Composting transforms organic waste into a nutrient-rich soil amendment through decomposition driven by microorganisms. These microbes require specific conditions to thrive. Many new composters wonder if covering their pile is beneficial or necessary, and the answer depends on the type of cover used and the environmental conditions. This article clarifies how covering affects the composting process and what materials are best suited for the task.
The Role of Moisture and Heat Retention
A cover is a valuable tool for managing moisture and heat, the two most important factors in decomposition. Microorganisms require a moisture content similar to a damp, wrung-out sponge (50% to 60%). In hot, dry climates, covering the pile retains moisture, preventing premature drying and slowing microbial activity.
Conversely, a cover protects the pile from excessive saturation during heavy rain. Too much water fills air pockets, risking the leaching of soluble nutrients and creating undesirable conditions. A cover also helps maintain high temperatures generated by microbial metabolism, ensuring efficient breakdown in cooler weather.
The Risk of Anaerobic Conditions and Airflow
The primary risk of using a non-breathable cover, such as a plastic tarp, is restricting necessary airflow. Composting is an aerobic process, meaning microbes require oxygen to perform their work. When a pile is covered too tightly or air spaces are filled with water, oxygen is rapidly depleted. This lack of oxygen shifts decomposition to an anaerobic process, which is significantly slower and less desirable.
Anaerobic conditions are identified by foul odors, often described as rotten eggs or a sour smell. This is caused by the production of compounds like hydrogen sulfide and organic acids. Another consequence is the production of methane, a potent greenhouse gas. If an impermeable tarp is used, the pile must be turned frequently to forcefully introduce oxygen and prevent these negative conditions.
Alternative Materials for Covering
Since the ideal cover must manage moisture and temperature without blocking oxygen exchange, permeable materials are the better choice over plastic tarps. Specialized compost covers made from UV-stabilized polypropylene fabric are designed to be water-repellent while still allowing gas exchange. These breathable fabrics shed rainfall but let water vapor and air move freely, protecting the pile without suffocating the microbes inside.
For a lower-cost, practical solution, a thick layer of organic material serves as an effective biocover. These materials offer insulation and moisture retention while maintaining a porous structure for airflow:
- Straw
- Wood chips
- Old burlap sacks
- Shredded leaves or thick cardboard (to reduce surface evaporation)
The choice depends on the specific need, such as protecting an active pile from a deluge or insulating a curing batch over winter.