foul odors. Sometimes the mix will be a compromise between C:N ratio and moisture content, but often moisture content will be the deciding factor. Carbon feedstocks such as straw, wood shavings, and leaves, are typically dry unless stored uncovered. Therefore, if a nitrogen feedstock is very wet, the mix will likely have a C:N ratio above optimal, thus prolonging the time it takes to compost. One method composters will use in such cases is to incrementally add the wet nitrogen feedstock to an active compost pile. A hot compost pile loses moisture quite rapidly. Frequent turning also helps reduce moisture content.
Different carbon feedstocks decay at different rates. Wood shavings and wood chips in particular are very slow to decompose due in part to the presence of lignin. Leaves are very quick to decompose when mixed with nitrogen feedstocks, but there are differences mainly in C:N ratio between freshly raked leaves, and leaves that were stored for several months or years.
The following are ratios of mixes that have been tested at Michigan State University and are representative mixes:
Many yard clippings compost operations will set up piles or windrows of leaves in the fall, and then incrementally add grass during the spring. Early in the spring the C:N ratio of the mix is very high but decreases as more grass clippings are added. This same type of approach can be followed with any carbon and nitrogen feedstocks used in farm compost mixtures.
There are also cases where poultry manure is partially composted without any carbon feedstocks and sold as a fertilizer. Poultry manure is considered a very hot nitrogen feedstock in that it heats up very rapidly, and the heat drives off moisture to the point that it dries the manure out before it has fully composted. While a significant portion of nitrogen is lost as ammonia, the resulting product is still high in nitrogen.
The organic standard also provides rules for when animal manure is applied directly to cropland without composting. There must be a ninety day period for above ground crops or 120 period for crops in contact with the ground from between the time manure is applied and crops are harvested. It seems logical that as long as these same time criteria are met from the starting point of addition of animal manure to the compost pile until the compost is used, the spirit of the law is met.
All of this is relevant because high quality compost can be made quickly with frequent turning or slowly with less frequent turning. The more frequently piles are turned and mixed, in general, the greater the availability of food, air and water, and the faster the rate of microbe growth. Microbe growth and composting will occur with less turning, but it will take more time. A minimum pile size (usually 3' x 3' x 3') is necessary to obtain adequate heating. In very large piles, pipes or tubes like 4" plastic drainage pipe available for about $20/100 feet may need to be added to provide adequate aeration. Placing them on an angle so that warm air rises and creates a draw helps pull more air through the pile.
Some farmers have set up windrows in fallow fields. This is an acceptable practice as long as the field has a minimum of 2% slope, and the windrows should be covered (e.g. fleece blankets) during periods of heavy precipitation to avoid leaching of nutrients into the groundwater.
When time and space is not critical, a farmer has significantly greater flexibility in how they use their compost. A batch of compost set up in the spring, even if not fully cured, can be applied to fields in the fall. However, batches set up in the summer and fall may not be mature enough for spring application, hence experienced farmers often wait a full year before using a batch to ensure the compost is fully cured and mature, especially when used on high-valued crops or in transplant medium.
A windrow is just a long pile. There are two main limitations to how big to make a windrow--aeration, and method of turning. Mechanical turning with a front-end loader is a good option because farmers often already have that equipment on hand. When mixing and turning piles/windrows with a front-end loader, dig into the bottom of the pile lift the bucket, and drop the material. The main drawback with front-end loaders is if the farmer intends to sell the compost. Front-end loaders do not break up and mix the materials as thoroughly as a dedicated turner, which will result in some clumps and a product that does not have the "look" consumers have grown accustomed to with compost. For the farmer, passing compost through a spreader can break up these clumps.
Plant material typically has better structure than manure, which is critical in providing porosity and hence, aeration. For manure compost, bulky materials like straw or wood chips are often used to provide and maintain structure and porosity. They also are a suitable carbon source. These materials, or bulking agents, will allow for manure compost to be piled higher, but the drawback, especially in the case of wood chips, is they decompose much slower than other types of carbon feedstocks. Leaves provide structure early in the compost process, but tend to lose their structure later in the process.
The volume reduction of bulky mixes can be over 50%. The volume reduction of denser mixes tends to be no greater than 40%. Because of the volume reduction, piles and windrows can be combined at some point during the process, which frees up space. It is also a good idea to not let piles or windrows get too small or else they may not be able to retain the heat generated, and they may dry out very quickly.
Well-aerated compost will reduce the amount of turning and improve compost quality, but can also lead to rapid drying of the compost before it has fully decomposed. In the case of heavier, denser, and less porous mixtures such as wet manure and sawdust, aeration can be enhanced by building the piles/windrows on a base of porous material like wood chips, and also placing perforated pipes in the base. In this type of system, known as passively aerated composting, the compost is not turned. This requires very thorough initial mixing of the materials, which can be accomplished by passing the mixture through a manure spreader. Some farmers use a feed mixing wagon to mix compost. If the mixture contains manure, either power wash the mixer wagon before using it to mix feed, or dedicate the wagon for just mixing compost. After the mixture has been piled, cover the entire pile/windrow with a layer of finished compost, peat, mulch, or wood chips. This layer acts as insulation, which is needed to ensure uniform heating and decomposition throughout the whole pile/windrow. Using finished compost or peat as a cover is preferable because it can be incorporated with the compost pile/windrow without diminishing quality. Mulch and wood chips are not fully decomposed and would either need to be screened out or, if left in the mixture, the compost would need to be cured for a longer period of time.
Turning is probably the most demanding part of composting in terms of time and energy. If the mix has good structure in terms of porosity, then the pile or windrow will only need to be turned two to three times during the entire process, to incorporate the material on the edges into the inner regions where composting happens at a much more accelerated rate. Denser compost mixes need to be turned more frequently to allow for better airflow, but the mix tends to settle within two to three days of turning, so it's a short lived benefit. It was commonly believed that turning introduces air into the pile, but several researchers have found that the microbes consume the new air within hours.
A compost mixture that exceeds 160°F should be turned to release heat. At such temperatures the beneficial microbes needed for decomposition begin to be killed off. If a pile is not heating, the food supply (carbon and nitrogen), moisture level, and aeration needs to be considered. If the pile is mixed and heating occurs, then either aeration (oxygen) was limiting, or the layers and sources needed to be mixed. If no heating occurs and water may be limiting, add water. If both air and water are present and the pile has been mixed, either composting is finished or more nitrogen or carbon needs to be added.
We created three piles at MSU with the same recipe but different management methods. Ingredients for the first pile were put in a manure spreader and mixed as the windrow was formed. Two additional piles/windrows (5'x5O' base) were formed by layering the materials mentioned above. The first and second piles were turned about every two weeks with a compost turner. The third pile was rolled or turned about every three weeks with a front-end loader. The pile mixed with a manure spreader and turned with a turner was done cooking in about eight weeks. Nitrogen tested at 2.5%, the pH was 6.5, and the C:N ratio was 12:1. The batch that was layered first and then turned with a turner took about two to three weeks longer to finish heating. The final pile took much longer, about 16 weeks, and still had some hay that was not decomposed. Eventually the quality of the compost was similar, but the equipment and effort were much different.
Temperature will be the easiest indicator to monitor. Compost thermometers with over 12" probes are available for under $20. When a human hand can be placed into the pile and not burned, the temperature is below 130°F. When a hand cannot be held in the pile for longer than a few seconds, the temperature is above the 130°F mark. Temperature above 160°F is not desired because certain beneficial bacteria are lost. The location in the pile will influence the temperature. Normally it is recommended to test the internal temperature about 2/3 down and l/3 in from the side. The hottest spot in a windrow will be the top of the pile where convection currents exit the pile. This is the place to check for excessive heating. The graph on page seven provides some ideas of how temperature patterns are influenced by the C:N ratio.
The moisture content of compost during active composting should be at a point that some water can be squeezed out by hand. There is not an easy or affordable way to accurately measure water content other than using a scale. For example, if a sample of compost weighs ten pounds moist and six pounds after oven drying, then the moisture content is four pounds (10 - 6) or 40%. Moisture content in the 50 to 75% range would be recommended. A bucket and a spring scale may be the easy way to measure a larger sample. The weight of five gallons of compost can be multiplied by forty to provide an estimate of the weight per cubic yard. A bucket of compost spread out in a thin layer can air dry in two to three days in a bright sunny location. This will provide a reasonable estimate, although oven-dried compost would be more accurate. If the compost is being dried for nutrient analysis, do not dry on an absorbent material that will remove water and nutrients.
be measured with oxygen probes but the cost is prohibitive for most
small farms and the equipment is not likely necessary. Oxygen levels
above 5% are desired. Levels of 2% or less usually indicate turning
is necessary. The appearance of the pile and sour or foul odors
will indicate low oxygen.
For container plant production, the availability of fertilizer in soilless media is monitored using an electrical conductivity meter (EC). Pure water does not conduct electricity and has a conductance of zero. The more salt (positive cations and negative anions) in the water, the greater the conductance (mhos or millisiemens). High conductance readings indicate a high level of soluble salts. The salts can be beneficial such as nitrate, ammonium, potassium, calcium, magnesium, or the salts can be undesired such as sodium and chloride.
While more detailed testing is necessary to determine what salts are present, measuring EC is a quick way to monitor both progress of the process and potential problems. Usually a volume of soil or compost is diluted with two parts (volumes) of distilled or RO water. A reading of zero to two mmhos or millisiemen would be low, between two and four would be moderate, and greater than four would indicate a high level of soluble salts that might be a concern. Ideally nitrate-nitrogen could also be measured by ion selective electrode or color indicator strips. (For monitoring equipment contact Spectrum at 800-248-8873, www.specmeters.com). More complete testing of soluble or total nutrients can be performed by many private or university soil testing labs. The carbon content can also be analyzed to provide C:N ratio. Interpretation guidelines for nutrients are available.
Compost maturity is usually measured by the rate of carbon dioxide evolution. [One type of test kit to measure maturity is sold by Woods End Institute (www.woodsend.org)]. Immature compost may also be losing nitrogen in the form of ammonia, which can be measured colorimetrically. It is usually recommended that at least two months pass between the end of heating and use of the compost. Four to six months would likely improve the quality of the compost assuming it is protected from leaching rains but moisture is maintained.
Another way to test the maturity of compost is to germinate seeds in the compost or compost mixed with a soilless root medium. Seedling bioassays provide a reasonably fast and very inexpensive measure on nutrient availability. A seedling bioassay can also indicate the presence of any phytotoxic chemicals or compounds in the compost. Recommended species include tomato, peas, corn, sunflower, and beans. This provides a range of plant types with known sensitivities to herbicides (tomato) and other compounds (peas). Corn and sunflower are very responsive to the amount of nitrogen present. A comparison to plants grown with added water soluble fertilizer like fish emulsion will give an indication if nitrogen is limiting. Snap beans also respond to nitrogen, but seem to be more sensitive to soluble salts. If adding fertilizer does not increase growth, see if leaching the compost with water to wash out salts will help.
John Biernbaum, PhD., is a professor of horticulture at Michigan State University. Andy Fogiel, a graduate student in the MSU Department of Horticulture, is coordinator of the Michigan State Sustainable Agriculture Network and chair of the Michigan Composting Council. John, Andy, and UW asst. professor in soil science Leslie Cooperband taught a course in composting at the 2003 Organic University, presented by MOSES.Return to TOP