Tex. Const. art. VIII, § 1-d-1

(a) To promote the preservation of open-space land, the legislature shall provide by general law for taxation of open-space land devoted to farm, ranch, or wildlife management purposes on the basis of its productive capacity and may provide by general law for taxation of open-space land devoted to timber production on the basis of its productive capacity. The legislature by general law may provide eligibility limitations under this section and may impose sanctions in furtherance of the taxation policy of this section.

(b) If a property owner qualifies his land for designation for agricultural use under Section 1-d of this article, the land is subject to the provisions of Section 1-d for the year in which the designation is effective and is not subject to a law enacted under this Section 1-d-1 in that year.