Best Ways To Maintain Moisture In Compost Piles

In hot and dry weather climates, water is a critical ingredient for effective composting. The expression “water is life” certainly applies to microbial life as well.  Without adequate moisture, compost microbes cannot survive. An overly dry compost pile will experience a huge loss of microbial diversity and will be unable to maintain internally-generated composting temperatures of 120-140 deg F. Additional water is needed for evaporative cooling to prevent the compost pile from overheating.  

In this blog, we explore some key strategies for conserving and protecting the water that allows for successful composting in hot, dry conditions.

Made in the Shade

If we consider how composting happens in natural ecosystems, we might notice that it typically occurs in the shade. For instance, in a forest, leaves and other biological matter fall to the forest floor, accumulate, and slowly decompose. Natural ecosystems work to protect and conserve moisture by keeping the soil/compost covered and in the shade.  Trees provide multiple layers of shading for the forest floor and decomposition happens below the surface, where conditions are cool and damp. In contrast, in a desert, bare soil exposed to intense sun rapidly loses its moisture and dries out, becoming relatively lifeless and at risk of erosion.

To emulate natural composting processes, it is helpful to place the composting piles in a shady location or otherwise provide shade cover for all of the feedstocks and compost piles. Shading is a basic tool for slowing evaporation and conserving moisture in feedstocks and compost piles.

A second basic practice is to apply a thick layer of biocover over all compost piles. Feedstocks that are suitable for a biocover layer typically have a particle size of 4-6” or larger with a significant amount of fiber (e.g., wood chips, leaves, weeds or straw). A thick layer of at least 6-12” of biocover material is beneficial. A biocover provides “armoring” for the compost underneath, meaning protection from the drying effects of the sun and the erosive effects of wind. While the biocover layer on the surface may become quite dry, this biocover helps protect the moisture in the compost below. For positively aerated composting systems, this biocover also provides odor filtration for the compost.

Maintaining Minimum Moisture

A key strategy is to avoid letting feedstocks become too dry. The drier a feedstock, the more difficult it is to restore moisture, feedstocks that become overly dry will tend to become hydrophobic and actually resist absorbing water. In this case, water added to feedstocks tends to run off and not be absorbed.By maintaining reasonable moisture content in all feedstocks, moisture addition is more effective, efficient, and requires less water.

If feedstocks do become overly dry, one possible strategy is to pre-water feedstocks (for example with a sub-surface drip irrigation system), which increases contact time to absorb the water and rehydrate.

If one must cope with dry feedstocks, another helpful strategy is to add feedstocks that already contain significant proportions of embodied water (such as food waste or fresh cut green waste). Since these feedstocks hold a higher moisture content, they will help boost the moisture content in the overall mixture.

Watering Strategies

In hot, dry weather, regularly adding water to a hot compost process is often a necessity.  We recommend a minimum moisture content of 50% to support a hot compost process.  A hot compost pile can easily lose 1% of moisture content every day. Therefore, in warm climates, we recommend a starting moisture content of 60% for a blended compost recipe. This provides about 7-10 days before the compost will need to be mixed and rewetted in order to maintain compost productivity and temperatures.

Adding water to a compost pile in hot weather follows the same principles as watering a garden. The goals are to conserve water and minimize the amount of effort required to keep the microbes alive and thriving. These best practices for moisture addition include:

• Add water regularly (at least once per week)
• Add water slowly and for longer durations (e.g., drip irrigation)
• Install drip irrigation below the surface of the pile to protect the moisture against the drying effects of the sun
• Add water during the cool hours of the day (e.g., in the evening)

Aeration Strategies

During the compost process, heat generated by the metabolic activity of microbes accumulates and tends to rise up through the compost pile. This results in the hottest temperatures residing on the top of the pile. This is not convenient for hot, dry climates as it can cause the pile to overheat or lose moisture too quickly.

Mechanical aeration can help manage heat and conserve moisture in the compost pile. For positively-aerated systems, the air going into compost piles can be misted in order to add moisture and support evaporative cooling of the compost. In particular, negatively-aerated and reversing aeration systems can provide advantages for heat and moisture management. For instance, negative aeration can induce moisture to condense in lower parts of the pile and allow heat to be dumped into the thermal mass of underground piping.

For more ideas on aeration strategies to support commercial composting in hot, arid climates, please see our upcoming article in Biocycle Connect. And if you would like professional help with protecting your composting efforts, please reach out to us today!