Hot composting requires more work, but with a few minutes a day and the right ingredients you can have finished compost in a few weeks depending on weather conditions. The composting season coincides with the growing season. When conditions are favorable for plant growth, those same conditions work well for biological activity in the compost pile. However, since compost generates heat, the process may continue later into the fall or winter.
Hot piles do best when high-carbon material and high-nitrogen material are mixed in a 1 to 1 ratio. A pile with the minimum dimensions of 7.5 X 7.5 X 7.5 cm is needed for efficient heating. For best heating, make a heap that is 120 or 150 cm in each dimension. As decomposition occurs, the pile will shrink. If you don't have this amount at one time, simply stockpile your materials until a sufficient quantity is available for proper mixing.
Hot piles reach 40 to 70 ºC, killing most weed seeds and plant diseases. Studies have shown that compost produced at these temperatures has less ability to suppress diseases in the soil since these temperatures may kill some of the beneficial bacteria necessary to suppress disease.
- Choose a level, well-drained site, preferably near your garden.
- There are numerous styles of compost bins available depending on your needs. These may be as simple as a moveable bin formed by wire mesh or a more substantial structure consisting of several compartments. There are many commercially available bins. While a bin will help contain the pile, it is not absolutely necessary. You can build your pile directly on the ground. To help with aeration, you may want to place some woody material on the ground where you will build your pile.
- To build your pile, either use alternating layers of high-carbon (brown materials, such as dead leaves) and high-nitrogen material (green materials, such as grass clippings) or mix the two together and then heap into a pile. If you alternate layers, make each layer 5 to 10 cm thick. Some composters find that mixing the two together is more effective than layering. Use approximately equal amounts of each. If you are low on high-nitrogen material, you can add a small amount of commercial fertilizer containing nitrogen. Apply at a rate 125 ml of fertilizer for each 25 cm layer of material. Adding a few shovels of soil will also help get the pile off to a good start; soil adds commonly found decomposing organisms.
- Water periodically. The pile should be moist but not saturated. If conditions are too wet, anaerobic microorganisms (those that can live without oxygen) will continue the process. These are not as effective or as desirable as the aerobic organisms. Bad odors also are more likely if the pile is saturated.
- Provide aeration either by turning the pile or by using bins that allow air to enter the pile (i.e. Punch holes in the sides of the pile).
- The pile will heat up and then begin to cool. Start turning when the pile's internal temperature peaks at about 55 to 60 ºC. You can track this with a compost thermometer, or reach into the pile to determine if it is uncomfortably hot to the touch.
- During the composting season, check your bin regularly to assure optimum moisture and aeration are present in the material being composted.
- Move materials from the center to the outside and vice versa. Turn every day or two and you should get compost in less than 4 weeks. Turning every other week will make compost in 1 to 3 months. Finished compost will smell sweet and be cool and crumbly to the touch.
Adapted from the USDA Backyard Conservation Tip Sheet, a public-domain work of the US Government.