Pros and Cons of Hot Composting

Most gardening books focus primarily (sometimes exclusively) on hot composting methods. They describe elaborate procedures for turning out the perfect batch of compost, including balancing nitrogen content and monitoring temperatures.

There is another way to compost—cold composting. The most obvious difference between the two methods is the temperature of the pile.

Each method has its advantages and disadvantages, making each one suitable for different circumstances. This week, we will examine the key features and pros and cons of hot composting, giving you an idea of the best applications for this technique. Next week, we will tackle cold composting.

Let’s get started!

Introduction to Hot Composting

The right balance of ingredients is key to the hot composting process. Every hot compost pile needs some materials high in nitrogen (called “green materials”) and some materials high in carbon (called “brown materials”). The green materials feed the microbes that will do the work, thereby fueling the whole process. The brown materials add bulk and air flow, preventing anaerobic decay from taking over and creating a foul smell. Experts vary in their opinions of the proper balance between browns and greens. A ratio of two browns to one green or even one to one consistently works for most home gardeners. For faster, hotter decomposition, chop the ingredients into small pieces. Layering greens and browns is not essential.

Most gardeners also like to add an activator of some sort to the pile to ensure that the necessary organisms are present and ready for work. Many expensive preparations are available for purchase, but good-quality soil is sufficient for the purpose.

The compost pile should be a three-foot cube to allow the center to hold heat efficiently. A four-foot cube is even better, while a five-foot-cube is about the maximum size for effective composting.

Once the ingredients are properly balanced, special microorganisms take over the pile and begin the process of decay, emitting heat in the process. The heat builds up quickly in a well-made compost pile, ideally reaching temperatures between 120°F and 150°F.

From time to time, the microbes will use up the oxygen in the center of the pile, causing the temperature to drop. For this reason, you will need to turn the pile once a week or more, giving the microbes access to fresh air and bringing organic matter from the outside of the pile into the center for processing.

At the same time, you will need to consider the moisture content of the pile. It should be moist, but not soggy. In dry weather, you will probably have to spray the pile down with a hose while turning it. In wet weather, you may want to place a tarp over the pile or make your compost in a fully contained bin to avoid drowning the microorganisms.

After about a month, the compost should look dark and crumbly (think chocolate cake crumbs), with no recognizable remnants of the former ingredients. At this point, the pile will no longer heat up when turned. The compost is technically finished, but it will benefit from one or two weeks of curing. This allows beneficial organisms a chance to repopulate the compost.

Pros

Speed. Hot compost does its thing in short order, making the finished product available within three to four weeks.
Reduced space needs. A hot compost pile is finished in as little as a month. A cold compost pile can take up garden space for as long as a year, requiring a commitment in advance.
Full control. Some gardeners like the amount of control hot composting gives them over the finished compost. They can easily adjust the temperature or decomposition speed by just turning the pile. To them, monitoring the pile is not a chore; it is an art.
Sterilization. This is probably the #1 reason that gardeners choose hot composting methods. As the compost pile heats up, many pathogens and weed seeds fry, rendering them harmless.
Toxin degradation. Are you using ingredients that contain chemicals? Hot composting can break down and neutralize the pesticides present on that orange peel and the residue of any medications you might have administered to your horse that are now lurking in the stable bedding.
No pests. Assuming the composting process is working correctly, that is. Most insects and rodents will leave a hot compost pile alone. A poorly made compost pile can still attract every type of pest from ants to raccoons.
Helpful heat. The heat put out by a good compost pile can be put to work. One of the most common uses for this composting byproduct is heating a greenhouse.

Cons

Difficulty. Unsuccessfully battling a compost pile that refuses to heat up or that smells and draws flies is frustrating. Hot composting is part art, part science. It takes practice. (Here’s a hint—make your compost in summer, when naturally hot temperatures will give you a head start.)
Need for ample resources. A hot compost pile cannot be built in stages. You must have all of the necessary ingredients on hand and in the right balance from day one.
Time and effort. A hot compost pile needs quite a bit of supervision. Besides weekly turning and watering, it will need to be monitored to ensure it is not too hot or too cold.
Lack of natural equivalents. One question that recurs periodically is why hot composting is so widely advocated if there are no equivalents in nature. It appears that most of the composting done in the wild uses the cold method. There may be benefits of cold composting that are lost in the process of hot composting.
Microbe and worm death. Any small organisms that don’t like heat will die or escape the compost pile as it heats up, including worms, beneficial fungi, and some low-temperature bacteria necessary to fight garden disease. Proper curing can remedy this. However, hot compost still tends to favor heat-friendly bacteria instead of the diverse, fungus-based biology commonly found in natural decomposition processes.
Nutrient loss. Valuable nutrients are lost if the compost pile overheats. If you smell a strong odor or see clouds of vapor rising from your pile, you can assume that most of the valuable nutrients are escaping into the atmosphere. Some gardeners feel that a certain degree of nutrient loss occurs in hot composting even if the temperatures stay below 150°F.
Fire hazard. While not a common occurrence, spontaneous compost pile combustion can and does occasionally happen, for the same reason that wet hay bales sometimes burst into flames. Avoid this problem by monitoring the temperature and keeping it no higher than 150°F. Making sure there are no pockets of dry material helps, as well. Also, if using wood ashes, make sure they are completely cool before adding them to the pile.

Conclusion

There are two main reasons that some gardeners prefer hot composting:

To destroy weed seeds and pathogens.
To have finished compost in a short amount of time.

In exchange for these two primary benefits, the gardener must devote considerable planning, attention, and effort to the project. For a very busy gardener, this is sufficient reason to turn to cold composting.

If, however, you have the time and resources, you may find hot composting to be rewarding and effective, especially for disposing of materials with potential chemical, disease, or weed seed residues. However, don’t let your pile get over 150°F, and allow it to cure for at least a week before using. This will help ensure that the finished compost retains as much of the nutrition and biological activity necessary for plant health as possible.