Composting transforms organic waste into a soil amendment through a managed biological process. This decomposition relies on countless microorganisms, primarily bacteria and fungi, to break down complex materials. Turning the compost pile is a mechanical action that significantly influences this microbial activity. It is a fundamental technique used to create the optimal environment for these organisms to work efficiently.
The Essential Functions of Turning
Turning the pile serves two main purposes necessary for rapid, odor-free decomposition. The first function is to replenish the oxygen supply for aerobic microorganisms. These microbes quickly consume oxygen as they metabolize organic matter, and turning re-fills the pore spaces, preventing oxygen levels from dropping below the optimal 10% concentration.
The second function is to redistribute heat and moisture throughout the compost mass. The center of an active pile heats up due to microbial metabolic processes, often reaching temperatures between 135 and 160 degrees Fahrenheit. Turning moves the cooler, less-decomposed material from the outer edges into this hotter core. This ensures all organic matter is exposed to the high temperature needed to kill pathogens and weed seeds, promoting uniform breakdown.
Consequences of Turning Too Infrequently
When a compost pile is neglected and not turned often enough, the decomposition process shifts from aerobic to anaerobic. Without sufficient oxygen, the beneficial, heat-producing microbes slow down or die off. This transition leads to slowed decomposition rates, often taking months longer to complete.
Anaerobic conditions result in the production of intermediate compounds that create foul odors. Instead of carbon dioxide and water, anaerobic organisms produce compounds such as organic acids, hydrogen sulfide, and ammonia. The pile may develop a rotten-egg smell from sulfur compounds or a pungent ammonia smell, which signals nitrogen loss. This lack of proper aeration often causes the material to become a slimy, dense sludge rather than the desired crumbly humus.
Is There Such a Thing as Turning Too Much?
While the instinct to turn a pile frequently to avoid anaerobic issues is understandable, it is possible to turn compost too often, which can be counterproductive. The primary drawback of excessive turning is the repeated interruption of the thermophilic phase, leading to significant heat loss. The beneficial high temperatures, which should be maintained between 131°F and 170°F to sanitize the compost, are quickly dissipated when the pile is broken apart and exposed to cooler air.
When the pile cools too frequently, the process slows down, and heat-dependent sanitization is less effective. Constant mixing also leads to moisture loss through evaporation. Over-aeration can dry out the material prematurely, halting microbial activity, as organisms require 40% to 60% moisture content. Constant physical disruption can also inhibit the growth of beneficial fungi, which are slower to re-establish their networks than bacteria, thus slowing the breakdown of woody materials.
Finding the Optimal Turning Schedule
The best turning schedule is dictated by the condition of the pile, primarily its temperature and maturity. During the initial, most active decomposition phase, the pile should be turned when the internal temperature peaks and begins to decline. This drop signals that the available oxygen has been consumed. A good rule is to turn the pile when the temperature reaches the high end of the thermophilic range, around 160°F, to prevent overheating that can kill beneficial microbes.
Alternatively, if the temperature drops below 104°F during the active phase, turning is necessary to reintroduce oxygen and mix in new food sources to reignite the process. Once the pile no longer heats up after turning, usually after four to five cycles, it has entered the curing phase. At this point, little further turning is required, and the compost is allowed to mature without intervention.