Moisture Management – The Key to Successful Composting

by Steven Wisbaum, Updated January 2019

Water is essential to all life on earth, and this obviously includes the myriad of microbes that make the compost process possible. Yet, over the past forty years of making compost, operating a "custom" composting operation, making tens of thousands of tons of compost for my own business, and selling ComposTex compost covers to composters around the world, I’ve learned how important it is to have adequate moisture throughout the compost process. But I’ve also discovered that there’s a significant lack of understanding regarding both the importance of, as well the techniques for managing moisture in composting operations. In my opinion, the reasons for this lack of understanding include:

  1. Few composting instruction manuals and articles explain that compost microbes primarily exist in the moist outer layer of the materials they’re metabolizing, which means that a compost pile that has insufficient moisture simply won’t be able to support optimal microbial activity.
  2. Many instruction manuals and articles often use the term “damp sponge” to explain how much moisture a compost pile should have. But “damp” is a subjective term that is used to describe many different amounts of moisture. For example, a “damp” sponge can be just barely
  3. damp (ie. no water drops can be squeezed out), damp (ie. it’s possible to squeeze out a few drops), moderately damp (ie. it’s possible to easily squeeze out more than a few drops), or very damp (ie. lots of water drips out when squeezed).
  4. In the instruction manuals and articles that do provide a quantitative value for the ideal amount of moisture in a compost pile, the moisture levels specified are often in the range of 40 to 60%. Yet, it’s my experience that 60% moisture is just barely enough moisture to support microbial activity during the “active” or “hot” compost stage, with levels below 50 or 55% being completely inadequate.
  5. Similarly, most instruction manuals and articles don’t even mention that because composting is a highly dynamic process, there’s different moisture levels required at different stages of the compost process. For example, while relatively high moisture levels are required at the beginning (ie. “active” or “hot”) stage of the compost process, much less moisture is required at the “curing” or “finished” stage.
  6. For all these reasons, it often seems that the authors of these instruction manuals and articles simply don’t have much of their own real-world composting experience, so are just repeating what they’ve read or heard somewhere else, possibly from other authors who also lack real-world composting experience. And/or it may be that few of these authors have actually measured and compared the moisture content of compost piles at different stages with varying amounts of moisture.

So, the first step in understanding how much moisture is required during the various stages of the compost process is to know what the term “moisture content” means, and how it’s calculated:

Moisture content is calculated by weighing a sample, drying the sample, and then reweighing the dried sample, with the resulting difference being the weight of the water that was in the original 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 is 50%, or .5 lb divided by 1 lb = .5 (or 50 %).

Too Much Moisture

As mentioned above, compost microbes need moisture to survive. And while some compost microbes can survive periods of limited oxygen availability, because they are primarily “aerobes” they require a steady supply of oxygen to replace the oxygen they metabolize. In a compost pile, this oxygen is supplied “passively” via “diffusion” and “convection”. Diffusion is the process wherein oxygen in the atmosphere outside a pile slowly “diffuses” into the interior of the pile where there is less oxygen. Enhancing this process of diffusion, heat generated inside an active pile rises (via “convection”) which pulls in cooler outside air - a process referred to as the “chimney effect”. In “aerated” composting systems, this chimney effect is mechanically aided by air being pushed or pulled into a pile using either positive or negative pressure using a system of fans, pipes and holes in the compost pad under the pile.

Whether due to forced, or passive aeration, the speed at which oxygen can be replenished is limited by how easily gases (including oxygen) can move through the pile, and that ability depends on the amount of “pore space” in a pile, which is largely determined by particle size, shape, and how densely packed the materials are, a measurement known as “bulk density” (ie. the weight of a sample for a specific volume). But particularly relevant to this discussion, another important factor that determines bulk-density is the amount of moisture present in a pile, because as the moisture level rises, water will fill the available pore space, thereby displacing and restricting the movement of gases.

While the primary purpose of “turning” a pile (either with a “compost turner” or a bucket loader) is often thought of as ADDING oxygen to a pile, in-fact, there’s only a relatively small amount of oxygen that’s added during turning and that oxygen is actually used up quite rapidly, likely within minutes. Instead, the primary value of turning (at least in terms of oxygen supply) is the restoration of pile “porosity” that’s lost over time due to the reduction in particle size and settling. But, it’s also worth noting that very high quality compost can in-fact be made with minimal or no turning, assuming a pile contains an optimal amount of moisture and porosity. This concept is described in detail in my article “In Defense of the Pile Less-Turned – A Case for Low-Input Composting”.

What Are the Causes of Excess Moisture?

Excess moisture in a pile can be caused by either using a mix of raw ingredients that were too wet to begin with, or by allowing a pile to be exposed to more moisture in the form of rainfall than can be 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 saturation. Characteristics of a saturated pile include water seeping out the bottom (ie. “leachate), the interior of the pile appearing wet and slimy, and unpleasant odors that are the by-products of anaerobic decomposition.

What Are Some of the Ways to Avoid Excess Moisture?

Start out with Optimum Moisture Levels
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.

Active Compost – 66% Moisture (Dried and Fresh Sample)

Active Compost – 66% Moisture “Squeeze Test"

Modify Pile Size and Shape
If there’s a high risk that a compost pile will be exposed periods of intense and/or prolonged rainfall, 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. If possible, building a pile with a peaked top will also shed water better than a broad flat top.

Protect Piles with Compost Covers
Excess moisture can also be avoided by protecting piles with a specialized compost cover such as ComposTex™, which is a non-woven, polypropylene, UV-protected, macro-porous fabric that sheds rainfall but is permeable to oxygen. More information about ComposTex including “Questions and Answers” and "Use and Care Instructions" can be seen here.

What Can be Done to Reduce the Moisture in a Compost Pile That’s Too Wet?

Add Dry Ingredients
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), more dry ingredients with low bulk density could be mixed in to absorb the excess moisture and reduce the pile’s bulk density. Examples of the materials that could be added includes horse manure with lots of dry bedding, dry leaves, finely shredded, dry wood waste, etc.

Protect Piles with ComposTex
As needed, a wet pile can also be protected from additional moisture from rainfall by covering it with a compost cover such as ComposTex™.

Use Turning to Accelerate Evaporation
A wet pile can also be dried out over the course of a week or two by allowing its surface to dry out, then turning it to mix the dry materials on the outside of the pile with the wetter materials from inside the pile, followed by another period of drying, followed by another turn, etc. And if heavy rains are expected during this drying out process, the pile should be covered to prevent exposure to this additional water.

Modify Pile Size and Shape
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.

Not Enough Moisture – A More Common Problem

While most experienced composters know that compost microbes need moisture to survive, it’s my experience that many compost operations still 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 optimum 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 when one considers the fact that compost microbes primarily inhabit the thin moisture layer on the surface of the organic materials they’re consuming, and that the ingredients taken from inside a pile containing 50% moisture feel dry to the touch, it’s obvious that 50% moisture is NOT sufficient.

Making matters worse, most compost manuals and articles also fail to mention that because composting is a highly dynamic process, more moisture is required at the beginning stages of the compost process, and that there are many factors effecting the moisture content during the entire process, including:

  1. Changes in the levels and types of microbial activity, the amount of heat generated by those microbes, and the amount of evaporation caused by that heat;
  2. The rate of evaporation caused by turning, or by forced aeration;
  3. The rate of evaporation caused by exposure of the pile to sun, wind, and the ambient (ie. outside) humidity and temperature;
  4. The frequency and intensity of rainfall and/or snow-melt and the effect of pile shape in regard to the amount of water that can penetrate the interior of the pile;
  5. The width and height of a pile which determines the amount of surface area for a given volume of material. This characteristic is described as the “surface-to-volume ratio”, with a HIGH surface-to-volume ratio describing the relative amount of surface area that’s produced as a result of putting a given volume of material into a long, narrow pile versus putting that same volume of material into a short, wide pile, that will create a comparatively LOW surface-to-volume ratio pile that has more volume and less surface area.
  6. Most carbon-rich materials become increasingly hydrophobic (ie. resist wetting) the drier they become. And conversely, their ability to both absorb and retain moisture increases as they decompose.
  7. Because low moisture levels are associated with a low bulk densities (ie. there’s less weight for a given volume of material), and lower bulk densities are typically associated with higher rates of passive aeration (because there’s more pore space), as moisture levels continue to drop, there’s increased passive aeration which allows for increased evaporation, which leads to increased moisture loss.

Characteristics of Insufficient Moisture
The characteristics of insufficient moisture in a compost pile include: excessively hot pile temperatures (e.g. 150 to 160 degrees F); ingredients inside the pile are light brown or yellow (versus dark brown or black); there are large swaths of a white mold that thrives in environments containing low moisture and high temperatures; no water drops can be squeezed out of a sample removed from the interior, and; a sample easily falls apart after being squeezed together.

Horse Manure - 45% Moisture (Dried and Fresh Sample)

How to Ensure Optimum Moisture Levels
Start out with Optimum Moisture
The easiest way to ensure that there’s sufficient moisture throughout the compost process is to begin with a moisture content of between 65 to 70%. The presence of this amount of moisture will rapidly engage the decomposition process so that pile ingredients start turning into compost as quickly as possible, thereby improving their water-holding properties, reducing the rate of moisture loss, avoiding the risk of pile ingredients becoming hydrophobic, reducing process time, and improving the quality of the finished compost. And since comparably higher rates of moisture loss are associated with piles that start off with low moisture levels, starting the compost process with optimal moisture levels also reduces the chance that water will need to be added manually during the compost process.

And to gain practical experience with different moisture levels, I recommend that composters collect samples from different piles of compost and raw materials that contain different amounts of moisture, noting the characteristics of those piles (e.g. temperature, color, aroma, presence of a moisture “sheen”, the look and feel of individual particles, the number of water drops that can be squeezed out of a sample, the ability of a sample to stick together after being squeezed tightly, etc.), and then calculating the actual moisture content of these samples by weighing, drying and then re-weighing them.

Make the Maximum Use of Rainfall to Increase the Moisture Content of Dry Feedstocks
If a compost operation is located in an area that experiences periods of high and/or prolonged rainfall, and uses a large proportion of dry feedstocks, those materials should be stored in piles with broad flat tops with the maximum amount surface area possible to absorb as much rainfall as possible and make use of those wet conditions.

Manually Add Moisture to Dry Stockpiled Feedstocks and Active Compost Piles
If taking advantage of rainfall isn’t possible due to the lack of rain and/or space limitations, then moisture needs to be added manually. There are basically two ways to add moisture to either stockpiled feedstocks or active compost piles. One way is to use sprinklers that wet as much of the entire pile surface as possible, allowing that water to slowly seep into the interior of the pile. And while using drip irrigation would seem to be the best way to add water, it’s actually NOT due to the very narrow wetting pattern that occurs in dry, hydrophobic, loosely-packed compost or feedstocks, versus the much wider wetting pattern that occurs in more densely packed soil. In-fact, when a drip system is set up on the top of a compost pile, the water from each emitter tends to “channel” straight down through the pile and out the bottom with relatively little actual wetting of ingredients occurring within the pile. What’s worse, as this water channels down through the pile it absorbs and removes soluble nutrients creating “leachate”.

Therefore, a faster and more effective method to add water is to do that while turning, either using the “watering manifold” installed on many compost turners, or by using a high-pressure/volume hose to manually spray a pile as it’s being turned with a turner or bucket loader. But whatever method is used to add water to a pile, the total volume of water needed is likely much larger than most people would imagine. For example, if a pile contains 300 cu yd of material, weighs 180,000 lbs (@ 600 lb/cu yd) and has a moisture content of 50%, that means the pile contains 90,000 lbs of water (or 300 lb of water/cu yd). Therefore, to increase the moisture content of that pile to 65% will require adding around 75,000 lb of water, which would be just under 9,000 gallons (@8.34 lb/gal).

Here’s the math:

  1. 300 cu yd of material weighing 180,000 lbs
  2. At 50% moisture, 180,000 lbs x 50% (.50) = 90,000 lbs H2O (with 90,000 lbs of “dry matter”)
  3. 90,000 lbs (original) H2O + 75,000 lbs (added) H2O = 165,000 lbs H2O (new total);
  4. 90,000 lb (original) dry matter + 165,000 lb (total) H2O = 255,000 lbs (new pile weight);
  5. 165,000 lbs H2O / 255,000 lb (total) = 65% H2O.

What this means is that for every 3% increase in the moisture content of this 300 cu yd pile, approximately 15,000 lb (1,799 gallons) of water is needed. So, assuming a flow rate of 50 gallons per minute, it would require 3 hours to supply the full 9,000 gallons of water, or 1.5 hours at 100 gallons per minute. This also means that to maintain optimum decomposition, disrupt the self-perpetuating cycle of excessive moisture loss, and reduce the total amount of water that will need to be added, it’s best to add this water as early as possible, rather than waiting until moisture levels have dropped precipitously and a pile has already become too dry.

Modify Pile Size
If a compost site is located in a region that experiences long periods of hot, dry weather, evaporative losses can be minimized by building taller and wider piles (versus narrower, and longer piles) to minimize the amount of exposed surface area.

Reduce Turning Frequency
As evidenced by those classic billowing clouds of steam released during turning, large quantities of moisture are lost when the hot moist air from the interior of the pile is exposed to the cooler and drier outside air. But moisture also continues to be lost well after turning as the moist ingredients that were previously inside the pile are now brought to the surface where they are exposed to the evaporative effects of sun and wind. Therefore, along with the other advantages of reduced turning frequency that are described in my article “In Defense of the Pile Less-Turned – A Case for Low-Input Composting”, turning less frequently is the easiest way to minimize moisture loss.

As a Last Resort, Protect Active Piles with a Compost Cover
While there are some composters in very dry climates who use compost covers to protect piles from the drying effects of sun and wind, there’s actually a relatively small reduction in water loss when using these covers for that purpose. Furthermore, while compost covers such as ComposTex are typically treated to be resistant to UV-light degradation, the lifespan of the fabric will still be reduced when used for prolonged periods over many summers. But, if an operator does need to use a compost cover to reduce evaporation (or for some other reason), I recommend protecting it with shade cloth (90 to 95% shade) which will significantly extend the life of the fabric.

The Connection Between Excessive Temperatures, Insufficient Moisture and Turning
Many compost instruction manuals and articles either directly or indirectly equate achieving high temperatures in the range of 150 to 155 degrees F with optimal composting. But aside from the fact that there are other reasons to avoid such high temperatures (as described in my article “In Defense of the Pile Less-Turned-…”), piles with temperatures of 150 to 155 degrees F are only 5 or 10 degrees away from becoming too hot for even the most heat-tolerant compost microbes. But I suspect that the main reason that these high temperature ranges are considered “normal” and therefore desirable, is that the problem of compost piles not having enough moisture is so pervasive.

In-fact, due to the cooling effect of moisture, active compost piles that have an optimal amount of moisture (e.g. 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 spiking oxygen concentrations and further reducing moisture levels.

Finally, for those who’d worry that adding water to achieve an initial moisture content to 65 to70% would be a waste of water, as described earlier, it’s actually quite the opposite. Specifically, since piles starting out with optimal moisture levels will gain water-holding properties faster and will have lower temperatures with reduced evaporation rates, these piles will lose LESS water than piles that start out with sub-optimal moisture levels, and they’re also less likely to need additional water later during the compost process.

Moisture Levels in Curing or Finished Compost
As the process moves from the active phase to the curing and finished compost phase, there’s less microbial activity, less heat being generated, and less evaporation. Therefore, a lower moisture content in the range of 55 to 60% is both adequate and desirable since this will reduce the weight of the finished compost, will make the compost easier and less expensive to screen, bag and/or transport, but there will still be sufficient moisture to support some microbial activity. A moisture content of 55 to 60% would be the amount of moisture found in a “damp” sponge, which would be enough water to make it possible to squeeze out one or two drops.

“Finished” Compost – 39% Moisture (Too Dry)

Finished Compost – 56% Moisture (Dried and Fresh Sample)

Finished Compost - 56% Moisture (Squeeze Test)

Protecting Curing or Finished Compost from Excess Rainfall
Because curing or finished compost is cooler and has greater water-holding capacity, its evaporative moisture losses are lower, making it considerably more susceptible to becoming too wet from rainfall. For these reasons, protecting curing or finished compost from excess rainfall is the most common use of compost covers.