In order to be shown on FMMP’s Important Farmland Maps as Prime Farmland and Farmland of Statewide Importance, land must meet both of the following criteria:
Has been used for irrigated agricultural production at some time during the four years prior to the Important Farmland Map date. Irrigated land use is determined by FMMP staff by analyzing current aerial photos, local comment letters, and related GIS data, supplemented with field verification.
-- AND --
The soil must meet the physical and chemical criteria for Prime Farmland or Farmland of Statewide Importance as determined by the USDA Natural Resources Conservation Service (NRCS). NRCS compiles lists of which soils in each survey area meet the quality criteria. Factors considered in qualification of a soil by NRCS include:
Water moisture regimes, available water capacity, and developed irrigation water supply
Soil temperature range
Soil sodium content
Flooding (uncontrolled runoff from natural precipitation)
Rock fragment content
Soil rooting depth
The Prime Farmland and Farmland of Statewide Importance soil lists for each county are available on the FMMP web site in PDF format. Any questions specific to soil qualifications for Prime Farmland or Farmland of Statewide Importance lists should be directed to your local NRCS office. For questions on compilation of the Important Farmland Maps, contact FMMP. The NRCS Web Soil Survey hosts original soil data in GIS format--see links below.
Other 'Prime' Land Definitions
The term 'Prime' as it refers to rating for agricultural use has two meanings in California. FMMP determines the location and extent of 'Prime Farmland' as described above; while under the state's Williamson Act, land may be enrolled under the 'Prime Agricultural Land' designation if it meets certain economic or production criteria.
NRCS link to download soils GIS data | NRCS California Home Page