RESEARCH & ARTICLES

In Defense of the Pile Less Turned –A Case for "Low-Input" Composting

by Steven Wisbaum, Updated February 2016


Although frequent turning regimens for turned-windrow composting operations continues to be promoted by some compost consultants, equipment manufacturers, educators, and the USDA* there's significant scientific and anecdotal evidence showing that compost of equal or even better quality can be made for a fraction of the cost and energy inputs using just a minimal number of well-timed and thorough turns. And in-fact, when building a relatively small (ie. 5 to 10 cu yd) compost pile carefully by hand, it's also possible to make excellent compost without ANY turning. As discussed in detail below, compost operations using this low-input method will not only enjoy the benefits of reduced labor, fuel, and equipment repair costs, but they also can significantly reduce water use, minimize the release of unpleasant odors, reduce the "carbon footprint" of their compost operations, conserve nitrogen, and produce compost with a higher organic matter and humus content due to reductions in CO2 and VOC emissions.


* Under the current USDA National Organic Program (NOP) standards, compost produced using the "turned-windrow" composting method must be maintained at a temperature of between 131°F and 170°F for 15 days during which period the piles must be turned a minimum of five times (7 CFR 205.203(c)(2), USDA, 2000).

However, in order to achieve optimal decomposition regardless of the number of times a pile is turned, the compost process must be proactively managed to optimally support the myriad of organisms that carry out the decomposition process. These management strategies include:



  1. Use as large a variety of ingredients as possible and ensure these ingredients are well mixed, either by turning with a compost turner or bucket loader, or by applying them in thin alternating layers.
  2. If the ingredients contain primarily plant material (ie. no animal manure), it's useful to inoculate the pile with a small amount of finished compost or biologically active topsoil.
  3. Because compost microbes require moisture to thrive and because this moisture also helps keep pile temperatures from becoming and/or staying too high (e.g. over 150 to 155 degrees F.), ensuring the pile starts out with an optimum moisture content is critical. It’s my experience that an optimum moisture content is between 60 and 70%, which is similar to a VERY damp (but not sopping wet) sponge. On the other hand, a pile that becomes excessively moist, or “saturated”, not only generates nutrient-laden leachate but also slows down the compost process by reducing the amount of pore spaces and thereby limiting the potential for "passive aeration". However, as the pile enters the “curing phase” and cools down, the moisture level should ideally be allowed to drop into the range of 55 to 60%, which is similar to the moisture content of a slightly damp sponge.

    For a variety of reasons it is even more important to make sure ingredients are thoroughly moistened at the beginning of the process when excessively dry weather conditions are anticipated. The reasons for this are as follows: 1. Undigested organic matter becomes increasingly hydrophobic as it dries out; 2. A negative feedback loop is created where-in the moisture content in a pile decreases which in-turn increases the temperature which then increases the evaporation rate which further decreases the moisture content, and; 3. The hotter a pile becomes, the more water is lost to evaporation which results in more water being needed to complete the process. And finally, since biological activity inside the pile will slow down or stop when moisture is a limiting factor, the entire process will take more time and/or the finished product will simply not be properly decomposed if there’s insufficient moisture.
  4. Create an optimal pile width and shape by taking into account the moisture and density (ie. Particle size) of the ingredients as well as the advantages and disadvantages of building a wide pile versus a narrow pile. For example, since very little decomposition occurs on the surface of a pile, for a given amount of material, a short wide pile will have a relatively smaller surface-to-volume ratio and therefore will expose a larger percentage of the ingredients to the most biologically active areas within the pile compared to a long narrow pile made with the same amount of material. A wider pile will also have greater insulation properties and therefore will lose less heat and moisture compared to a narrower pile. On the other hand, since the rate of gas exchange is inversely proportional to the depth (ie. There's less gas exchange in the center of the pile) and is also impacted by particle size and moisture content, when a pile contains a relatively large percentage of ingredients that have a high bulk density and are wet (e.g. food waste, dairy cow manure, etc.) there's an advantage to making a narrower pile (e.g. 6 to 9 ft wide) to account for the reduced gas exchange rate associated with these conditions. Conversely, a pile that contains a higher percentage of ingredients with a low bulk density such as horse manure or shredded yard debris can be made wider (e.g. 15 to 25 ft) with minimal reduction in passive aeration.
  5. Construct and turn the pile in ways that will minimize compaction and thereby maximize passive aeration.
  6. Throughout the entire compost process, check the interior of the pile regularly to monitor changes in color, moisture content, and odor. The interior of the pile should remain sufficiently moist, become increasingly dark in color, and take on a more "earthy" odor. A long-stemmed compost thermometer should also be used to monitor changes in the internal temperature. However, it's worth noting that temperature alone is an imperfect indicator of DESIRABLE biological activity since a pile containing excessively dry, carbonaceous ingredients (e.g. Horse manure, yard debris, etc.) will also heat up and become colonized by an undesirable white mold sometimes referred to as "fire blight" that looks similar to ash. In extreme cases, a dry pile can become excessively hot and become a fire hazard.
  7. If the interior of a pile begins to dry out before the active decomposition process is complete, water must be added as soon as possible.  This can be done using a compost turner equipped with a "watering manifold" or by using a sprinkler system that wets the entire surface AND interior of the pile, versus using a "drip" system which will likely result in the water simply "channeling" down through the pile and thereby only wetting only a relatively small percentage of the pile contents.
  8. Minimize turning. From my experience, turning is only needed: A. To mix the drier, less-digested outer material with the wetter/more digested inner materials. B. To restore porosity within the pile which is reduced over time as the pile settles from the force of gravity and the reduction in particle size. C. To break apart large clumps of material. D. To add moisture (as needed), and; E. To expose as much of the pile ingredients as possible to the hottest and most biologically active areas within the pile.

    If turning is being done primarily to restore porosity and keep the decomposition process moving at a moderate pace, turning can be timed to occur after internal temperatures have dropped 20 to 30 degrees F. from a high of 131 to 145 degrees F. And although turning with a specialized compost turner provides the most effective and efficient mixing, agitation, and aeration, turning can also be done using a bucket loader or excavator, or even by hand if the piles are small enough.
  9. Compost operations can significantly improve product quality while reducing both equipment use as well as labor inputs during both the active phase and on finished compost by using "macro-porous" compost covers (also known as compost "fleece") such as ComposTexTM. As described in detail on the CV Compost website (http://www.cvcompost.com), ComposTex is made of 100% UV-protected polypropylene fibers that allow the fabric to shed rainfall while remaining completely permeable to oxygen and carbon dioxide. These covers are also used to reduce unpleasant odors both by preventing saturated/anaerobic conditions from excess rainfall and by trapping odor-containing vapors on and within the fabric. However, to both minimize the labor required to manage the covers and to avoid premature degradation of the polypropylene fibers due to unnecessary exposure to UV-light, compost covers should only be used when absolutely needed.

    For example, while ComposTex is most commonly used to prevent excess moisture conditions in wet weather, it’s also sometimes used to protect piles from the drying effects of sun and wind in very dry weather. However, because moisture can still slowly exit the pile as water vapor, covered piles should have sufficient water initially and then regularly be monitored. And since the durability of ComposTex is significantly reduced when used for prolonged periods in the summer months and the water savings are relatively minor, other methods to reduce moisture loss are often more effective such as turning less frequently, reducing air flow (aerated systems), and/or building larger piles to reduce the surface-to-volume ratio. But if covers are to be used for long periods of time during the hot summer months, the author recommends using agricultural "shade cloth" (e.g. 85 to 95% shade) to reduce the amount of UV-light reaching the cover.
  10. Finally, since an important objective of composting should be to make material handling as efficient as possible, it's helpful to locate the compost operation to minimize the amount of handling/transportation of the ingredients and to ensure they can be easily turned, especially if using a bucket loader to "roll" the pile over, or pick it up bucket by bucket to build a new pile next to the original pile.


Debunking the supposed benefits of frequent turning regimes:
The defining feature of high-input composting is frequent turning and its supposed benefits include: ensuring optimum oxygen and carbon dioxide levels inside the pile; preventing excessive temperatures inside the pile; promoting high concentrations of beneficial microbes, and; improving compost quality. However, as described below, both field observations and scientific studies show that frequent turning is actually NOT the best way to achieve these goals, and in many cases, actually works against them:

1. Frequent turning is the best way to ensue optimum oxygen concentrations: It's been well established that most of the oxygen added to a pile during turning is used up by the microbial populations within hours, or even minutes of turning. Therefore, unless additional oxygen is being provided under pressure (ie. via "forced aeration") or a pile is turned hourly, the purpose of turning is NOT to add oxygen but rather to restore pile porosity which allows oxygen and carbon dioxide to naturally move into and out of a pile through a process called "passive aeration". And while decomposition and gravity-induced settling does reduce pile porosity over time, in piles that are not water logged or excessively dense and/or wide, adequate porosity is easily maintained in a pile for two to four weeks following each thorough turn, as evidenced by monitoring internal pile temperatures and the appearance and smell of the pile interior.

2. Frequent turning is the best way to prevent excessively high temperatures: Excessive temperatures (ie. over 155 degrees F.) can suppress certain beneficial microbes and even cause a pile to combust. However, such excessive temperatures are easily avoided by ensuring adequate moisture levels, pile porosity, and a reasonable carbon to nitrogen ratio. In-fact, trying to reduce excessive pile temperatures by frequent turning versus ensuring proper moisture, porosity, and C:N, will more likely have the opposite effect by both increasing moisture loss and by forcing more oxygen into the pile, similar to fanning a smoldering fire. By contrast, simply leaving a pile undisturbed will allow internal temperatures to drop naturally as the decomposition process reaches an equilibrium with the amount of oxygen that can enter the system naturally through passive aeration.

3. Frequent turning is the best way to promote high concentrations of beneficial microbes: There are a three critical flaws in the premise that frequent turning promotes high concentrations of beneficial microbes. The first flaw is associated with expected changes in oxygen content within the pile from turning. While it IS true that strictly aerobic microbes will die when oxygen concentrations drop below certain levels in a compost pile, it's also well known that many aerobes can simply reduce their metabolic activity and/or switch temporarily into an anaerobic state. And even though a small percentage of beneficial aerobic microbes might die in the oxygen-depleted and/or excessively hot core of a compost pile, a sufficient number of aerobes will easily survive in the outer layers of a pile to readily repopulate the pile core as the temperature drops and oxygen levels increase through passive aeration.

The second flaw relates to expected changes in the internal temperature related to turning. Specifically, while it is true that frequent turning tends to promote higher temperatures and elevated temperatures are associated with pathogen destruction, it is also well know that high temperatures will also destroy many beneficial microbes as well. Therefore, assuming a compost pile has sufficient moisture and achieves temperatures over 131 degrees F for at least a short amount of time, minimizing the amount of turning should also help moderate temperatures which will promote a greater diversity of beneficial microbes.

The third flaw is associated with the physical impact of turning on microbial diversity. In particular, scientists and farmers alike are increasingly appreciating fungi for their critical role in maintaining the health of the soil/plant ecosystem. However, since turning destroys hyphae, which are the branching filamentous structures that serve as their main mode of vegetative growth, compost piles that are turned less frequently will simply have higher fungal populations compared to piles that are turned more frequently.

4. Frequent turning is the best way to improve compost quality: There is little, if any, independent scientific data showing that the compost made using frequent turns is of higher quality than that of compost made with a minimal number of thorough turns done over a two to three month period. On the contrary, both practical experience and available scientific research show that compost made with fewer turns often contains higher nitrogen concentrations, organic matter content, and fungal populations than compost that has been turned more frequently. Other compost quality parameters that could be affected by turning frequency are as follows:

Phytotoxic Substances: Although some phytotoxic substances (e.g. sulfides, volatile organic acids, etc.) will likely be produced in the very center of a pile (the pile "core") due to anaerobic conditions that could develop between turns, these compounds are readily broken down when aerobic conditions are restored after turning and/or during the "curing" phase.

Weed Seeds: While it's commonly thought that weed seeds are primarily killed by heat in a compost pile, weed seeds are also destroyed by decomposition (rotting), and/or sprouting followed by agitation. Therefore, compost operators using only a minimal number of turns are able to achieve the same degree of weed seed destruction as compost operators who turn more frequently.

Pathogens: While it is widely accepted that pathogens are inactivated by high temperatures and that turning a pile to expose the entire pile contents to temperatures over 131 degrees F will substantially decrease the risks from pathogens, it’s also well known that there are many other mechanisms of pathogen inactivation including predation and competition. Therefore, assuming a compost pile has adequate microbial activity, there will likely be no measurable difference in pathogen survival rates in a pile that has been turned frequently versus in a pile that has reached proper temperatures but has only had a minimal number of properly timed, thorough turns.

Since 1996 Steven Wisbaum has produced and sold tens of thousands of tons of high quality compost using the low-input, minimal turn method described in this article. He has written numerous on-farm composting articles and guides including the "Horse Owner's Guide to Composting". He is also the largest worldwide distributor of ComposTex compost covers used by compost operators to protect active and finished compost from excess rainfall or to reduce moisture loss from sun and wind. He can be reached in the U.S. at 802-363-3930 or through his web site at www.cvcompost.com.

References:

Brinton, William F. What's Your Compost Energy Index? BioCycle Magazine, January/February 2008. Pg 53 – 54. http://cvcompost.com/research

Compost Fact Sheet #3. 2004. Improving and Maintaining Compost Quality
Cornell Waste Management Institute, Cornell University.
http://cwmi.css.cornell.edu/compostfs3.pdf

Harrison, Ellen. July 2003. The Quality OF NYS Agricultural Composts, Final Report of the Compost Marketing and Labeling Project, by the Cornell Waste Management Institute, NYSERDA Report 6593. http://compost.css.cornell.edu/MLfinalreport.pdf

Harrison, E., 2007: Compost Facilities: Offsite Air Emissions and Health. Cornell University Waste Management Institute. Ithaca, NY, USA. p 4.
http://cwmi.css.cornell.edu/compostairemissions.pdf

Hepperly, P. and Ziegler-Ulsh, C., 2007: Studies and Advances in Composting Technology. Acres, September 2007, p 16-23.

Hill, D., 2008. Composting and Climate Change; Opportunity in A Carbon Conscious World, GOC Technologies.
http://cvcompost.com/research

Jenkins, J.,2005: Humanure Handbook, 3rd Edition. Chelsea Green Publishing. Vermont, USA.p 48–52. http://www.gubaswaziland.org/files/documents/resource9.pdf

Michel Jr., F. et al, 1995: Effects of Turning Frequency, Leaves to Grass Mix Ratio, and Windrows versus Pile Configuration on the Composting of Yard Trimmings. Michigan State University Press, Ann Arbor, Michigan, USA for the Southeast Oakland County Resource Recovery Authority. p 1, 5, 6, 7. https://www.msu.edu/user/michel/swap2.htm

San Diego State University, March 2007. Contractors Report - Comprehensive Compost Odor Response Project, Integrated Waste Management Board Public Affairs Office, Publications Clearinghouse Pub #: 442-07-001

Tiquia, S.M. et al, 2002: Carbon, Nutrient, and Mass loss During Composting. Nutrient Cycling in Agroecosystems, 62: 15-24.
http://www3.abe.iastate.edu/hoop_structures/swine/pdf/recent/manure_mgmt/compostingloss.pdf

Tirado, Sandra M. 2008. Effects Of Turning Frequency, Pile Size and Season On Physical, Chemical And Biological Properties During Composting Of Dairy Manure/Sawdust; Ohio State University.
http://etd.ohiolink.edu/view.cgi/Tirado%20Sandra.pdf?osu1218133685