Composting is the natural process where microorganisms break down organic materials into a nutrient-rich soil amendment. Successful decomposition relies on a delicate balance of carbon, nitrogen, air, and moisture. Managing these variables involves controlling the external environment surrounding the organic matter. Deciding whether to cover the compost bin is central to managing this environment effectively.
Managing Temperature and Moisture Levels
The decomposition of organic matter is an exothermic process, releasing heat as the microbes consume the materials. Microbes require temperatures between 131 and 160 degrees Fahrenheit (55 to 71 degrees Celsius) for the thermophilic stage, which rapidly breaks down material and kills pathogens. A cover acts as insulation, helping to trap this heat, which is beneficial for maintaining the necessary temperature during cooler external conditions.
Moisture content is equally important for microbial activity, ideally maintained around 40 to 60 percent, often described as the consistency of a wrung-out sponge. In hot or dry climates, an uncovered pile can lose water rapidly through evaporation. Covering the bin significantly reduces this moisture loss, ensuring the necessary water is available for the organisms driving the decay process.
Conversely, a cover prevents excessive saturation during periods of heavy rain or snow. A waterlogged pile displaces the necessary air pockets, leading to anaerobic conditions and a stalled process. By shielding the materials from direct precipitation, the cover maintains the optimal moisture range and prevents the leaching of water-soluble nutrients.
Preventing Pests and Odor Problems
A securely covered compost bin serves as a physical barrier against nuisance wildlife and insects. Uncovered food scraps, especially those high in protein or fat, attract scavengers like rats, raccoons, and opossums. By limiting access, a cover mitigates the risk of these animals establishing residence or reproducing within the warm environment of the pile.
House flies and fruit flies are particularly drawn to exposed, decaying food waste to lay their eggs. A tight-fitting cover prevents adult flies from reaching the materials, thereby interrupting their life cycle and controlling potential infestations. This physical separation is a simple, non-chemical method for maintaining sanitary conditions around the bin.
While a healthy, aerobic pile should smell earthy, exposed materials can sometimes release undesirable volatile organic compounds. Covering the bin helps contain these compounds, preventing nuisance odors from attracting pests or bothering neighbors.
When Open Air Access is Beneficial
Access to oxygen is necessary for successful aerobic composting, despite the benefits of covering. Microorganisms require oxygen to efficiently break down organic material, and an airtight seal is detrimental to the process. If a pile becomes starved of air, it shifts to anaerobic decay, which is slow and produces foul-smelling compounds like hydrogen sulfide.
Therefore, any covering must allow for some degree of ventilation or be easily removed for regular aeration, such as turning the pile. In climates experiencing high humidity, temporarily lifting the cover can allow excess moisture to evaporate, preventing saturation. Conversely, in extremely dry conditions, a cover might be briefly removed during a light rain shower to intentionally capture needed moisture.
Selecting Appropriate Covering Materials
Selecting the right material for covering a compost bin depends on the bin’s structure and the local climate. A suitable cover must be heavy enough to resist being displaced by wind, ensuring continuous protection. It should also be non-absorbent and designed to shed water away from the pile, similar to a roof structure.
Many composters use heavy-duty plastic sheeting, a waterproof tarp, or a solid piece of exterior-grade plywood as effective coverings. A dedicated lid, often supplied with commercial compost tumblers or bins, is engineered to fit snugly while sometimes incorporating vents. The goal is a protective layer that maintains the internal environment without creating a fully sealed, oxygen-deprived chamber.