It shall be the duty of every assessor and board, in estimating and determining the value of lands for the purpose of taxation, to consider and give due weight to every element and factor affecting the market value thereof, including its location with reference to roads and streets and the location of roads and streets thereon or over the same, and to take into consideration a reduction in the acreage of each tract or lot sufficient to cover the amount of land actually used for any improved public highway and the reduction in area of land caused thereby. It shall be the duty of every assessor and board, in estimating and determining the value of lands for the purpose of taxation, to consider and give due weight to lands which are comparable in character, quality, and location, to the end that all lands similarly located and improved will be assessed upon a uniform basis and without discrimination and, for agricultural lands, to consider and give recognition to its earning potential as measured by its free market rental rate.
When mineral, clay, or gravel deposits exist on a property, and their extent, quality, and costs of extraction are sufficiently well known so as to influence market value, such deposits shall be recognized in valuing the property; except for mineral and energy-resource deposits which are subject to taxation under section 298.015 , and except for taconite and iron-sulphide deposits which are exempt from the general property tax under section 298.25 .