Someone wrote today asking:

i recently installed a 16x32 foot garden with 9inches of dirt that came from an old hay field (high clay content, sticks to shoes when wet).  trying to find best worm to place in soil, not in bins.  what kind would you recommend and where to purchase up to a pound of them?  will be out of town for a week but look forward to an answer and getting the right worms to working.  Live in san antonio, tx.

My answer:

Oh yes, the San Antonio garden, it's not too user friendly for happy worms. Great shock to me to find myself living in a place where I could barely turn the soil, much less, once I did, could find not a single worm in it. Back in the DC/MD/VA area, worms are just something that you find in the ground. Not so here. Here you have to work for worms. So, what to do, and what type to get.

There are many species ranging in size, color, soil preference, and life expectancy, but they are all beneficial.

There is no one worm type or species that is generally "best" for building soil. While worm people break them
into categories, the defining lines between species categories are not firm, but are rather blurry in many cases.  My advise is get as big a variety as
possible.  Unfortunately, one of the easiest to get a hold of, the Red Wriggler, is probably the least effective because they are used to growing in compost bins
and may not be up to the harsher realities of Texas soil. Generally, night crawlers are good.

Earthworms need moist soil (not just clay) to survive. Our dry Texas summers are lethal to many, especially those that happen to end up in the lawn
areas that receive full exposure to the merciless summertime sun. It is no wonder that people have so many problems with their lawns - they are
inhospitable to the creatures that do the most to benefit the soil, and without healthy soil you won't have healthy plants.

During periods of drought, the earthworms recede deeply into their burrows, which can be six feet deep. 
The mucous from their exterior helps to provide structure to their burrows so that they do not collapse.

Rototilling the soil can be harmful to earthworms. Not so much from the machine, as they flee from vibration, but
mostly because their food supply is depleted rapidly when exposed to oxygen. If you rototill, you should immediately cover the area with partially finished
compost or waterlogged leaves.

The most common garden earthworm, the Night Crawler (Lumbricus Terristris) is not native to North America but this is
one case of where the accidental introduction of a foreign species has been beneficial.

Garden earthworms are not to be confused with composting worms that can be kept in containers. The Night Crawler is a burrower, and will
kill itself trying to burrow in a worm bin.  Composting worms (Red Wrigglers) cannot tolerate temperature extremes, and though they can survive inside a
compost pile, will perish if left to their own in garden soil. 

Worms are hermaphroditic, meaning that they possess both sets of sex organs. However, they
still need to mate with another worm in order to produce offspring. The raised band that encircles the worm is actually a carrying case for its eggs. Once the
eggs are formed, the band migrates along the worm's body and it is then shed and the eggs left to hatch.

A few more facts about our subterranean allies...     

It takes worms from all categories working in all areas of soil to maximize soil health.  Determining what kind is akin to trying to
determine which came first, the chicken or the egg.  Endogeic worms feed in soil that has decayed organic material worked into the matrix; epigeic and anecic
species decay that surface debris so it can be worked into the soil. Which should come first?  That is a question only the worms can answer.  For this
reason, adopting the "if you build it they will come" approach is usually the best idea. Create conditions suitable to worm activity; i.e. keep the soil well
mulched and moist, avoid chemicals and tilling where possible. 

So the important thing is, mix it up if you can afford to.  And create soil conducive to their (and your plant's) health.  They'll do the rest.
Maintaining a healthy population of worms is easy. Just use common sense and see that they have a moist, well-mulched habitat with plenty of decaying matter. Take care
when turning the compost pile. A pile interior might contain dozens of worms, so try to relocate them to an area where they will have a good chance in which to
burrow deeply. Worms are resilient, have a high reproduction rate, and can survive without us.  But with just a little help they can be made to flourish. 
And you can forget about ever needing a rototiller if you get them going.  So, mulch, mulch, mulch, get roots in the ground (plant roots encourage soil
biology), avoid disturbing the soil as much as is practical, and let the worms do what worms do!

Worm sites:

For Night Crawlers (for the garden)

Shady Creek worms

DQ Bait Shop

And should you ever decide you need Red Wrigglers (for composting)

4SeasonGreenhouse.com
Composter HQ

Interesting Worm facts:
Earthworms..
- have been kept alive for 6 years, but in the wild probably live two years at the most.
- do not have lungs, breathe through their skin, and can live under water for a while.
- do not come to the surface during rain to escape drowning, but rather to find a mate since their mobility is much better on wet ground.
- lack eyes but are light sensitive.
- can detect the motion of a robin (that can hear the earthworm in its burrow).
- may survive being frozen if the freeze is not too rapid.

Hope that helps!