It kind of depends on what your soil needs, and you're not really going to know that without a soil test.

SoCal usually has sandy soil that's rather low in nitrogen and is somewhat alkaline. If you could find some composted manure, it would provide some pretty good nutrients. Got any horse people around you? (Cow manure is better, but most dairies were long gone even when I lived there). Composted manure usually has fewer weeds.

Your compost doesn't have to be completely finished for you to use it. It will continue breaking down in the soil.

Don't use lime or ashes in your soil, or they will make it more alkaline, which you don't want.

Kelp meal has a lot of micronutrients in it, so that would probably be of benefit, too. Go by what the directions say.

It's really hard to add too much organic matter to soils there, as the heat breaks it down fast, and the nutrients wash through the sandy soil.

Rototill as little as possible. Too much 'fluffing' of the soil destroys the soil microbes that are so important to healthy soil. The best thing to do to open the soil is to use a spading fork. Push the fork down into the soil, then rock it back and forth a bit. Then move the fork back a few inches and repeat. Don't turn the soil over if you can avoid it. If you put manure, your compost and the sprinkling of kelp on top of the soil and then use the fork, as you rock it back and forth some of the nutrients will fall into the cracks. More of the nutrients will wash down to the plant roots when you water. Don't mix your manure and compost too deep, just in the top 6" or so is enough.

The top of the soil dries out quickly there, so a top mulch around the plants would be of great benefit, and will help keep the weeds down, too. Add a deeper layer as the plants get taller, once the plants are tall enough, eight inches of protective mulch is not too much. Add more as it breaks down and gets thinner. Straw or dried grass clippings make fine mulch. (Dry the grass clippings on your driveway, turn and fluff a couple of times a day, then apply to the top of the soil.) Don't use fresh, wet clippings or they'll stink!

Don't overwater. Stick your fingers down into the soil near the roots. If the soil is still damp, that's fine. Too much water in sandy soil just flushes the valuable nutrients deep into the soil, out of reach of the plants.

Avoid the chemical fertilizers. If you want to use an organic fertilizer, that's okay (it should say OMRI somewhere on the label). But the harsh, microbe-killing chemical fertilizers do nothing but harm to the soil. That's one of the reasons CA is having such problems in the drought. Organic matter holds moisture in the soil, chemical fertilizers don't.

Have fun!

Sue, formerly of Azusa, CA

Sue said everything I was going to. Excellent advise there. I work at an organic-only nursery and that's the advice I give, as does my boss (John Dromgoole). Compost, compost, compost is the mantra at work ~ and it really does work. It's the original "slow release" fertilizer, releasing nutrients over the course of months as it breaks down.

Fertilizing used to stymie me, too, Molly. Then I decided to just amend with lots of compost to get the benefits of moisture retention as well as good drainage from more friable soil, then sit back to watch the plants and let them tell me what they needed. I figured that it was like us ~ if we were force fed a lot of unneeded food, we'd get fat and unproductive, while if we were starved we'd show signs of weakness. Sure enough, that's what I saw in my plants. Too much nitrogen made my tomatoes big and full of gorgeous green leaves, but not many fruit. Too little made them grow weakly (an aside ~ if I'd added enough compost, I rarely saw this last one).

Now, years later, I topdress with an inch or two of composted manure when the plants are a few inches tall (I use it as mulch) and again near harvest time. I do this for all seasons ~ spring growing season, fall garden and winter garden. I usually don't add anything else and still get plenty of veggies, but occasionally I drench the soil with fish emulsion (for the nitrogen) and/or seaweed (for the micronutrients), all depending on what my plants tell me they need. Go Googling for pictures of nitrogen deficiency and micronutrient deficiency and you'll be able to recognize them easier later.

Something I plan to try this year is adding molasses. This helps the beneficial soil bacteria thrive, and I'm thinking if they thrive, they'll break down organic matter better, thereby fertilizing my plants even better and helping them through stressful times. I plan to do it when I plant and again as the heat sets in.

I would turn into the soil a couple of bags of composted cow manure.

Here is a link that might be useful: Win Free Garden Stuff

I am so thankful for all the great advice! I have a much clearer picture of what to do now. I just love gardening so much and I am so grateful for this site which is always a friendly place for some shop talk.
Molly

Molly,
Every year I put down at least 2" of composted manure prior to planting. It's amazing at how much gets used during the year. Luckily, it's free here since we have so many horse farms near us. My neighbor relies on chemical fertilizer and he has some luck when it's a rainy year. Last year was aparently a bad year for tomatoes around here but I didn't notice anything other than a huge crop. We still have sauce canned from last year. Bottom line...the more organic material you can work into you soil, the better.
Rick