The Role of Water in Decomposition
Bacteria and fungi that drive composting require moisture to function. At very low moisture levels, their activity drops dramatically and the pile may remain visually unchanged for months. At very high moisture levels — when material is waterlogged — aerobic bacteria are displaced by anaerobic ones that produce hydrogen sulfide and other compounds with unpleasant odours.
Decomposition proceeds most efficiently when moisture content is in a range that allows aerobic bacteria to remain active while still holding enough water to support biological processes. The texture comparison most often used is a wrung-out sponge: damp but not dripping when squeezed firmly.
Practical Moisture Tests
There is no standard home instrument for measuring compost moisture precisely, but two manual checks provide useful information:
The Squeeze Test
Take a handful of material from the center of the pile and squeeze it firmly. If several drops of water fall out, the pile is too wet. If the material crumbles and feels dry after squeezing, it needs water. If you feel dampness but only one or two drops appear, moisture is roughly in the right range.
Visual Assessment
A pile with correct moisture will look darker and more cohesive than dry material but will not look muddy or saturated. Steam may rise from the center on cool mornings when a pile is active — this is a sign of microbial heat production, which requires adequate moisture to sustain.
Adjusting Moisture Through Canadian Seasons
Canada's climate produces wide seasonal variation that directly affects compost moisture. Understanding the patterns by season helps anticipate adjustments rather than react to problems.
Spring
Snowmelt and rain can saturate an outdoor pile, particularly if the bin has no lid or drainage. Turning the pile and adding dry brown material — such as stored autumn leaves or cardboard — absorbs excess water and reintroduces air pockets. Covering the pile during heavy spring rain prevents over-saturation.
Summer
In many parts of Canada, summers bring extended dry periods. A pile exposed to full sun may lose moisture faster than kitchen scraps can replenish it, particularly in BC's interior, the Prairies, and southern Ontario during drought years. Watering the pile with a garden hose and turning material to distribute moisture evenly is often necessary between July and September. Placing the bin in partial shade reduces evaporative loss.
When watering a compost pile, apply water gradually and turn the pile as you add it. Pouring a large amount onto the surface can pass through without wetting the center of compacted material.
Autumn
Autumn brings falling leaves — the main carbon input for many backyards — and cooler temperatures. Leaves added dry will absorb water from kitchen scraps over time, but the transition period before sufficient nitrogen is available can result in a very dry pile. Mixing leaves with fresh garden material and watering moderately keeps conditions suitable for continued decomposition before temperatures drop.
Winter
Below freezing temperatures stop microbial activity in most outdoor bins. The pile retains its moisture through the frozen season and resumes decomposition when temperatures rise in spring. Some gardeners continue adding kitchen scraps through winter; the material freezes and begins breaking down when the pile thaws, often generating significant heat in April as bacteria resume activity on a large accumulated input.
Common Causes of Excess Moisture
- Insufficient brown material relative to wet kitchen scraps
- Bins without lids exposed to sustained rainfall
- Poor drainage beneath the bin — concrete pads or compacted clay soil can trap water
- Adding large amounts of high-moisture material (watermelon rinds, cooked food) without balancing browns
Common Causes of Insufficient Moisture
- Piles composed mostly of dry leaves or straw with minimal kitchen waste addition
- Bins placed in full sun on sandy or well-drained soil during dry months
- Extended periods without any green material input
- Over-turning in dry conditions, which accelerates evaporation
Interaction with Carbon-Nitrogen Ratio
Moisture and the carbon-nitrogen ratio interact. A pile with good C:N balance but inadequate moisture will not decompose noticeably. A moist pile with poor C:N balance may become anaerobic or simply inactive. When troubleshooting a stalled pile, checking both moisture and material composition is more reliable than adjusting one alone.
References
Royal Horticultural Society — Composting
Environment and Climate Change Canada — Composting