I don't know about California, but here the county agents can help with soil samples and they aren't very expensive. They come back with specific instructions to what is needed, based on what you are growing. You most likely amend the soil with compost or some other organic materials before you plant, so I'd test it after that was added, however it takes a while for that to get broken down, so I'd take a few samples from the beds that have been around a while and see what's up. I've never used the soil test kits. I don't think they test for anything but Nitrogen, Phosphorus, and Potassium Oxide, and you don't know about some of the trace minerals. (at least that's what I think, maybe someone who has used them should weigh in on this. )
