Most California lowlands like the Central Valley sit on alluvial plains where water runoff from the mountains evaporated over millions of years, depositing nutrients dissolved from rock. It’s like the Nile river delta but over time instead of flooding every year. They were also once an ocean, so there’s a very deep layer of fossilized organisms that provide nutrients too.

There’s other reasons like California’s climate supporting a double growing season for many plants but the fertility is what really makes it so economically competitive. Farmers still have to use tons of NPK fertilizer like everyone else but most of the micronutrients are already in the ground so it’s a lot easier to get high yields with low risk and little micro optimization.