Composting is a biological process where microorganisms break down organic materials into a soil-enriching product. The speed and quality of this process depend heavily on providing the right environment for these microbes. Turning the compost pile is a technique synonymous with aeration, which mechanically introduces oxygen into the decomposing mass. A proper aeration schedule controls the rate of decomposition and can dramatically reduce the time it takes to create finished compost.
The Essential Function of Aeration in Decomposition
Turning is necessary because beneficial microorganisms consume oxygen rapidly. These microbes are obligate aerobes, requiring an oxygen-rich environment to thrive and perform breakdown functions efficiently. When oxygen is abundant, these organisms generate heat, driving decomposition quickly and without foul odors. This heat is a byproduct of their metabolic activity as they break down carbon structures.
If oxygen is not replenished, the pile’s center becomes depleted, shifting the process toward anaerobic decomposition. Anaerobic organisms work much slower and produce undesirable byproducts, such as hydrogen sulfide gas, which causes the distinct, putrid smell of rotten eggs or ammonia. Turning reintroduces oxygen, sustains the faster aerobic process, and prevents the compaction of materials that restricts airflow.
Determining the Optimal Turning Schedule
The frequency of turning is determined by the specific composting goal. For rapid, or “hot,” composting, the pile must be turned whenever the internal temperature begins to drop. A drop below 130°F (55°C) signals slowing microbial activity due to oxygen depletion. Reaching this temperature is desirable because it is sufficient to kill most weed seeds and pathogens.
Actively managed piles are commonly turned every three to seven days, depending on how quickly the temperature rises and falls. Using a long-stemmed compost thermometer is the most reliable method to indicate when microbes need fresh air. If the temperature exceeds 160°F (71°C), the pile should be turned immediately to cool it down, preventing the heat from killing the organisms driving the process.
Passive or slow composting requires less labor, as the goal is simple breakdown over time, not speed. Turning is only necessary a few times over the entire process, perhaps once every few weeks or months. The primary reason to turn a passive pile is if a foul, putrid odor develops, signaling anaerobic conditions. Turning immediately aerates the materials and restores the desirable aerobic state.
Modifying Factors That Change Turning Frequency
The ideal turning schedule is flexible and influenced by the pile’s physical characteristics. Moisture content is a major factor because excessively wet materials quickly saturate air pockets and become anaerobic. A pile that feels too wet, like a saturated sponge, requires more frequent turning to help dry out the materials and introduce air. Too little moisture, however, can halt decomposition altogether.
The physical composition of materials also affects turning frequency. Piles made up of fine, dense materials, such as grass clippings or fine sawdust, tend to compact easily. This compaction restricts airflow, necessitating more frequent turning to fluff materials and restore porosity. To reduce constant turning, coarse, bulky materials like wood chips or straw can be introduced as a bulking agent to create natural air channels.
The overall size of the pile plays a role in heat retention and turning frequency. To achieve the high temperatures required for hot composting, a pile needs to be at least one cubic yard in volume. Larger piles retain heat more effectively. Very small piles lose heat rapidly and may not reach the desired temperatures, potentially requiring less frequent turning since the thermophilic phase is not fully sustained.