Water is essential to all life on earth, and since the compost process depends on the activity of microbes, it’s critically important that compost piles contain ENOUGH moisture. And for the reasons explained below, it’s also critical that compost piles don’t contain TOO MUCH moisture. Unfortunately, many new compost operators aren’t aware of the importance of ensuring optimal moisture throughout the entire composting process, what the moisture content should be, and/or the various methods for ensuring this optimal moisture content. Some of the reasons for this lack of information include:
So, the first step to understand the optimum amount of moisture required during the various stages of the compost process is to know how to actually calculate “moisture content”:
Moisture content is calculated by weighing a sample of compost, completely drying the sample, and then reweighing the dried sample. The moisture content of the original sample is expressed as a percentage, and is calculated by dividing the weight of the water that was lost through drying by the weight of the original sample. For example, if the original sample weighs 1 lb and then weighs .5 lb after it’s been dried, the weight of the water that was lost is .5 lb, and the moisture content of the original sample would be 50%, or .5 lb divided by 1 lb = .50 (or 50 %).
Compost microbes need moisture to survive, but since they’re primarily aerobes, they also need adequate supplies of oxygen. In a compost pile, this oxygen is primarily supplied “passively” via “diffusion” and “convection”. Diffusion is the process wherein higher concentrations of oxygen in the atmosphere slowly “diffuses” into the interior of the pile where there’s a lower concentration. Enhancing this process of diffusion, heat generated inside an active pile rises (via “convection”) which pulls in cooler outside air from the bottom and sides of a pile. In “aerated” composting systems, oxygen is supplied using either positive or negative pressure via a system of fans and pipes.
However, with both passive and forced aeration, the supply of oxygen is limited by how easily it can move through the pile, which depends both on the width of the pile, and the amount of “pore space” within the pile, which is largely determined by particle size, shape, and how densely packed the materials are, a measurement known as “bulk density”. But another critically important factor that determines how easily oxygen can be replenished is the moisture content, because as the amount of moisture in a pile increases, available pore space is filled with water, thereby displacing and restricting the flow of oxygen. And when a pile becomes saturated with water, aerobic microbes are replaced by anaerobic microbes, resulting in the production of unpleasant odors, and phytotoxic substances. Wet compost is also heavier and denser, making it more difficult and expensive to turn, screen, and transport.
And while there’s a common misconception that the primary purpose of “turning” a pile (either with a “compost turner” or a bucket loader) is to “add” oxygen, in-fact, the oxygen that’s added during turning is actually used up very quickly. Instead, the main benefits of turning include: mixing and homogenizing ingredients from different zones of the pile; redistributing moisture; breaking apart clumps, and; restoring pile “porosity” that’s lost over time due to the reduction in particle size and settling. Additionally, if a pile contains too much moisture, turning can actually compact the ingredients into wet, sticky “clumps”, which even further limits oxygen availability. It’s also worth noting that very high quality compost can in-fact be made with minimal or no turning, assuming a pile starts out with an optimal amount of moisture and porosity. This idea is described in more detail in another article by Steven Wisbaum titled “Low-Input Composting”.
Excess moisture in a pile can be caused by either using a mix of raw ingredients that are too wet to begin with, and/or allowing a pile to be exposed to more rainfall than is being lost through evaporation. Different piles at different process stages, at different temperatures, with different types of raw materials having different bulk densities, will all respond differently to increasing moisture levels, but a pile having a moisture content beyond 75% is likely approaching “saturated” conditions. Characteristics of a saturated pile include water seeping out of the bottom as “leachate”, the ingredients becoming wet and slimy, and unpleasant odors that are the by-products of anaerobic decomposition.
One of the easiest ways to avoid excessive moisture is to start off with a proper mix of dry and wet ingredients to achieve a moisture content of 65 to 70%, which is the amount of moisture that’s contained in a moderately damp, but not “sopping wet” sponge. At this moisture content, you’ll be able to easily squeeze out multiple drops from a sample taken from inside the pile, but you should see little to no leachate flowing out from the bottom of the pile.
If there’s an elevated risk that a compost pile will be exposed periods of intense and/or prolonged rainfall, and pile size is not limited by the width of a compost turner, the potential for excess moisture conditions can be reduced by building wider and taller piles (versus narrower and shorter piles), which will minimize the amount of surface area for a given amount of material, also known as the “surface-to-volume ratio”. If possible, building a pile with a peaked top will also shed water better compared to a pile with a broad flat top.
Excess moisture can also be avoided by protecting compost piles with a specialized compost covers such as ComposTex, which is a non-woven, polypropylene, UV-protected, macro-porous fabric that sheds rainfall but is permeable to oxygen. And although it may sometimes be necessary to protect active piles from excess rainfall, compost covers are most commonly used to protect curing or finished compost because compost at this stage is much cooler, has greater water-holding capacity, has minimal evaporative losses, and is more susceptible to becoming too wet. Conversely, as described above, active piles are more likely to need to be exposed to as much rainfall as possible to make up for evaporative losses.
If a pile contains too much moisture and is in the early stages of decomposition (ie. it still contains a relatively large amount of raw or semi-composted ingredients), adding more dry ingredients with a high carbon content and low bulk density will absorb the excess moisture and reduce the pile’s bulk density. Examples of the materials that could be added include horse manure with lots of bedding, chopped straw or hay, dry leaves, wood shavings/sawdust, etc.
As described above, a wet pile can also be protected from additional rainfall by covering it with a specialized compost covers, such as ComposTex.
A wet pile can sometimes also be dried out by allowing its surface to dry out, then turning it to mix the outer dry materials with the wetter materials inside the pile, followed by another period of drying, followed by another turn, etc. And if heavy rains are expected during this process, the pile should be covered to prevent exposure to additional water. However, with some materials such as food waste and animal manure, turning can compact materials into wet, dense clumps that will resist decomposition and become hard to break up as they dry out.
If possible, in hot dry weather, evaporation can be accelerated by reforming a pile into a longer, narrower pile to maximize the surface area that’s exposed to the evaporative forces of sun and wind.
While most experienced composters know that compost microbes need moisture to survive, it’s my experience that many compost operations operate with sub-optimal moisture levels. One reason for this is that many compost instruction manuals and articles say that a moisture content as low as 40 or 50% is sufficient for composting. And intuitively, it would seem that a moisture content of 50% IS sufficient, since that means that HALF the weight of a pile is water. But considering the fact that compost microbes primarily inhabit the thin moisture layer on the surface of the organic materials they’re consuming, that piles with a low-moisture content tend to be hotter and lose water at a faster rate than a wetter pile, and that ingredients taken from inside an active compost pile containing 50% moisture look and feel dry, it’s obvious that a 50% moisture content is NOT sufficient for creating an optimal environment for compost microbes.
Unfortunately, many compost manuals and articles also fail to adequately communicate the fact that because composting is a highly dynamic process, more moisture is required at the beginning stages of that process compared to the end of the process, and that there are many factors that have an impact on the moisture content, including:
The characteristics of insufficient moisture in a compost pile include: excessively hot pile temperatures (e.g. over 155 degrees F) for extended periods of time; ingredients inside the pile feel dry and are light brown or yellow; few if any water drops can bet squeezed out of a sample taken from inside the pile, and; the ingredients easily fall apart after being squeezed together.
One of the best ways to ensure that there’s sufficient moisture throughout the compost process is to begin with a moisture content of between 65 to 70%. As described above, the presence of optimum moisture will rapidly engage the decomposition process so that pile ingredients start decomposing as quickly as possible, thereby improving their ability to absorb and retain moisture, reduce the rate of moisture loss, reduce process time, and contribute to creating a high-quality finished compost. Starting the compost process with optimal moisture levels can also reduce the need for water to be added manually during the compost process.
As evidenced by billowing clouds of steam released during turning, moisture is lost when the hot moist air from the interior of the pile is exposed to the cooler and drier outside air. Turning also creates additional moisture loss as the moist ingredients that were previously inside the pile are brought to the surface where they’re now exposed to the evaporative effects of sun and wind. Therefore, along with the other advantages of reduced turning frequency that are described in the article “Low-Input Composting”, reducing turning frequency is one the best ways to minimize moisture loss.
Piles should be checked regularly to monitor changes in temperature, color, aroma, moisture content, the size and integrity of individual particles, and the ability of a sample to stick together after being squeezed.
If a compost site is located in a region that experiences long periods of hot, dry weather, and pile width is not limited by the size of a compost turner, evaporative losses can be minimized by building shorter and wider piles (versus narrower, and longer piles) to minimize the amount of exposed surface area. Whenever possible, feedstocks should also be stored in piles with broad flat tops to absorb as much rainfall as possible.
While there are some compost operations in very dry climates that use compost covers to protect piles from the drying effects of sun and wind, despite the fact that ComposTex compost covers are treated to be resistant to UV-light degradation, the lifespan of this fabric will still be reduced when used in this way. Therefore, if there’s a need to reduce evaporation, instead of using ComposTex for that purpose, it’s more cost effective to use shade cloth (90 to 95% shade), which is designed for extended periods of high UV-light exposure.
If either stockpiled feedstocks or active compost piles become too dry and there’s insufficient rainfall to increase the moisture content, then moisture should be added manually. There are a variety of ways to add moisture, including:
But whatever method is used to add water to a pile, the volume of water needed to appreciably increase the moisture content of a dry compost pile is likely to be much higher than most people would imagine. For example:
Many compost instruction manuals and articles describe temperatures in the range of 150 to 155 degrees F for active compost piles as both desirable and normal. And while there are many disadvantages of such high temperatures as described in the article “Low-Input Composting”, I suspect that the main reason that these high temperatures are considered normal and desirable, is that the problem of compost piles not having enough is so common.
In-fact, due to the cooling effect of moisture, active compost piles that have a moisture content of 65 to 70% will rarely have temperatures above 140 or 145 degrees F. Yet, inexplicably, rather than recommending higher moisture levels to avoid or reduce excessive temperatures, turning is often recommended, despite the fact that after a brief cooling effect, turning will actually INCREASE temperatures by both temporarily spiking oxygen concentrations, and accelerating evaporation rates.
Finally, for those who might worry that adding water to achieve an initial moisture content to 65 to70% would be a waste of water, since piles starting out with optimal moisture levels will gain water-holding properties faster and will have lower temperatures and reduced evaporation rates, these piles will actually lose LESS water compared to piles that start out with sub-optimal moisture levels.
As the process moves from the active phase to the “curing” and “finished” phase, there’s less microbial activity, less heat being generated, and less evaporation. Therefore, assuming the compost has truly entered the “curing” and “finished” phase (ie. it has a dark color, has an “earthy” aroma, and relatively few identifiable particles, a lower moisture content in the range of 45% to 50% is both adequate and desirable since this will reduce the weight of the finished compost, will make it easier and less expensive to screen, bag and/or transport, while still providing sufficient moisture to support some microbial activity. For reference, a moisture content of 45% to 50% would be the amount of moisture in a just slightly “damp” sponge.
Steven Wisbaum has produced thousands of tons of high quality compost through his business Champlain Valley Compost Co. based in Northwest Vermont. Since 1996, he has also been the largest worldwide distributor of ComposTex compost covers.