There are more living organisms in a yard of healthy soil than all the humans on earth. Soil is nature transforming through the process of weathering the rocks and minerals on the earth's surface. This process is different in every place, because the rocks and minerals are not identical, and the weathering process is anything but a straight line process. Which explains why there are so many different soil types, even in a single vineyard, let alone an appellation.
There are more soil types in the Russian River Valley than all of France. This diversity is key to the appellation, and why so many stellar wines are produced here. Some of the familiar types of soil in the appellation are:
Yolo - well-drained silt loams to silty clay
Goldridge - sand/silt clay, quite well drained
Arbuckle - older alluvial bench
Laughlin - sandy clay loam on top of sandstone and shale
Sobrante - cobbly, clay loam
The job of soil is to provide water and nutrients to vines. An important factor to a vine is drainage—how well does it retain water (retention) or allow water to drain (permeability). Too much drainage results in stunted growth, and too little drainage gives wet feet and potentially too much growth. Clay soils tend to hold water, while sandy soils favor drainage.