Cal. Gov. Code § 51230
Beginning January 1, 1971, any county or city having a general plan, and until December 31, 1970, any county or city, by resolution, and after a public hearing may establish an agricultural preserve. Notice of the hearing shall be published pursuant to Section 6061, and shall include a legal description, or the assessor's parcel number, of the land which is proposed to be included within the preserve. The preserves shall be established for the purpose of defining the boundaries of those areas within which the city or county will be willing to enter into contracts pursuant to this act. An agricultural preserve shall consist of no less than 100 acres; provided, that in order to meet this requirement two or more parcels may be combined if they are contiguous or if they are in common ownership; and further provided, that in order to meet this requirement land zoned as timberland production pursuant to Chapter 6.7 (commencing with Section 51100) may be taken into account.
A county or city may establish agricultural preserves of less than 100 acres if it finds that smaller preserves are necessary due to the unique characteristics of the agricultural enterprises in the area and that the establishment of preserves of less than 100 acres is consistent with the general plan of the county or city.
An agricultural preserve may contain land other than agricultural land, but the use of any land within the preserve and not under contract shall within two years of the effective date of any contract on land within the preserve be restricted by zoning, including appropriate minimum parcel sizes that are at a minimum consistent with this chapter, in such a way as not to be incompatible with the agricultural use of the land, the use of which is limited by contract in accordance with this chapter.
Failure on the part of the board or council to restrict the use of land within a preserve but not subject to contract shall not be sufficient reason to cancel or otherwise invalidate a contract.
Ca. Gov. Code § 51230