Composting occurs when helpful bacteria found in the soil consume the carbon and nitrogen found in decaying organic matter and, from it, produce nutrient-rich soil. A natural process, composting needs only a mixture of nitrogen- and carbon-rich ingredients to happen. However, composting may take several months to produce soil under normal conditions. A method called "hot composting" speeds up the composting process.
Cold Versus Hot Composting
Most gardeners simply pile together carbon- and nitrogen-rich ingredients and let nature do the rest. Also called cold composting, this method is inexact and requires little labor, but it also works more slowly. Gardeners do not need to worry about achieving an exact ratio of ingredients or frequently turning or aerating the pile.
Hot composting, on the other hand, optimizes the conditions soil bacteria need to grow, allowing the bacteria to reproduce quicker and work harder at turning your lawn clippings and kitchen scraps into fertile soil. Hot compost piles may reach internal temperatures of up to 160 degrees. According to "Rodale's Ultimate Encyclopedia of Organic Gardening," hot composting produces compost in as little as 8 weeks, giving gardeners six batches of compost per year.
Weeds and Disease
The heat generated by a hot compost pile effectively kills most plant diseases, pathogenic microorganisms and weed seeds. However, the high temperatures also destroy beneficial microorganisms that help protect your plants from disease. Hot composting is advantageous when composting weeds or diseased plant parts. However, if your plants are plagued by diseases and you hope that compost will help build their resistance, hot composting may not be the best method to choose.
Nitrogen-rich ingredients, such as grass clippings and egg shells, provide the nitrogen that soil bacteria need to reproduce. Because hot composting encourages bacteria to reproduce quickly, they deplete the nitrogen in your compost pile faster than they would during cold composting.
Nitrogen is an essential nutrient for plants to grow. Many plants, such as tomatoes and lettuce, quickly deplete the soil of nitrogen. Adding compost to your garden is one way to restore nitrogen to the soil, so if boosting nitrogen levels is a goal for you, consider the possible negative impact that hot composting may have on nitrogen levels.
During cold composting, gardeners add ingredients to the pile as they become available. For example, if you mow the lawn, you toss the clippings onto the pile. In tidying the home office, you mix the paper shreds into the pile. When hot composting, the pile must be built all at one time, requiring gardeners to stockpile large quantities of carbon- and nitrogen-rich ingredients. Nitrogen-rich ingredients like food scraps can develop an odor and draw scavengers. Additionally, although it can be done year-round, hot composting works best in warm weather.
Building a Hot Compost Pile
To maintain the correct temperature, a hot compost pile must be large enough to retain heat at its core and small enough that it can be properly aerated. The USDA's Natural Resources Conservation Service recommends a minimum of 3 feet of height, depth and width for a hot compost pile and suggests 4 or 5 feet as optimal. To accommodate the increased need for nitrogen, add carbon- and nitrogen-rich ingredients in a 1-to-1 ratio, either layering them in 2-inch layers or mixing them together. Every few days, move materials at the center of the pile to the outside and vice versa to assure even composting. The pile should be kept moist but not sodden.