It is hereby stated to be the policy of the State of California to encourage and protect the right of agricultural employees to full freedom of association, self-organization, and designation of representatives of their own choosing, to negotiate the terms and conditions of their employment, and to be free from the interference, restraint, or coercion of employers of labor, or their agents, in the designation of such representatives or in self-organization or in other concerted activities for the purpose of collective bargaining or other mutual aid or protection. For this purpose this part is adopted to provide for collective-bargaining rights for agricultural employees.