Worm casting shelf life

Is there a rule of thumb for how long worm castings are still “magical” after having been sifted and removed from the bin and worms?

As I understand it, the microbes are the key part of the magic - do they tend to be long-lived and hardy, or go dormant rather than die? Does anyone know if the microbes can survive being frozen, like some bacteria can?

Any advise, pointers, success stories, or general musings on the subject would be warmly regarded.

Thanks in advance, permacomrades.
-J

In biologically active soil, edible carbon residues are generally exhausted in 8 weeks, after which the microbial activity is curtailed.  In other words, if there are not roots in the soil pumping sugar/starch exudates, the microbial herd will exhaust the available food in about 2 months.  If there is a heavy layer of (for example) leaf litter on the soil surface and there are worms and other biota available to carry that biomass down into the soil profile, then the microbes will continue to eat beyond the 2 month window.  

So, imagine a bag of worm castings.  How much edible carbon residue remains for the microbial community to continue to eat?  With each passing day, less and less.  Obviously, there would be no living root to continue to feed the microbes, and no mulch layer on top of the bag of castings.  The microbe buffet gets leaner and leaner by the minute.

If I were to venture a guess to your question, I'd say that within 2 months, a large number of those bagged-casting microbes would be starving, at which point they may go into a state of dormancy, or perhaps go so far as die.  Clearly, the fresher your worm castings, the higher and healthier the population of microbes.  So while the nutrients should still be there in an old bag of worm castings, much of the biologic life would be dormant.

That is not a carefully researched answer—just me thinking aloud.

No expert here.  I have always tried to keep my worn castings moist until used.  Have always had better luck.  I believe that drying your casts either kills off or makes go dormant the bulk of the benificial microbes.
I have used dry and wet casts from the same bin harvested at the same time in side by side rows in the same raised bed. Let a buckets worth of casts dry out accidentally.  The row with the wet casts showed a noticeable difference in the size and vigor than the rows that I used the same but dried casts.  This was done on beans, tomatoes and cucumbers.
I don’t know if it kills off the microbes or by making the microbes go dormant it creates the noticeable lag in the above mentioned veggies.  Not that the dry casts didn’t provide an adequate fertilizer, but the wet cast rows were better across the board; size, color, number of fruit, and less bug damage were observed.
I really enjoyed the carbon info Marco posted above.

Microbes are incredibly hardy. Make sure they don't get completely desiccated (dried out) and they'll stay alive for a very long time (decades). They can absolutely survive being frozen, but but but… remember this is all a very contextual scale. Different vermicompost starts out with different numbers and types of active organisms. The time and conditions in between becoming worm castings and you putting them in the soil and your plants benefiting from these organisms are all factors in this equation.

My advice: don't worry about it too much. Fresher is better, but remember that you don't know when "fresh" was — was it when you harvested it? When the worms created the castings? Who knows? I try to use mine within 6 months.

Marco makes very great points and I think that Kevin's experience is very valuable and worth noting. It seems fairly obvious that there is an element to fresh worm castings that is lost in storage. The only thing I would like to add is that there is more to castings than their microbial content, there is also the digested and bioavailable minerals as well as the humic structure of worm feces.

If the question is - I want to buy a tote of castings, one is 2 weeks old and the other is 3 months old, they cost the same, which should I buy? - the obvious answer is the fresher batch.
If the question is - I can buy a tote of castings that is 2 weeks old for $100 or I can have this tote of castings that is 6 months old for free, which should I choose? - I think the answer is much less obvious. If your soil is missing biology than fresh castings have a lot to offer. If your soil is active but would benefit from some P, K, and some secondary minerals then the old castings could be just what the doctor ordered. This really comes into focus if you're talking about keeping your own worms and trying to value the castings you don't sell. It makes a lot of sense to make more castings than you would ever apply in a season, so what is the value of the left overs? after a year? after 2 years?

I would suggest that there is a large value in older castings and if they were significantly cheaper than fresh castings that, once you got life into your soil, the older and cheaper model would be the better choice.

Mostly I hope that fears about the shelf life of castings won't stop anyone from keeping worms and sifting an excess of castings. Even a freezerfull surely isn't excessive

Thank you all so much for the detailed and thoughtful feedback!