Composting is a natural process where microorganisms break down organic materials, which requires four main ingredients: carbon, nitrogen, oxygen, and water. Moisture is a fundamental component, as the beneficial bacteria and fungi that drive decomposition rely on it to live and move. If the compost pile is either too dry or too wet, microbial activity slows down dramatically or stops entirely, halting the conversion of waste into beneficial soil amendment. Maintaining the correct moisture level is required for successful composting.
Defining the Optimal Wetness
The ideal moisture content for a compost pile falls within a range of 40% to 60%. This percentage ensures the organic materials are damp enough to support the metabolic functions of the microorganisms. A practical way to visualize this level is to imagine a fully saturated sponge that has been thoroughly wrung out. The material should feel moist to the touch without leaving an excessive wet sheen on your hand.
This ideal range creates a thin film of water around each particle without filling the air space between them. Decomposition is an aerobic process, meaning the microorganisms require oxygen to break down matter effectively. If moisture exceeds about 65%, the water displaces the air in the pore spaces, leading to oxygen-starved conditions. If the moisture drops below 40%, the microbes become dormant and cannot access nutrients, causing decomposition to slow significantly.
How to Check Moisture Levels
The most effective, hands-on method for checking moisture is the “Squeeze Test.” This technique provides an immediate and tactile assessment of the pile’s condition without needing specialized equipment. To perform the test, reach into the center of the compost pile, typically 18 to 24 inches deep, to grab a representative handful of material. The center is the most active and least affected by surface evaporation or rain.
Squeeze the material firmly in your hand and observe the result. If the compost is at the ideal level, only a few drops of water should be released between your fingers, similar to a wrung-out sponge. If no water emerges and the material crumbles easily, the pile is too dry. If a steady stream of water runs out when squeezed, the moisture content is too high and requires immediate attention.
Issues Caused by Incorrect Moisture
Falling outside the optimal moisture range leads to two distinct sets of problems that severely impede the composting process. When the compost is too dry, microbial activity is inhibited because the organisms cannot move or feed without water. The pile will appear dusty or flaky, decomposition will slow to a near halt, and the internal temperature will remain cold, signaling stalled biological activity.
The more common issue is a pile that is too wet, which is typically caused by adding excessive “green” materials like food scraps or fresh grass clippings. The surplus water fills the necessary air pockets, resulting in a lack of oxygen and the onset of anaerobic decomposition. This oxygen-starved environment promotes microbes that produce foul-smelling compounds. These include hydrogen sulfide (rotten eggs smell) or ammonia gas (a strong, pungent odor). A wet pile often becomes slimy, dense, and compacted, preventing air circulation and exacerbating anaerobic conditions.
Methods for Adjusting Wetness
If the “Squeeze Test” indicates the pile is too dry, introduce water while simultaneously turning the material. Water should be added slowly, ideally using a watering can with a rose attachment, to ensure even distribution rather than simply soaking the surface. Turning the pile helps break up dry clumps and ensures moisture penetrates the core of the compost.
If the compost is too wet, the immediate action is to introduce dry, carbon-rich materials, often called “browns,” to absorb the excess liquid. Effective materials include shredded cardboard, dry leaves, straw, or newspaper. The pile must then be thoroughly turned, which serves the dual purpose of mixing the dry materials in and introducing fresh air to re-establish aerobic conditions. This turning helps the excess moisture evaporate and restores the oxygen levels necessary for the beneficial microbes.