I have seen worm farms for sale down at the hardware store but have always been discouraged by the price. I know the benefits of using worm poop/tea on your garden, but I'm wondering whether it is actually just better to encourage worms into your soil naturally.
What are the benefits of using a worm farm over encouraging them naturally?
What can you do to attract worms into your garden?
And how do you keep them?
If your soil is in good condition, with plenty of humus content, and it isn't too dry most of the time, worms will be present. Lack of worms in the soil, assuming it's not dry as dust, is a good indicator of soil fertility - few or no worms means it's poor.
Adding humus rich materials regularly (certainly yearly) such as composted animal manures, good garden compost, leaf mould, spent mushroom compost, anything like that, either dug in or as a mulch, will increase the fertility by improving bio diversity - including worms.
There is no point in buying in worms and adding them to the ground; if they're not there in the first place, the soil needs emending, and when the soil is good, the worms will appear anyway.
Bamboo's answer is what I came here to write, excellent advice there.
One other thing worth adding is to avoid doing things that will send worms away: harsh chemicals and overuse of rototilling. (For the reasons that it's hard to live somewhere that the soil is literally killing you, and tilling will tend to shred worms, which is the opposite of your goal.)
A few years ago, the space where my garden is had hardly any worms. After a few years of adding composted horse manure, household compost, and leaves, the worms have arrived in force. I've noticed that in the spots where I added shredded leaves, and one spot where I added some shredded paper mulch, the worm population is extra heavy. I mostly avoid rototilling -- mainly just to kill winter rye cover crops.
