In all counties of the second and third class, having a population of fifty thousand or more and an assessed valuation of seventy million dollars or more, the county highway engineer shall be a resident of the state of Missouri, skilled and experienced in general road, bridge and culvert work, and authorized to practice engineering under the laws of this state providing for and requiring the registration of professional engineers. He shall be active and diligent in the discharge of his duties and personally attend to them.