On the return of such appraisement, the amount of the appraised value of improvements on each tract or parcel shall be divided by the number of acres contained therein, and the result, together with the minimum price per acre of unimproved lands of the same description as established in this chapter, shall be the specific minimum price per acre of such tract or parcel, the improvements upon which shall have been so appraised, until the same shall be changed by a subsequent appraisal.