Depending on where you are located in Los Angeles, you might consider having an Agricultural Suitability soils test done to determine heavy metals levels (particularly lead) in your soils. This will also determine if your soil is suitable for growing produce. Mainly its important to know about lead levels because certain plants will uptake and store higher levels of lead than others (leafy greens). It will also tell you what pH your soil is at and macro/micro nutrient levels. This test typically runs about $150.00 from a private soils lab (such as Soil and Plant Lab in Orange). Occasionally your local extension can help you out. Contact Los Angeles County Agricultural Extension in South Gate and ask how to proceed.
I am not sure who the extension agent in LA is that would handle that, but, if you are on the West side, you can try to reach Ventura Extension, I think Dr. Ben Faber, may be helpful.