I see a lot of people wanting to buy composting worms to add into their garden soil, or raised beds. There are huge benefits from getting these guys in your soil, but to have success with that a few things need to be understood first.
The Different Kinds of Earthworms
There are 3 main types of earthworms that live in the soil profile. They do not all have the same ecological niches/natural habitats.
Composting worms are 'Epigeic' Earthworms. They prefer to live only the top several inches of the soil profile - in decaying organic matter rather than soil itself. Because they prefer to live in leaf litter, they do not make burrows like other kinds of earthworms.
The kinds of earthworms that do prefer to live in the soil, are Endogeic and Anecic Earthworms. These are the ones you see in your yard as you are digging, or you find after a heavy rain. They make burrows throughout the soil that can be multiple feet deep, and are the "earth worker" earthworms.
In the North Americas, these Anecic earthworms are invasive and do harm to our forest soil profiles by mixing the different horizons together. Epigeic worms do not pose the same threat as they remain in just the O horizon.
Composting Worms in soils/raised beds
Composting worms are the fastest worms for eating organic matter and creating worm castings. If an area is suitable for them, they will improve a soil tremendously.
However, their success depends on how suitable their environment is for them.
If, for example, you have a raised bed that has been filled with compost, leaves, straw, manure etc., the composting worms can do extremely well.
If you have soil with very little organic matter, they will not be in their natural habitat and won't be able to demonstrate their effectiveness nearly as much.
Earthworker Worms in soils/raised beds
"Earthworker" worm's are especially effective at aerating soil through their burrowing/tunneling. They also produce worm castings, but not as fast as composting worms can.
Unfortunately it is difficult to grow endogeic/anecic earthworms in mass like composting worms due to their natural lifestyle, and I would be hesitant to purchase them to apply to your soil anyway because it could be difficult to get them established.
Fortunately, if you're growing in the ground directly, or your raised bed is connected to the ground, you have access to the largest earthworm farm that sells their worms for free!
You can help attract these worms to your soil, and encourage them to reproduce too, in the same way that you can help establish composting worms.
How to Establish a Worm Population in Your Soil
For all kinds of worms the method is the same.
Moisture and Organic matter!
The more aerobically decomposing organic matter in (or on) your soil, the more worms will be attracted to it to it. However worms don't want to risk dehydration, and will choose water over food so make sure there you have moisture to help encourage them too.
Moisture will stay more consistent deeper in the soil, so pay extra attention to moisture levels in the top 3-6 inches for your epigeic/composting worms because they won't go much deeper in soil.
Encouraging Earthworker Worms
For endogeic and anecic earthworms, my recommendation is "If you build it, they will come."
These worms are practically everywhere, just in higher or smaller densities. They will come to you.
I suppose you could go dig up these worms and add them directly as long as you make sure they are able to get underneath the soil and build themselves a burrow, but I really don't think that would be time efficient and I'm not sure what the survival rate would be - but I'm not really an expert on these kinds of worms specifically so maybe it could work well.
Add organic matter, do what you can to keep the soil moist, and you'll see them show up.
Establishing Composting Worms
There are a few ways I do this in a soil.
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Adding them directly by mixing them into the soil and hoping for the best. The more organic matter and moisture, the better their chances and the better the chances that earthworker worms show up.
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Use an in-ground worm composter such as the Sub-Pod. This will give you a defined area that you can optimize for worms, but will still allow for them to travel throughout the rest of your soil as they choose. It'll also be easier for you to check on the general health of the worms because they will be a lot easier to find in the subpod instead of digging through the soil to look for them. The Sub-pod will also help attract earthworker worms already in your soil.
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(What I feel is the best) Start a normal above-ground worm bin to help you learn how to keep worms happy in general, easily harvest pure worm castings directly, and then (the main reason I like this method) you can use that as a renewing source of worms to fuel method 1 or 2. You can also use the worm castings harvested from this bin to add to your soil to improve it's quality.
Whichever method you feel is best for you, you can find the worms or bins that you need here at the shop!