Iowa Code § 426.3
The agricultural land credit fund shall be apportioned each year in the manner hereinafter provided so as to give a credit against the tax on each tract of agricultural lands within the several school districts of the state in which the levy for the general school fund exceeds five dollars and forty cents per thousand dollars of assessed value; the amount of such credit on each tract of such lands shall be the amount the tax levied for the general school fund exceeds the amount of tax which would be levied on said tract of such lands were the levy for the general school fund five dollars and forty cents per thousand dollars of assessed value for the previous year, except in the case of a deficiency in the agricultural land credit fund to pay said credits in full, in which case the credit on each eligible tract of such lands in the state shall be proportionate and shall be applied as hereinafter provided.
Iowa Code § 426.3