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
1997

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).

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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!

Written by Bentley on November 11th, 2010 with 8 comments.
Read more articles on Vermicomposting Research.

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• Vermiculture Technology (January 11th, 2011)
• Steinernema feltiae and Red Worms (November 12th, 2008)