Farmers, municipalities, and home gardeners alike understand the concept and value of composting, but they may not understand how to make good compost. The National Organic Program (NOP) offers guidelines for composting to eliminate pathogens and weed seeds. However, following these guidelines does not guarantee the creation of quality compost. To address this concern, composting research at Rodale Institute is geared towards developing methods that will produce quality compost and clarify some of the best practices in order to bring greater success in implementing the NOP guidelines.

Composting is the oxidative decomposition of a mix of organic materials. Oxygen is required to support the growth of beneficial organisms and to eliminate the risk of pathogens and other toxic compounds. Waste materials are blended together and then mixed on a regular basis to maintain proper oxygen levels throughout all parts of the pile. The oxygen is constantly being used up by the microorganisms that are breaking down the organic material, and there is a limited volume of space between the material particles of the pile to hold oxygen. As a result, a compost pile must be turned many times to “re-fill” those in-pile spaces with oxygen. This turning process ensures that oxygen levels do not drop low enough to kill the good organisms and grow pathogens.

NOP guidelines require compost to be turned a minimum of five times within a 15-day period, during which time the temperature must be maintained between 131- and 170-degrees F. While the guidelines act as a safety net to ensure pathogens are destroyed, they do not ensure that a diversity of beneficial organisms will be produced in the end product. In some cases, during the active phase of composting when organisms are using lots of oxygen, five turns in 15 days may not be enough to maintain proper oxygen for growing beneficial organisms. In other cases, the pile may not need to be turned as often as five times in the 15-day period.

This is a critical issue that may leave composters feeling uncertain. Instead of a general rule for turning, guidelines that help composters know exactly when the pile needs air would improve the end product. By observing biological indicators of compost activity in a pile, a compost pile can be turned at the exact moment that the organisms need more air.

Establishing the research: May

Last May, as part of a research project funded by a Natural Resources Conservation Service Conservation Innovation Grant (NRCS-CIG), six piles were built on concrete pads to investigate issues with compost management according to the NOP guidelines. The piles were made out of identical materials: leaves as a carbon source, food waste as a green material, and chicken manure as a high nitrogen input. Three piles were managed according to NOP guidelines and the other three piles were managed by a process known as TAT (turn according to temperature).

The activity and growth of organisms produces vast amounts of heat and causes the temperature of a compost pile to rise quickly. This heat is essential to killing harmful pathogenic bacteria, fungi, protozoa, worms, and other parasites as well as weed seeds in the pile, and it can also tell us when the pile needs to be turned. We use thermometers to measure the temperature of our compost piles as an indicator of the activity of the microorganisms in the piles. When a pile reaches 160 degrees F (or above), we know it is time to turn the pile because high temperatures indicate that the activity of organisms is so high that the oxygen in the pile is being used up faster than it can diffuse into the pile from the outside air. This loss of oxygen, coupled with the high temperatures, can actually kill the good organisms if it isn’t addressed quickly enough.

The TAT piles were turned whenever temperatures reached this critical threshold, whereas the NOP piles were simply turned five times in 15 days (typically once every 3 days) as the guidelines suggests. Our goal was to investigate if turning according to temperature would result in a higher quality of compost with more beneficial microorganisms. We were also hoping to clarify whether the TAT method would allow the pile to be managed more easily than the NOP method, which requires mandatory turning but not always necessary or timely turning.

Concrete pads: The issue, the resolution

The design of the concrete pads used as platforms for our six compost piles caused quality issues in our end product. The design of the concrete pads inhibited mixing because the sides of the pads were angled slightly towards a drain at the center of the pad to allow any water leaching out of the pile to drain out into an underground holding tank for testing. As mentioned above, compost must be thoroughly mixed to maintain aerobic conditions, but the slight V-shape of the pads prevented proper aeration of a portion of the compost at the very bottom of the piles. Our flat-bladed compost turner was unable to reach those bottom-center materials, allowing that small portion of the pile to go anaerobic and generate pathogens. Even though harmful pathogens where destroyed throughout the rest of the piles, that small anaerobic portion of material at the bottom center was a continual source of pathogenic contamination for the rest of the pile.

The picture above illustrated the area of the concrete pads that could not be properly mixed and aerated. Properly aerated compost will be the brown color of 75% cocoa. Note how the color is dark black in the center of the concrete pad, indicating that the material became oxygen deprived and went anaerobic.

Luckily, the ability of the concrete pads to drain liquid was determined to be unnecessary, which allowed us to resolve the turning issue easily. There was no liquid collected from the holding tanks below the concrete pads at any point during the process. This result illustrated another valuable aspect of making quality compost. A compost pile shouldn’t produce leachate if you’ve used a proper mixture of starting materials. It is absolutely necessary to use enough brown materials, such as leaves or woodchips, so moisture levels are managed properly. The brown materials also provide the piles with adequate structure to allow oxygen flow throughout. Too much wet waste material will cause the pile to become compacted due to lack of structure, preventing the circulation of oxygen.

Similarities in TAT and NOP piles: Actinobacteria

In both the NOP and the TAT piles, we ran into problems with the growth of actinobacteria. These bacteria are easily recognized as a powdery or ashy white growth that can show up in a compost pile with reduced oxygen. We determined that the standard compost recipe we used was too low in brown materials to deal with the quantity of food waste in the mix, leading to the overgrowth of actinobacteria throughout the trial. Although we didn’t have enough food waste to create leachate, it was still too much for proper oxygenation.

When a pile isn’t turned frequently enough, or if the pile becomes matted down by too much food waste, the oxygen levels will decrease, and these facultative anaerobic bacteria will take over. The problem is that actinobacteria suppress the growth of other beneficial organisms such as mycorrhizal fungi. Most agricultural soil is lacking a healthy fungal community and has too much bacteria to begin with. So adding heavily bacterial compost to a soil that already has plenty of bacteria doesn’t really benefit the soil microbial population, or result in improved plant growth. Ideally you’ll want to make compost very high in beneficial fungi so that you can begin correcting imbalances in your soil.

Differences in TAT and NOP piles: Actinobacteria

Visibly white-ish areas are actinobacteria.

At the end of the composting process we analyzed all six compost piles and found that, while all piles contained actinobacteria, the TAT piles had lower levels. By turning according to temperature, we were able to introduce oxygen into the pile at the crucial times when oxygen levels where beginning to fall. Turning according to the NOP guidelines did not always coincide with the critical temperature window when oxygen was most needed in the pile.

Furthermore, the TAT piles only needed to be turned four times instead of 5 within the first 15 days. This demonstrates an interesting point. We were able to turn less often yet produce higher quality compost because we turned at the critical time when oxygen demands where highest.

Replicating the research: July

Our next step was to repeat the experiment in July by building six new compost piles. This time we decided to move the trial off the concrete pads so we could eliminate possible contamination. The six piles were built on level ground to ensure that every inch was being aerated by our Global Repair Sittler1014 compost turner.

Beneficial fungi in a compost sample (400 magnification).

We also decided to make an adjustment to our compost recipe. We doubled the amount of leaves so our piles would have better structure and allow more airflow through the pile. We learned from our May piles that food waste can be very high in moisture and that there must be a substantially high amount of brown material in the pile to properly absorb and manage the moisture. We also suspected this new recipe would grow more beneficial organisms, especifically fungi. Since fungi live on logs, sticks, leaves and other woody material in the environment, we hoped that the extra leaves would help us make compost with higher numbers of fungi.

The management of the new compost piles was also slightly modified from the previous trial. As before, three piles were turned according to NOP guidelines and three piles were turned when temperatures reached 160-degrees F or higher. However, we decided to also turn the TAT piles whenever a significant amount of actinobacteria was visually apparent. We hypothesized that if the temperatures reached the critical threshold during the nighttime hours, depleting oxygen levels, then we would see the effects in the morning by looking for actinobacteria.

May versus July

Analysis of the data shows that the July NOP and TAT piles had significantly lower action bacteria and significantly higher fungi than the May NOP and TAT piles. This indicates that the adjusted compost recipe with higher amounts of leaves contributed greatly in solving the actinobacteria problem. The data also shows a trend of the May TAT piles having lower amounts of actinobacteria compared to the May NOP piles, suggesting that the TAT management practices may be beneficial in reducing these bacteria.

The data shows that there were slightly higher fungi levels in the NOP piles, but this difference was not statistically significant. The number of fungi in all piles was still much lower than is needed to build better soil fungal populations and grow healthy plants.

We have learned that it may be possible to make quality compost by turning only at critical stages in the composting process exactly when the biological activity is reaching a crashing point, rather than turning a pile a prescribed number of times. We did discover, however, that actinobacteria may indicate that we have reached this turning threshold during the night when nobody is available to monitor temperature.

We have also demonstrated that the starting ingredients in the compost pile must be in the right balance to allow for proper moisture balance and adequate airflow. You must understand the nature of the waste resources you have and carefully plan out a recipe that will address potential problems later down the road. We quickly learned that much more leafy brown material is needed to compost wet food waste, which help with the in-pile dispersal of oxygen, the critical component to making compost.

Looking to next year

Next year, we plan to build six more compost piles and repeat the experiment with the goal of finding more effective ways to promote fungal growth in the piles. This time, we will be monitoring the temperature throughout the night via a newly purchased recording thermometer, to determine if a correlation can be made between the visual appearance of actinobacteria and peaks in temperature at night. This will hopefully allow us to understand what is happening to the compost at night when nobody is watching. We can then determine better management strategies to mitigate problems that may occur.

We were able to reduce the problem with actinobacteria by adjusting our compost recipe to include more leaves. However, one problem that can’t be fixed so easily is the amount of garbage that can end up in compost made with food waste. Waste hauling companies are struggling to eliminate trash contamination, such as plastic bags and other non-compostable materials, so we have decided to use alternate waste materials for our green pile component until the contamination problem is solved.

Next year, instead of using food waste, we will use aged cow manure and rotten hay. These are both easily obtained farm waste products. We will also continue to use leaves because they are readily available. We predict that we will be able to further reduce actinobacteria and hopefully grow much more fungi when we make these adjustments.

This material is based upon work supported by the Natural Resources Conservation Service, U.S. Department of Agriculture, under Grant Agreement Number 69-2D37-11-499. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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