restrictions.] Except by the express written consent of the commissioner, improvements upon leased state lands held under one lease shall be limited as follows: upon those leased for grazing purposes, fences only, at a cost not exceeding one hundred and fifty dollars ($150) per mile, and necessary corrals, at a cost not exceeding two hundred dollars ($200); upon those leased for agricultural purposes, fences at a cost not exceeding one hundred and fifty dollars ($150) per mile for exterior boundaries and seventy-five dollars ($75) per mile for inside cross-fences; barns, dwellings and all other buildings, at a total cost not exceeding six hundred dollars ($600); wells and irrigation systems at a total cost not exceeding one thousand dollars ($1,000); and for improvements to the land, such as orchards, plowed land, crops, etc., the amount allowed shall be only the amount added to the natural value of the land by such improvement, but in no case to exceed a total of ten dollars ($10.00) per acre for lands actually so improved. For the purposes of this chapter fences and growing crops shall be considered as movable improvements, and all other improvements as permanent improvements.