Moisture Content and Stocking Densities – Impact on Growth and Maturation

One of the things I really enjoyed doing during my ill-fated masters degree program (apart from playing with my worms, this was probably one of the ONLY things I enjoyed doing! haha) was reading academic vermicomposting literature. I certainly don’t say that to look all sophisticated or anything like that. I’ll be the first to admit that it’s the summary (“abstract” – kinda like the “Cliff Notes” version of the article), intro and discussion sections that always hold the greatest appeal for me. I am definitely not a numbers guy, so my eyes tend to glaze over while reading through the “results”! haha

Anyway, this morning when I happened upon one of the many vermicomposting journal articles I have printed out, it reminded me that there has been a lot of really interesting research conducted in this field. Unfortunately, a great deal of it is not really all that accessible to the public at large. As such, and since I know a lot of readers are interested in the experimental side of things, I thought it might be fun to start writing posts about particular journal articles I have found to be interesting – perhaps it will help to stimulate some cool discussions!

On that note – here is the first article I have selected:

Effects of stocking rate and moisture content on the growth and maturation of Eisenia andrei (Oligochaeta) in pig manure

Authors: Jorge Dominguez and Clive Edwards
Journal: Soil Biology and Biochemistry, Volume 29, Number 3/4. pp. 743-746

As the title implies, in this study the authors set out to determine what sort of impact moisture content and stocking densities had on the growth and development of E. andrei. Just so you know, this species is another very common variety of “Red Worm”, and while they have been determined to be different than Eisenia fetida (species most commonly thought of as “Red Worms”), the two species are very often together in a given culture and can only truly be distinguished from one another via high-tech laboratory methods.

The authors conducted two main experiments in this study:

Experiment #1 – This examined the impact of different moisture contents. Four juvenile worms were placed in plastic dishes and fed 100 g of pig manure mixed with maple leaves (85g + 15g, respectively). Six different moisture content treatments were established (via the addition of water) – 65%, 70%, 75%, 80%, 85% and 90%. Four replicates were created for each treatment, and all dishes were left to sit for 44 days without further (food) addition. Treatments were monitored every 4 days, with a focus on three key parameters: 1) survival, 2) biomass of worms, 3) presence/absence of clitellum (indication of maturity). Moisture content was monitored every two days and water added as necessary (to maintain treatment level).

Experiment #2 – This examined the impact of different stocking densities at a consistent moisture content (80%). Each dish received 150 g of the manure/leaves mix mentioned above. The stocking densities were: 1, 2, 4, 8 and 16 worms per treatment – again there were four replicates for each. Every 4 days the worms were weighed and their level of maturity determined. The duration of the experiment was 48 days.

Both experiments were conducted at 20 C (68 F).


It was determined that moisture did indeed have a significant impact of the growth and maturity of the worms (although they were found to grow and mature in all treatments). The “optimal” moisture level was found to be 85%.
It was determined in the second experiment that stocking density also has a significant impact on the growth and maturation of Red Worms. The researchers found that 8 worms per dish was the most favorable stocking density (of those tested), with growth/maturation dropping off at the higher stocking density.

Interesting tidbits
– the authors state (in discussion) that temperature and moisture content of waste materials are the two most important environmental factors affecting the vermicomposting process.
– in the intro, however, they mention that moisture preferences can vary from substrate to substrate (so don’t assume this applies to all situations).

As is always the case with scientific experiments (or any vermicomposting results for that matter), I strongly recommend that the results of this study not be viewed as “set-in-stone” rules to follow. Many people may not realize it, but 80-90% moisture content is REALLY high – there is no way in a million years I would recommend that anyone keep their system this wet, UNLESS there is also excellent oxygenation (remember, moisture and oxygen content often tend to be somewhat mutually exclusive) – a prime example of a situation where this could work would be vermiponics. Worms can basically live in an aquatic environment if there is enough oxygen present.

In this study the authors were using tiny little dishes, so it’s not too surprising that the worms were able to survive in really wet conditions (since ample gas exchange still possible). I can only imagine how quickly they would have perished if they had been put in larger bins containing the pig manure mix at some of those moisture levels!

The results of the stocking density experiment were interesting. Clearly, Red Worms can do well at higher densities, BUT there is a point where the performance drops off. For those of us who want to grow lots of worms, this suggests that we should be splitting our bins (starting new systems) before conditions become too crowded. Similarly, it might not be a bad idea to avoid starting with a really high density of worms.

It all depends on your goals though – if, for example, you want to quickly produce lots of castings (and/or process lots of waste materials), starting with higher worm densities might make more sense.

Anyway – that’s basically it for this particular research article – hope you found this new (review) approach interesting. I had fun with it, and look forward to digging up some more articles to write about here!

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    • Anna
    • November 11, 2010

    Bentley, I no longer have free access to academic articles, so I’m thrilled you’ve decided to undertake these reviews.

    So now for the questions…did the Dominguez and Edwards happen to mention the exact size of the containers they used? I think that having an ideal worm-to-volume ratio might help me better visualize the ideal population density in there study.

    • Jorge
    • November 12, 2010

    Hi Bentley,

    I’ve been trying out Clive Edwards’ suggestions of ‘optimal’ conditions for a worm bin and one thing that has been really surprising is how much moisture vermicast/vermicompost can retain and not seem ‘too wet’. My bin at school is kept between 80-85% by monitoring the moisture using
    gravimetric analysis
    , and yet it still seems ‘dryer’ than any plastic bin I’ve ever had.

    Definitely one of the great perks to still being in school is the access to unlimited scholarly journals. I don’t know if all schools have this but the City Univ. of New York let’s me order books via WorldCat from practically any institution in the world – which I would never be able to afford myself. I just got Zenjhun’s Vermiculture & Vermiprotein – can’t wait to dig into it.
    Great Post!


    • Larry D.
    • November 12, 2010

    I found out with a euro test about moisture.I have no way to measure it.But one is wetter than the other.The real wet one has a little smell with fewer babies.The other is still wetter than you would normally keep a bin.But it has three times the babies it appears.Is it 85 percent moisture?I would guess yes.But have no way to check it.It is neat to have the same amount adults sitting on top of each other,and get two different results!
    They say euros like it wetter though.But my EFs like some moisture too!

    • Bentley
    • November 12, 2010

    ANNA – was silly of me not to include that info! Thanks for asking.
    The containers were 250 ml. Again, I wouldn’t assume that one could come up with an “ideal” density for a regular worm bin based on this (since a completely different system) but you could at least get some idea.
    JORGE – Thanks for sharing. That book actually sounds really interesting. Are you saying it could be ordered via a university fairly easily (my father is a semi-retired prof so could help me out there).
    LARRY – I’ve seen some evidence to indicate (and have read) that Euros like it wet, but as you say, Efs sure like the moisture as well!

    • Kator
    • November 12, 2010

    Great idea Bentley .. and the material is very interesting. Please keep it coming.

    Every paper that I’ve reviewed points to moisture control as being very relevant to a productive operation and as you refer, I also experimented with wigglers proving that they can survive (not necessarily thrive) in water. That experiment ran for 43 days before I terminated it (Bti testing).

    I now have three Rubbermaid bins and one stacked wooden flow-through indoor setup going, all initiated from my original 1 pound supply of wigglers – into my fifth month. My original Rubbermaid bin is my primary breeder and the moisture in that bin averages 80% (meter read). All feed is blended and fed in fashioned cardboard troughs. Bin three is receiving quite a bit of pumpkin these days with about a four month supply in reserve.

    Your point on moisture, which you cautioned when I first started, is really important. In my case, every bin requires individual attention. It has been my limited experience that moisture retention/environment is dependent in large part on the maturity/type of bedding and quantity of mature/processed VC present. Controlling that is a little labour intensive but rewarding, my goal being expansion/optimum breeding conditions.

    I mist spray for moisture control and really have to watch the wooden flow-though as it will not retain a similar level of moisture like the rubber bins, but does provide much better air flow. It’s relatively new to my little operation and it will take a few months to assess productivity and make comparisons to rubber bin production. Rubber bins work if you are attentive and keep the material aerobic. I may eventually be converted, but for now it’s working very well.

    • Jorge
    • November 14, 2010

    Bentley – I’m pretty sure that is the case; most universities are connected through an ILL (interlibrary loan) system. Because I’m currently researching issues like moisture and pH re: growth of worms, I’ve been looking through a lot of literature and creating pdfs of studies I will be referencing. I can send them your way if you like.

    Also excited for this upcoming release:
    Vermiculture Technology: Earthworms, Organic Wastes, and Environmental Management

    • Kator
    • November 15, 2010

    I feel that my comment on wiggler survival in water warrants a little detail. I stated, “I also experimented with wigglers proving that they can survive (not necessarily thrive) in water.”

    The experiment involved hatching numerous eisenia fetida cocoons in Petri dishes which contained water, a feed mix and several types of insect larvae. The eisenia fetida not only hatched while submerged but also fed and grew in size. Growth was not as fast as observed in a “bin” environment, however the experiment was not designed to optimize growth. The project is being repeated and if successful it may be of value with the control of fungus gnats. It’s not a “normal” environment for eisenia fetida and full life cycle aquatic reproduction may not be possible.

    • Bentley
    • November 15, 2010

    KATOR – Thanks for sharing your experimental results/insights.
    I definitely DO caution people about moisture because it can be so easy to overdo it when using an enclosed plastic bin (the most common type of system for newcomers). With draining systems and/or those with LOTS of air flow (and just generally, once one gains more experience) it’s definitely not as much of a concern.
    I think Red Worms could complete a full lifecycle in an almost-aquatic habitat for sure. I think that as long as there is ample oxygenation and some good habitat structure (such as would be found in a vermiponics bed or water filter) they would thrive. If the liquid is at all stagnant I suspect the worms wouldn’t do all that well.
    Anyway – do keep us posted! Very interesting stuff
    JORGE – Sounds great! I’d love to learn more about what you are doing, and what you’ve come across in your literature digging!

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