Full power and authority are hereby given and granted to the general court to prescribe, for the purpose of developing and conserving agricultural or horticultural lands, that such lands shall be valued, for the purpose of taxation, according to their agricultural or horticultural uses; provided, however, that no parcel of land which is less than five acres in area or which has not been actively devoted to agricultural or horticultural uses for the two years preceding the tax year shall be valued at less than fair market value under this article.