Composting is the natural, aerobic process where microorganisms break down organic materials into a nutrient-rich soil amendment. Successful decomposition relies on maintaining a delicate balance of four primary ingredients: carbon-rich materials (browns), nitrogen-rich materials (greens), oxygen, and water. Since this process is managed by living microbes, their environment must be kept within specific parameters to thrive. Whether a compost pile needs protection is determined by how covering it affects this internal balance, acting as a tool to regulate moisture, temperature, and external factors.
Why Covering is Essential
A cover is frequently employed as a fundamental tool for moisture management. Microorganisms require a specific moisture content, ideally between 40% and 60%, which feels similar to a wrung-out sponge. In hot or arid climates, an uncovered pile can quickly dry out, causing microbial activity to go dormant and halting decomposition. A cover helps to prevent this rapid evaporation, keeping the internal environment consistently damp for the aerobic bacteria.
The physical barrier of a cover also plays a significant role in temperature regulation, especially in cooler weather. When a pile reaches high temperatures, often between 131°F and 160°F, it enters a thermophilic phase. This high heat is beneficial because it destroys pathogens and neutralizes weed seeds. Covering the pile helps to trap this biologically generated heat, ensuring the heap reaches the necessary temperature thresholds.
In addition to climate and temperature control, a cover is effective for deterring unwanted visitors and managing odors. The presence of food scraps attracts pests such as rodents and insects. A physical cover acts as a barrier, denying easy access to the organic matter. Furthermore, covering the pile, especially with a layer of material like finished compost or straw, helps to suppress the volatile organic compounds that cause unpleasant smells.
When to Adjust or Avoid Covering
While covering offers many benefits, an overly restrictive or tight cover can quickly lead to undesirable conditions. Aerobic composting relies on sufficient oxygen flow, and a completely sealed cover can lead to an anaerobic environment. When oxygen is depleted, decomposition slows significantly, producing organic acids and foul odors. This happens when the pile becomes too wet or compacted, restricting the air pockets needed for the aerobic process.
Managing moisture intake must also be adjusted based on the local climate and the type of cover used. In regions with persistent, heavy rainfall, an impermeable cover is necessary to prevent the pile from becoming waterlogged. Excess moisture displaces the air within the pile, leading to anaerobic issues. Conversely, in a climate with moderate rain, occasionally removing the cover may be beneficial, allowing natural precipitation to maintain the moisture level without manual watering.
The composition of the cover material itself can dictate the need for adjustment. Using a loose, porous cover like straw or shredded leaves may allow enough air and small amounts of rain to pass through. However, a non-porous plastic tarp or sheeting requires regular monitoring to ensure the pile is still receiving adequate air and moisture is not condensing excessively on the underside.
Practical Covering Materials and Techniques
For a loose, temporary cover that encourages air exchange, a thin layer of carbon-rich materials works well. A simple “cap” of dried leaves, straw, or corrugated cardboard can be placed on top to deter flies and minimize surface moisture loss. This type of cover is easily incorporated back into the pile later.
For full protection against high winds or heavy, sustained rain, an impermeable cover like a plastic tarp or plywood is a better choice. When using these non-porous materials, avoid laying them directly onto the pile, which seals the surface. A good technique is to use a cover that is raised slightly above the compost, or to secure it loosely, leaving small gaps along the edges. This gap allows for passive air exchange, maintaining the necessary aerobic conditions.