How long that poison would stay active in the soil depends on what it was, although most should be washed out of the soil by now (and in to our drinking water). The only way to know for sure would be by having that soil tested for the poison.

That is a great idea. It would also probably help me determine any needed amendments. I'm in the LA area. Any ideas on where to have the soil tested? Is there an on line resource directory maybe?

The only way a lab will test a soil for "poisons" is if the customer states which "poison(s)" to test for.

Each test (one "poison" each) is costly.

I agree with lazy gardens - you are likely overthinking this. Even a certified organic farm only requires 3 years of chemical free activity before certification.

But getting a generalized soil test is a very good idea. It will provide a starting point for understanding your soil and provide direction on how and what to amend with, if amending is necessary. Like many other states, CA no longer provides testing services through their extension service - you need to send samples to a professional testing lab. This is the listing of labs suggested for homeowners by the CA Master Gardener program: Soil Testing Labs

Very helpful feedback. I just scored a large load of free dirt - woot! Thanks everyone!

yay....common sense and basic knowledge prevails! Congrats on the soil, jkm, and I wish you luck with the weed seeds (eeeek)