Which cation is not considered dominant in California soils?

In the context of California soils, the dominant cations are typically calcium (Ca), magnesium (Mg), and sodium (Na). These cations play significant roles in soil chemistry, structure, and fertility. Calcium is particularly dominant due to the prevalence of limestone and clay minerals in many Californian soils, while magnesium and sodium also commonly occur in significant amounts depending on the region and soil type.

Iron (Fe), while essential for plant growth and a crucial component of many biochemical processes, is usually found in much lower concentrations compared to the other cations mentioned. It does not dominate the soil cation exchange capacity or significantly influence soil properties in the way that calcium, magnesium, and sodium do. This makes iron the correct answer to the question, as it is not considered dominant in California soils like the other cations are.