At the double digging class last Saturday, Tom gave a link to the Center for Agroecology and Sustainable Food Systems at the University of California Santa Cruz. I tried it, but it didn't come up for me. I did a search for the center under it's full name and got this link:
And here is the section he said to look at specifically:
We'll be giving this class again in late spring or early summer, when we resume classes in May. Keep an eye on the events calendar at the Natural Gardener website. We'll post dates and times there.