Composting is a natural process where microscopic life transforms organic waste into a valuable soil amendment, creating humus. This breakdown is driven by bacteria, fungi, and other microorganisms that feed on the carbon and nitrogen in the waste. The goal is to produce a stable, dark, and nutrient-rich material that improves soil structure and fertility. Adding amendments like lime, commonly used to adjust soil chemistry, is frequently discussed regarding its benefit or harm to the compost pile.
The Critical Role of pH in Decomposition
The successful conversion of raw organic matter into finished compost depends heavily on the activity of microorganisms, which are highly sensitive to their environment. Key factors controlling their speed and efficiency are temperature and the level of acidity, measured as pH. The initial mesophilic phase often sees a temporary drop in pH because the rapid breakdown of materials produces organic acids.
For rapid decomposition, the microbial community thrives in a slightly acidic to neutral range, optimally between pH 6.5 and 8.0. If the environment becomes too acidic (below pH 5.5) or too alkaline (above pH 8.5), decomposer activity slows down significantly, stalling the process. An overly acidic environment is often indicated by a slow-breaking pile, pests, or an unpleasant, sour odor.
How Lime Affects the Compost Environment
In the context of composting, lime is primarily a chemical tool used to raise the pH level, making the environment more alkaline. Agricultural lime is most often calcium carbonate, though dolomitic lime or hydrated lime may also be used. The chemical function of lime is to neutralize acidity by reacting with hydrogen ions in the pile. This reaction increases the pH of the compost mixture. Lime also provides beneficial minerals like calcium and magnesium, which are necessary nutrients for microorganisms and the finished compost.
When to Use Lime and When to Avoid It
Lime should only be considered when specific materials have made the compost pile excessively acidic, such as large quantities of pine needles, citrus rinds, or coffee grounds. A light application of lime neutralizes these organic acids, returning the pH to the optimal range for microbial activity. Using lime can also help suppress odors associated with anaerobic, acidic decomposition.
Adding lime to a pile that is not acidic is detrimental because it pushes the pH too high into the alkaline range. This excessive alkalinity triggers nitrogen volatilization. When the environment becomes too alkaline, nitrogen compounds convert into ammonia gas, resulting in a strong ammonia smell and a significant loss of valuable nitrogen. Therefore, experts advise against adding lime unless a pH test confirms severe acidity. If lime is necessary, ground or dolomitic limestone is generally safer than the highly reactive hydrated lime.