There are various soil types found all over California. American Organics explains the process of soil development as a combination of environmental factors: weathered rock, volcanic ash and plant residues all combine to develop a particular soil type. Unless your soil already contains lots of organic matter, chances are you will have to amend it prior to beginning a garden. Contact your local county extension office for information on soil testing services.
Sandy soils are found near and around the mountain foothills, along rivers and streams, and in certain coastal areas. Sand is light and free draining, but has very little organic content and holds water poorly. Sandy soils will have to be amended with organic material, like compost. Its composition is 80 percent to 100 percent sand, 0 percent to 10 percent silt and 0 percent to 10 percent clay.
Loam can be found in valleys and flat areas, such as flood plains surrounding rivers and streams. The term loam refers to soil with a favorable mix of sand and clay. It is relatively fertile and holds moisture well, but loam soils may require the addition of organic matter. Loam's composition is 25 percent to 50 percent sand, 30 percent to 50 percent silt and 10 percent to 30 percent clay by volume.
Clay soils are commonly found in urban areas where soil fills have been used "to establish grade in subdivisions and developments," according to Rain.org. The U.S. Geological Survey reports that because of its low mineral content, shifty clay soils contribute to high levels of shaking during earthquakes. Clay is not free draining and allows large amounts of runoff. It is also heavy and difficult to work when dry.