In order to talk about optimal storage and moisture conditions for the finished product of a worm bin we first need to be on the same page as regards the nature of that finished product.
The material harvested from a "finished" worm bin is generally called vermicompost , and is a mixture of earthworm castings (earthworm poop), decomposed organic matter that did not pass through the gut of the worm (compost), and partially decomposed/undecomposed organic matter that is still (somewhat) recognizable. In most situations it is castings that comprise the bulk of the material, usually around 70-80% of the total, dependant upon system type, management method and residence time in the system.
Castings (also called casts and vermicast) are, as stated above, the manure of the earthworm, meaning they are the waste material generated after the worms have processed the organic matter through their gut. Castings are collected by sifting the vermicompost through a series of mesh screens, typically beginning with a 1/2" mesh, with the material that falls through then screened through a 1/4" mesh. Some castings producers screen the material that falls through the 1/4" mesh through a 1/8" mesh to produce an exceptionally fine, uniform material, but it is generally accepted that the material sifted through the 1/4"mesh is nearly pure earthworm castings.
Optimal storage methods will be influenced by whether one is storing earthworm castings or vermicompost.
Before discussing storage methods, it may be wise to reiterate that castings are not necessarily superior to vermicompost! There is some research and a great deal of anecdotal evidence suggesting that, all things being equal, vermicompost produces a better over-all plant growth response than does pure castings. It is believed that the varied particles size, water-holding ability, and level of stability in vermicompost more closely mirrors that of the active fraction of soil, creating a resource that well supports a wide variety of beneficial organisms, and better aids in the formation of good tilth than does uniformly sized, dense, stable castings. Further, all animals excrete excess salts through their manures, and earthworm are no different. Earthworm castings can be and usually are very high in salt relative to good soil, and, at sufficiently high concentrations in a growig medium, can cause plant damage due to salt toxicity. Vermicompost, by virtue of its lower castings volume, tends to have a lower salt level, and it is believed that the compost component may act as a salt buffer, further ameliorating any potential salt issues. Vermicompost is also not so dense as are castings, ensuring optimal oxygen and water penetration into the growth medium.
The take-home message is, castings are not better than vermicompost!
The additional steps taken by castings producers to ensure they package (essentially) pure castings is a marketing issue, not necessarily one of efficacy. For reasons surpassing understanding, the average gardener erroneously believes that soils and soil amendments that are fine and uniformly sized are generally superior to chunky, somewhat irregular materials. The size uniformity of screened castings appeals to most gardeners, thus castings producers give the gardening public what they want, not what is necessarily most beneficial to the plant or soil. This is a marketing decision, not one based on product performance.
It should be noted, too, that because of their stability, earthworm castings are a somewhat more predictable material than is vermicompost. By screening vermicompost to (essentially) pure castings the castings producer can ensure that each bag sold contains a product with a known chemical and biological character, and a relatively reliable plant growth impact.
Another issue associated with the sale of vermicompost is its stability and how it must or should be stored, and this leads us back to the original topic.
The undecomposed and partially decomposed organic materials in vermicompost continue to break down during storage. Placing actively decomposing organic materials in an airtight container encourages anaerobic organisms to take over decomposition with their cadre of foul-smelling, plant-toxic by-products; and drying vermicompost halts decomposition, and often causes some polymerization, meaning the material becomes impossible to re-wet (hydrophobic). What all of this means is that if one is storing vermicompost it is best to first dry it down so that it is damp, not wet, thus ensuring sufficient air penetration through the material to prevent anaerobia; and keep it in a non -airtight container. Stored in this way the vermicompost slowly stabilizes in an aerobic environment and has a shelf life of more than three years.
If one is storing castings, things become much less management intensive. Because castings are essentially stable they can be both stored in airtight packages and dried to 5% moisture with no loss of plant growth impact.
It is typically the differences between vermicompost and castings that lead to what appear to be conflicting recomendations in storage practices. If you are unsure whether you are dealing with castings are vermicompost in your system, simply seal a small handful in an airtight plastic bag, and after a few days, open it up and take a big ol' sniff. If it smells like soil, you're dealing with castings that can continue being stored in an airtight container. If it smells like ammonia, a sewer, vomit, vinegar, a dead body,or generally causes you to turn your head and say something best not said in the presence of a three year old, well, you probably get the idea...
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