Compost Moisture content

This page should be read in conjunction with those on  Compost Microbes, CN ratio,Compost Temperature  and Compost pH

 Water is essential for the survival and activity of the creatures, including microbes, required for composting.  The organic material being decomposed has to moist but not too wet.

Biological activity will slow if the compost heap starts to dry and will virtually cease if it dries out as most of the desirable compost creatures become dormant or die.

 In home composting moisture content of between 40% - 60% is normally recommended. Within the optimum in this range a thin water film will cover the particles of material being composted but will not fill the air spaces (pores) between and around the particles. The air spaces allow air and water to circulate through the organic material. At the lower end of this moisture range, between 40-45%, the compost heap will be able to absorb additional moisture, as greens and food waste is added, while providing allowance for a limited amount of rain to add extra moisture to the heap. It therefore provides target moisture content at the start of the wet season whenever that might fall.  As this implies monitoring, the moisture content of the heap or bin allows adjustments to be made during the composting process. During the main composting stage, higher moisture content, in the region of 50-55%, may be recorded. Increasing the moisture content to 55% - 60% is desirable at the start of the summer, or dry season, to allow for an increased rate of evaporation during hot and dry weather.

 During the maturation stage, lower moisture content is desirable, as the compost will become lighter making it easier to mix to make seed or potting compost or to use in the garden.

 In practice, the effective moisture range will vary with the size and type of the material being composted.  If fibrous materials are used, such as straw or Jerusalem artichoke stems, the moisture content can be higher without the material forming a wet, compacted, anaerobic mass. Work at the University of California showed that the moisture content can be as high as 85% to 90% and such material would still compost aerobically.  If grass cuttings, ash or paper form a significant part of the organic material anaerobic conditions can occur when the moisture content rises above 60% and will occur quite quickly at around 70%. As excessive water in the heap results in anaerobic decomposition it will be obvious that the aeration of the heap or bin is directly influenced to the moisture content. If the contents of the bin contain sufficient air spaces e.g. corrugated cardboard, egg boxes, hollow stems etc and enough Browns to provide the necessary C: N balance it may not be necessary to physically aerate the bin but if the moisture level is

The need to provide an adequate supply of moisture for the microbes breaking down  the organic material is why “hot heaps” are traditionally watered as they are  built,

 On the other hand if the compost is allowed to become too wet the water fills the air spaces between the organic particles being composted creating an anaerobic environment. If the spaces between the organic particles are flooded, with water there will be little airflow, there will be insufficient oxygen for aerobic respiration, and anaerobic microbes will start to dominate.

Worms require a moist environment and worm composting bins may run with higher moisture content than conventional composting bins. While it is true that composting worms are tolerant of a higher moisture levels,  some sources suggest between  50-70%, while another indicates up to  80% and yet another suggesting a range of between  50-90%

As a guide it is worth remembering that as worms breathe through their skin and it is estimated that the worms body consists of 75% water this might therefore indicate that  an  acceptable moisture content of 75 - 85%  for  the bedding.


High Moisture Content: Compost too wet

For normal composting the moisture, level should not exceed 60% or there will be a risk of water displacing air in the compost heap resulting in anaerobic conditions. If the compost is not in a sealed container nutrients may also drain (leach) out of heap.

Most food waste is wet and nitrogen rich, some having moisture levels of about 80% - 85% and will need to be mixed with carbon rich material to provide the correct C: N ratio and to absorb some of the excess moisture. Many cooked food waste composters are designed to work with the addition of a bulking agent such was woodchip, wood pellets or sawdust.

As high-moisture foods waste such as fruits may form a significant part of the waste added to worm bins they need careful monitoring to detect occasions where they become too moist. Rather than turning to aerate dry coir or shredded paper is normally added to absorb up excess moisture while stopping the supply of high-moisture foods.

In order to reduce the risk of excess moisture in my worm bins I leave the drainage tap open on the reservoir tray collecting the worm wee in a plastic container (a cut milk plastic bottle.

 When composting garden waste and uncooked kitchen waste the   compost may become too wet if too many “greens” have been added to the mix e.g.  Grass clippings.  More “Browns” should be added such as cardboard, paper, wood chip or chopped straw. Turning the composting material to mix the newly added Browns also aerates the compost and breaks up any compacted areas.

 If the compost is in an open heap or New Zealand bin, rather than in a bin with a lid, the excess moisture may be the result of rain rather than an incorrect mix and covering the heap should be considered

The moisture range of worm bins may be higher than for compost bins but they can still become too wet.



 At best, a wet worm bin will result in worm activity and reproduction being reduced and will eventually result in anaerobic conditions, with unpleasant odours and the death of the worms.

Opening the drain tap (if it is closed) and add dry bedding such as shredded paper and mixing it with the wet bedding to soak up the excess moisture. If the bin has draining holes rather than a  tap tip it  on its side so that the worm wee  drains out of the drainage (or lower air) holes if nothing comes out unblock the holes with a nail of drill. If it still does not drain, the wormery will need emptying.


Low moisture Content: Compost Too Dry

When the moisture levels fall below 40% decomposition will start to slow down and will virtually stop below 15%. This normally results from too many “Browns” being added to the mix or in hot climates it can be the result of environmental conditions.

  The use of a “corkscrew” type aerator  during composting will enable a sample of material to be removed from the centre and lower levels of the bin or heap to provide a sample for moisture checks or a probe moisture meter can be inserted from above or the through the side of the bin.

Moisture also has a role in the regulation of the temperature of the decomposing material compost and because of the type and activity of the compost microbes in the heap. When monitoring the temperature of a hot heap it will be found that a fall in temperature may be associated the compost drying out with lack of moisture reducing microbial activity.  In this situation adding water and mixing the contents should result in the temperature rising again Drier piles tend to heat up and cool down more rapidly than wetter piles, and excessive dryness makes piles susceptible to spontaneous combustion.  Usually dry heaps are the result of a poor Browns: Greens ratio and the use of too high a proportion of “Browns”. The addition of “Greens” or water will rectify the situation.

 In hot climates where the sun or high environmental temperature is causing the compost to dry out there are a number of options, in addition to adding greens and waste, these include composting in a pit, rather than a bin in full sunlight, and keeping the compost covered, so as to reduce water loss by evaporation.

 Where the top tray of a   wormery becomes too dry more “wet” food waste can be added or for a quick fix the dry surface of food and cardboard/paper top cover can be sprayed with water from a garden spray bottle. Spreading sheets of wet newspaper or cardboard over the top of the compost will help retain the moisture level in your worm bin.




Monitoring Compost Moisture

Visual and smell test.

 A simple visual check should show if the contents are dry. If dry it will also show no sign of decomposition, it will also lack heat, although this test is not helpful if cold composting.  If the compost is too dry it will lack heat and there will be little evidence of organic material break down. If you compost is too wet may appear slimy, be matted and smell unpleasant slight ammonia smell indicating anaerobic decomposition

 The Sponge Test

 A simplest hands-on test is to take a handful of compost from between 18-24 inches into the heap or bin, and squeeze it in a gloved hand. If the moisture content is between 40% - 60% it should react like a wet sponge staying compressed and releasing relatively little water.

The sponge test is difficult to use on the contents of the top trays of a wormery, as it would involve squeezing and possibly killing the worms.

 The sponge test provides a rough moisture measure but Will Bakx of Sonoma Compost has developed a system that refines the sponge test and classifies the moisture content into seven   categories based on the effects of squeezing the compost.

These ratings were confirmed as accurate by laboratory tests on the samples tested and the ratings may need adjusting when applied to compost from other sources and interpreted by another other individual. The article appeared in  BioCycle, a composting journal published monthly by The JG Press, Inc. in Emmaus, PA.

 Sponge Test Moisture Estimates

 Less than 40%. Compost too dry and does not form a ball when compressed and the hand is dry after discarding the material.

40-45%. The compost forms a ball when squeezed but does not remain in a ball when the hand is opened.  

45-45%.  A ball forms when the compost is compressed but it falls apart if tapped with the knuckle of the other hand. 

50-55%.  The ball of compost stays intact when tapped but no water is visible on the hands

55-60%. The ball stays intact and sheen can be seen on the skin but no water droplets are produced

60% upper limit.   Some drops of water are released when the ball is squeezed and can be seen between the fingers.

65%.  The compost is too wet with water running between the fingers when the compost is squeezed.

More details can be found at ( )


Compost moisture meters

  A wide range of compost and garden moisture meters are available for the home composter at between £7 and £40 this quick but not necessarily a very accurate measure. More accurate, and expensive, meters are available for commercial composters including digital recording meters.

As the moisture content with varying in different parts of the heap or bin, it is recommended to take multiple readings and, so obtain a wider picture of the activity in the heap, temperature (and the pH) be recorded at the same time

I would recommend using as long a probe as possible to reach the into the compost core of the compost and as well as the outer reaches. Most of the meters operate without batteries but some do require them. The cheaper meters often record on a scale rather than giving a direct moisture level. In many meters will record temperature and pH as well as moisture.

 The materials used in Vermiculture normally involves considerable variation in the moisture content of the feedstock   with pieces of material with a very high moisture content, such as food waste and fruit,  mixed in the tray with drier material such as shredded paper and cardboard. This makes sampling water content with a probe difficult as the results will differ considerably if the probe records the moisture from cardboard  compared with the results from the centre of a piece of melon or pumpkin.  This makes it essential to take a number of samples from each worm tray to give a representative result or to use the sponge test in preference to a probe.

  Drying to calculate moisture content

 Moisture meters give an immediate reading but these are said to lack accuracy. If a more accurate “laboratory” method is preferred (where the lab is the kitchen) the compost may weighed wet, dried and reweighed. Oven drying provides an accurate  method for measuring compost moisture content but inaccuracies can be result  drying it at a  so high a temperature as  it  loses  organic material or burns.


  • Weigh an empty ovenproof container of a suitable size to hold 10 -20g of compost. It will help if three or four similar, containers where available for use.
  •  Add approx 10-20g of compost taken from a number of sites in the bin or heap and dry the compost thoroughly in the oven at low heat overnight or until it ceases to lose weight.  
  • Weigh the dried compost
  • Subtract the weight of the dry compost from the wet weight given the weight of the evaporated moisture.
  • Divide by the wet weight and multiply by 100 to determine the Moisture content.
  • A variation of this technique is to adjust the weight of the wet compost to 10g when weighing.  Dry the sample in an oven at 110°C for 24 hours. Reweigh the sample, remembering to subtract the weight of the container, to obtain the dry weight. Calculate the moisture content as above.




Compost Moisture conten:t Sources and Further reading