You've harvested some worm castings, but you're not quite ready to use them.
No worries! There are many things you can do to preserve the quality of your castings.
Lets talk about the factors that affect how well your worm castings age, both in terms of nutrition and biology, and how to store them best, both for under 6 months, and over 6 months.
Oxygen
You never want to store your worm castings in such a way that there is no airflow. Sealing them in a plastic bag, or placing a lid over the container they're in (unless the lid has ventilation) will quickly decrease the quality of the castings.
If the worm castings do not have oxygen, all of the beneficial aerobic microorganisms will suffocate, and harmful anaerobic microorganisms will replace them.
The microorganisms that grow when there is little to no oxygen - anaerobic microorganisms - are the disease and pathogen organisms.
These organisms will further decomposition in the worm castings, and the chemical reactions that take place during anaerobic decomposition will cause your worm castings to lose nutrients as they are released through greenhouse gases.
This is the only time your worm castings will lose nutrients. In aerobic conditons, nutrients will be absorbed by microorganisms, and then released when they die, but not lost. Portions of carbon are always released during aerobic decomposition as CO2, and so over time your castings might have a higher C:N ratio, but this is not because there is more nitrogen (or any other nutrient), it is because there is less carbon.
Additionally, only in anaerobic conditions do pathogenic/disease causing bacteria grow. Though the worms are extremely efficient at sanitizing those microbes to nondectectable levels, if the worm castings get anaerobic enough even just one of those anaerobic microbes can reproduce rapidly.
So if your worm castings do not have oxygen, they will lose their value in both nutrients and biology.
Water
If you have too little water, your microorganisms will die and thus lose the biological benefits of worm castings.
If you have too much water, your worm castings will become waterlogged - and thus anaerobic, which we already know is bad after reading the section above.
If you are going to keep and use your worm castings within 6 months, you should aim to keep them at a similar moisture level you would for your worms - or a bit drier.
It is also quite likely that there are some baby worms or cocoons in your castings, so keeping a good moisture level will help those guys grow.
Aim for only 1 drop of water to come out when you squeeze a handful. You don't want more than a couple drops of water, and you don't want the material to crumble apart when you let go of it.
This moisture level will allow microbes to remain alive and continue the work of decomposition. Up until around the 6 month mark, the biology will become more diverse as the castings have time to mature.
Temperature
Ideally you can keep your worm castings at the same temperature as your worms. But its really not that important.
As temperatures get cooler, microbial activity can decrease. You don't want to let your worm castings freeze, though, because that can kill your microorganisms. You can use temperature to your benefit if you are storing your castings for more than 6 months.
Storing Worm Castings Long Term (6 Months +)
Of course, this isn't ideal, but stuff happens.
Worm castings have a shelf life because decomposition is occuring. So to extend that shelf life, decomposition needs to be inhibited. The idea is to wipe out the microbial life in the worm castings, so that there are essentially inert.
*Note: its better to not use these methods if the worm castings can be used within 6 months.
1.) Dry your worm castings so that all microorganisms die or go dormant.
a) The drier, the less decomposition can occur, and the longer they can last.
2.) Freeze your worm castings so that all microorganisms die or go dormant.
a) This one is especially useful because most of the time people arent using their worm
castings because its winter. If all available water/microbes are frozen, no decomposition can
occur.
You are essentially pressing pause on your worm castings, so that when you rehydrate them or unfreeze them, they are in the same condition as when you first dried/froze them.
However, the big problem with doing this is that you are wiping out the beneficial biology in the worm castings. When you rehydrate/unfreeze the worm castings, many of those organisms will be lost.
So to combat this, when you do rehydrate/unfreeze your worm castings, you need to reinnoculate them with the biology.
You will probably have fresher worm castings by the time you are ready to use them, so you will want to mix together your older worm castings with the newer.
Within a couple weeks, the older worm castings should have been mostly repopulated with biology, and they will be much greater quality than what they would have been if you hadn't frozen/dried them.