I found many years ago that tilling organic matter into sand was not a very good idea since that encouraged the soil bacteria, apparently, to digest that OM so fast that none was left to build up they humus level. There was no diggerence in plant growth if the organic matter was layed on the soil as mulch or tilled in as far as plant growth was concerned but if the OM was put on as mulch the humus level in the soil slowly built up.

So should I mix it with a corse mulch or something else? or use as is.

Sandy soils do benefit from adding OM but you just need to add it in larger quantities and with greater frequency than you do in finer particled, clay soils. The pore space in the soil is larger, resulting in more oxygen present, which speeds up the decompostion process. This also causes the soil to drain very rapidly and adding OM will improve moisture retention.

I'd look for some other compost, or composted manure to add. Or peat moss. You need at least 4-5 inches incorporated each season - more wouldn't hurt. I'd not stick just with the mushroom compost - it tends towards alkalinity and if you are located in California, that is already an issue with many native soils as well as the irrigation water. Peat moss is very slow to decompose any further, absorbs and holds a great deal of water and will help to acidify the soil and neutralize the effects of alkaline irrigation water. But its nutrient load is very low, so add some other compost product as well. Any municipal or commercial composting facilities would be the place to start looking.

Mulch after planting to further moisture retention. And plant off-season with a cover crop or green manure to continue to increase the soil's level of SOM (stable organic matter). In a few years you should have an organically rich, moisture retentive, productive soil and you can cut back on the amount of OM needed each season.

Thanks for the response. Very helpful.