Harvesting Vermicast

“It is said that we can continuously add raw substrates to the worm bin to feed the worms, while it is also said that we can harvest vermicast after 30 to 45 days.  Question:  If we continue adding raw substrates, how can we harvest vermicast after 30 to 45 days? My doubt is based on the fact that there are still raw substrates present in the worm bin after 30 to 45 days. Your advice please. Best regards.” ~ Adriano

Hi Adriano,

Your question is quite similar to Andrea’s, but it provides me with the opportunity to talk about this topic some more.

Firstly – thanks for the great question. I can see how that might seem confusing! You have highlighted one of the potential limitations of ‘continuous’ composting systems (as compared to ‘batch’ systems).

For starters, it IS important to make sure you know what sort of system people are referring to when they provide suggestions such as those mentioned in your question. The ‘rules’ of vermicomposting can vary considerably from one system to the next, so generalizations (like ‘harvesting in 30 to 45 days’) won’t necessarily apply to your particular system.

One thing is for sure – in order to ensure that you have quality vermicompost, you will indeed want to avoid having unfinished materials in it (a fact you obviously understand already). How is this accomplished? As mentioned in my previous response, one good option (and very likely the best option) is to use some sort of flow-through system. This way, worms will continue to move towards the zone with the newest food additions, leaving behind the finished compost.

In a system like this, it is not unreasonable to predict that some vermicompost will be ready to harvest after 30-45 days – especially if you are working with very high densities of worms (a practice I would only recommend for those with a fair amount of vermicomposting experience, by the way).

In systems of a more static nature, it simply comes down to manually separating the unfinished materials from the finished compost. Again, screening the material can certainly help, since newer materials tend to have a larger particle size than the processed materials (especially worm castings themselves).

If you were using a windrow system (another approach we’ll be talking about in a future newsletter), periodically removing the top half (which should be full of worms and unfinished material) and harvesting the lower zone should work fairly well. The unfinished materials (with worms) can simply then be used to start up a new windrow.

Lastly, you can also simply leave the system to sit for a period of time without any further food additions. The worms will convert all remaining food/bedding into castings and then you can simply (or not so simply – depending on your approach) separate the material from the worms.

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