Section 5A. In the case of a person engaged in agriculture, as hereinafter defined, and having a permanent hired work force of more than four agricultural workers who are not members of his family, the provisions of section five shall apply; provided that only the employer unit shall be deemed appropriate for collective bargaining purposes; and provided further that nothing in this section nor in said section five shall be construed as constituting authority for any action or proceeding to nullify, amend or otherwise modify any contract or agreement, or any provision thereof, which is reached by any such person for the seasonal employment of agricultural workers with the official sanction either of the government of any territorial possession of the United States or of the United States department of labor. As used in this section, the term ''agriculture'' includes horticulture, floriculture and any other commercial enterprise involving the production of food or fiber.