Composting is a process of controlled decomposition that relies on microscopic organisms to break down organic materials into nutrient-rich soil amendment. These microbes, primarily bacteria and fungi, require a specific environment to perform their work efficiently. Maintaining the correct moisture level in the compost pile is the single most important factor for supporting this microbial activity and regulating the overall speed of decomposition. Therefore, achieving the right dampness is more important than adhering to a fixed watering schedule.
Determining the Perfect Moisture Content
The ideal moisture content for a working compost pile is between 40% and 60% by weight. Within this range, a thin film of water coats the organic particles, allowing microbes to thrive without flooding the air pockets. If the material becomes too dry, the microbial population slows its activity and can become dormant, causing the decomposition process to stall completely.
The “squeeze test” provides an immediate assessment of the pile’s condition. To perform this, reach into the center of the pile and grab a handful of material, then squeeze it tightly. The compost should feel like a wrung-out sponge—damp, but not saturated.
If the moisture level is correct, only one or two drops of water should be released. If no water is released and the material crumbles apart, it is too dry, and decomposition will be severely limited. Conversely, if a stream of water runs out, the pile is too wet, displacing the necessary oxygen and creating anaerobic conditions.
Variables That Change Watering Frequency
The frequency of watering depends on several dynamic factors. Environmental conditions are a major influence, as hot, sunny weather and high winds increase the rate of evaporation, quickly drawing moisture from an exposed pile. Placing the compost in a shaded location helps to conserve water by minimizing these drying effects, thereby reducing the need for frequent additions.
The physical construction of the composting system also dictates moisture loss. An open, uncovered pile is subject to both evaporative drying and saturation from rainfall, necessitating constant monitoring and adjustment. In contrast, enclosed compost tumblers or bins with lids retain moisture more effectively, meaning they require water less often than an open windrow.
The specific materials being composted also play a large role in water management, particularly the ratio of carbon-rich “browns” to nitrogen-rich “greens.” Browns absorb and hold water, but they often require significant initial saturation. Piles heavily skewed toward dry browns will need more water added at the outset.
Greens, like fresh grass clippings and food scraps, are naturally high in water content. A pile that is overly rich in these materials can quickly become too wet, in which case the brown materials act as a necessary sponge to absorb the excess liquid. In a hot, active pile, up to 1% of the moisture content can be lost daily in warm climates, suggesting a check and potential rewetting every seven to ten days is a general rule of thumb.
Methods for Applying Water
The technique used to introduce water is important for achieving deep, even saturation throughout the pile. Watering should ideally be performed in conjunction with turning or mixing the pile to ensure the moisture penetrates the core. Applying water to the surface of a dry, compacted pile often results in runoff, leaving the interior largely untouched.
To prevent this issue, water should be applied slowly and deliberately, allowing the materials time to absorb the liquid. A simple garden hose with a gentle spray nozzle or a watering can works well for smaller bins and piles. For larger volumes, a sprinkler system can be used to ensure even distribution.
When dealing with an extremely dry pile, which can become hydrophobic, poke several deep holes into the top surface before watering. This breaks the surface tension and creates channels for the water to reach the center of the mass. Using a rain barrel or collected greywater is an effective way to apply moisture while conserving resources.
The most effective method is to water materials as the pile is built, dampening each layer of browns and greens. This ensures a consistent moisture baseline from the start, significantly reducing the need for extensive rewetting later. After the initial build, simply check the pile weekly with the squeeze test and only water when the result indicates the pile is too dry.
Correcting Overly Wet or Dry Compost
If the moisture level falls outside the optimal range, corrective action is needed to prevent decomposition from stalling or turning anaerobic. If the squeeze test reveals the pile is too dry, the entire mass needs rehydration. Begin by adding water slowly, combining this with a thorough turning of the pile to distribute the moisture evenly and break up dry pockets.
A dry pile can also be corrected by incorporating new, nitrogen-rich materials that are high in natural moisture, such as fresh food scraps or grass clippings. These additions not only supply water but also fuel the microbial activity, helping to restart the heating process.
If the compost is overly wet, often indicated by a foul, rotten egg, or ammonia-like odor, the water has displaced the oxygen, leading to anaerobic decay. The solution is to incorporate a large volume of dry, carbon-rich “brown” materials, such as shredded cardboard or dry leaves, to absorb the excess liquid.
After adding the dry carbon, the pile must be turned aggressively to introduce fresh air and distribute the absorbent material throughout the wet mass. This restores the necessary air pockets, allowing aerobic microbes to take over and eliminate the unpleasant smell.