These articles are quite explicit about methods to create the compost, and have good information about the processes of vermicomposting or composting.  They also do a good job of explaining some of the decisions a someone with horse manure will need to take (what kind of worms, how to set up windrows).

Here’s a great quote about the profligacy of E. foetida:

50 pounds of Eisenia will reproduce to 1,000 pounds of worms shortly after four doubling
periods (in just over 16 months). In two years, that initial 50 pounds of Eisenia will be more than enough (3200 pounds) to vermicompost the manure from 30 horses.

And the article also answers a very important question about horse dewormers and redworms:

Will horse dewormers kill red worms?

The most common wormer used is known by the brand name Ivermectin® made by the Merial Company. Merial’s research shows that the active chemicals in Ivermectin® are deactivated when manure is exposed to sunlight. Equine studies show that 95 percent of the active chemicals in Ivermectin® are deactivated in the horse before being passed in the feces. Leading experts in vermicomposting believe that the concentration of Ivermectin® in the horse manure is not high enough to seriously injure Eisenia.

Apparently the Ivermectin used for horses has been rebranded Zimecterin; it might be worth asking the vet about this situation because I couldn’t find anything.

These articles don’t, however, give much advice about what to do with the presumably vast amount of vermicompost your worms will be generating.  Assuming the worms reduce the weight of the manure by 20%, that’s still over 4 tons of vermicompost a week when your operation is fully going (assuming 30 horses).  This is about all the advice you get on what to do with castings:

Castings can be tilled into the soil directly from the windrow or screened for adding to greenhouse soil mixes, potted plants, lawns, and gardens.

Vermicomposting at this scale is an economic decision.  50 pounds of red wrigglers will probably cost you 500-700 bucks, and you have labor, land and equipment costs.  On the other hand, you have to get rid of your manure somehow and beyond castings, you might be able to start a business selling redworms to home wormkeepers like me.

To figure this, out, I filled a tray with vermicompost.  Then I manually separated the worms from the compost, using sunlight and time.  I ended up with a ball of squirmy worms.

I estimated the weight of the worms at 1.5 oz.  Then I measured the cubic volume of vermicompost I had separated, which ended up being 120 cubic inches.  This means that there was one ounce of worms for every 80 cubic inches.

I then measured the height and width of my worm bin.  It is 24 inches by 24 inches.  I then estimated how far I’d seen worms down in the bin.  Redworms typically stay in the first meter (approximately 36 inches) of soil, but I had recently dug out some castings from the bottom and hadn’t seen any worms there.  So I estimated the worms were in the upper 16 inches of the bin.  Multiplying these together gave a volume of 9,216 cubic inches.

Dividing the number of populated cubic inches in the entire worm bin by 80 gave me the number of ounces of worms in the bin, and dividing that by 16 indicated that there were approximately 7 pounds of worms in my bin.  Given I started with one pound two years ago, that is quite a return on investment (especially given that all the materials I’ve added to the bin were free).

Caveats: if I were serious about this, I would pull and harvest multiple samples, as that is the key calculation.  The above assumes that worms were distributed throughout the bin in the same manner as they were in the sample.

But, hey, it was fun to figure out–I was surprised!

Basically, the idea is to provide a safe place for worms to eat food scraps, while at the same time letting the worms move around your garden. When the worms move, they will distribute castings and aerate the soil Even red wrigglers and other epigeic species will move earth around to some extent.

I think this is a fantastic idea. It fits with the permaculture idea of using biosystems to minimize work. It enriches soil. If you make more than one worm tower, it creates redundant sources of worms. Depending on climate, it may make worm keeping outside a possibility.

However, there are some downsides as well. The PVC pipes recommended in the video are made of fossil fuels. Any substitute that will be durable probably has a high embedded energy. Depending on location and your type of worm, you may be introducing an exotic species. And obviously, a worm tower is not useful in an apartment or on a balcony.

With all that said, I’m grateful to the person who put the video on YouTube for introducing me to a new idea in such a pleasant manner.