When I was a kid, I remember my dad piling leaves and grass clippings into an area in the back of the yard. Eventually, the pile would yield a loamy substance called compost. As time went on, he began to use one of those compost tumblers, a large plastic vessel with a crank. Into the tumbler went leaves and grass clippings again, but the results were more rapid. The increased heat and mixing seemed to yield compost much faster.

What my father was taking advantage of was an application of the Berkeley method of hot composting. There are several ways to compost organic material:

• cold composting
• hot composting
• vermicomposting

The Berkeley method

Each has its advantages and disadvantages, but the trickiest in many respects is hot composting. For years, the Berkeley method, described by UC Berkeley professor Robert D. Raabe in this paper has defined the standard for hot composting methodology. The method has the following components:

• The material should have small particle size. Materials of smaller dimensions will breakdown faster.
• The carbon/nitrogen ratio (C/N) should be 30:1.
• The pile moisture content should be about 50%
• Heat is an essential element in the method. Care with the sizing of the pile is important to preserving heat. Piles that are smaller than 32" in any dimension can lose heat so fast that decomposition is slowed.
• The fastest decomposer organisms have an optimum temperature of about 160 degrees Fahrenheit.
• Frequent turning of the pile is necessary to maintain the temperature. If the pile is turned daily, the compost will be ready in about 2 weeks. If it is turned every other day, it will take about 3 weeks.
• No additions to the pile are recommended once the pile has been formed.
• No compost starter is required because the microorganisms that do the work of composting are ubiquitous in the environment.

Modified Berkeley method

Although the Berkeley method of rapid composting has been very helpful in processing large amounts of material, it has some disadvantages that I’ve tried to address in a series of modifications.

• The construction of the pile at a single point in time doesn’t coincide with the availability of organic materials. Household food scraps for example, become available on a daily basis in most homes. Stockpiling them for 2-3 weeks while waiting for the current pile to complete is impractical.
• The method cannot be used in the wintertime in colder climates. Once a pile freezes, it is nearly impossible to restart it; so continuous addition of material to keep it warm is essential.
• The C/N ratio is difficult to assess for most individuals and it doesn’t adequately predict the availability of carbon, in particular, at the level of the composting microbes. Some carbon sources, in other words, are most bioavailable than others. This seems to be a lesser problem with nitrogen-rich materials. However, my experience with hot composting suggests that the nitrogen content of these materials vary considerably over the course of the growing season.

Continual addition of new material

I’ve addressed the problem of availability of materials by allowing for the addition of new material after the pile has already heated up. In this way, I can continue to use organic materials, particularly household food scraps, as they become available. Of course, the duration of the process is considerably longer. But with a multiple bin setup, this is a lesser issue. Once the pile has attained is maximum size as permitted by the dimensions of the bin, it is allowed to continue at high heat levels until the temperatures begin to fall off from the peak. At that point, I move the material over to the next bin to complete the process while I begin work on the next pile.

C/N ratio and bioavailability

Availability of carbon in the source materials varies considerably. For example, the ratio for wood chips is about 500:1^[On-Farm Composting Handbook, Appendix A. Characteristics of Raw Materials. Accessed on 2016-09-03 at http://compost.css.cornell.edu/OnFarmHandbook/apa.taba1.html] However, wood chips breakdown rather slowly; so I’m not sure that the carbon is as available as it is in other carbon-rich sources. I tend to use carbon sources that have better bioavailability such as partially decomposed leaves, shredded paper and shredded corrugated cardboard.

Another bioavailability issue that I’ve uncovered is that the available nitrogen in grass clippings varies considerably. During the growing season, grass clippings are our main nitrogen source. But over the course of the season, the amount of nitrogen seems to vary. During very active growth periods after rainfall and after relief from a hot spell, the amount of nitrogen seems to be higher. During these conditions, even the same compost recipe results in much higher heat levels and stronger ammonia off-gassing. It is not so much a modification to the method as a note to consider the variability of nitrogen content in free materials.

Winter composting

I will admit that I have not successfully kept a hot composting pile going all winter. But this may be the first year! However, the unmodified Berkeley method won’t do in our Canadian winters. Since the method calls for allowing the pile to cool down, it would rapidly freeze if not kept warm by the addition of new source materials and turning