Bay Area clay soil is prone to compaction.
Healthy soil is loose enough to allow roots and worms to move around freely, while still providing support and structure. Unlike sand (which has its own problems), compacted soil has too few macropores and micropores (larger and smaller spaces) between soil particles. These spaces are needed to hold air, water, and nutrients for plant roots.
Compacted soil can prevent water from moving into the soil (infiltration), through the soil (permeability), and out of the soil (drainage). Standing water can drown plants and create mosquito habitat. It can also make life difficult for tender new seedlings trying to get a healthy start by reducing nutrient uptake and poorly anchoring plants to the ground. Soil compaction hurts matures plants, as well, by reducing nitrogen levels in the soil, as well as other nutrients.
What causes soil compaction?
Every step you take presses down on the soil beneath your foot. Healthy soil can spring back. Soil that is walked on too frequently loses that ability and it becomes compacted. Other common causes of soil compaction include:
Plants that counteract compaction
Deep taproots can help break up compacted soil. Put these plants to work for you, rather than compounding the problem with further digging. Adding these plants to your landscape can help reduce compaction and improve soil structure:
Other tips to reduce soil compaction
Finally, COMPOST! COMPOST! COMPOST!
I hope this information inspires you to grow more of your own food. You can ask your garden questions on my Home page.