Find an organic farm and get the soil from them, then mix that soil half and half with peat moss. This provides a loose, friable soil where sprout roots can easily take hold. There are a lot of organic farms around the Los Angeles area and you could probably convince one to sell you some soil. That's what I did here in NY before I had my own farm.

I don't believe in vegan soil if there even is such a thing. In Nature, all the animals, birds and insects leave their manure in and on the earth and I believe Nature knows best. I like to follow her teachings. You could add some rock dust to your soil mix if you think it needs some minerals. Usually organic soil that has had compost & seaweed added to it would have minerals, but it can't hurt to add some rock dust, though I haven't actually done that when growing wheatgrass but I have used rock dust on my farm. It is great stuff. It revived some apple trees that were no longer producing apples. I also feed my sprouts with Willard Water one time in their short growth span.

Check out some of the articles on my website's Articles section under farm and garden.

With holiday blessings,