California's farm and grazing lands decreased by 176,000 acres (275 square miles) from mid-2004 through mid-2006, according to the state Department of Conservation. Most of the agricultural land was lost to urban development (102,000 acres) and a little more than half of that urbanization occurred in only five counties – Riverside, Sacramento, San Bernardino, Kern and San Diego. Nearly all urbanization occurred in Southern California and the Central Valley.

"Housing developments were the most frequent and largest category of newly urbanized land," according to the recently released California Farmland Conservation Report 2004-2006. "Most of the increase was associated with single-family homes located at the periphery of existing cities, and to a lesser degree condominium and apartment complexes. Individual subdivisions ranged up to 300 acres in size."

The overall amount of farmland conversion increased by about 8,000 acres from the previous two-year period. A total of 81,000 acres of prime farmland were lost to urban development or other changes, such as idling, dry cropping, confined animal facilities and rural residential development, during the 2004-06 period. That's the greatest decrease in prime farmland since the state started the farmland mapping and monitoring program (FMMP) in 1984. In Stanislaus County, 81% of farmland lost to urban development during the 2004-06 period was prime farmland.

"During the 11 biennial reporting cycles since FMMP was established, more than 1.2 million acres of agricultural land in California were converted to nonagricultural purposes. Nearly 79% of this land was urbanized," the report concluded.

The report and detailed county-level information about farmland is available from the Department of Conservation website.