For the purpose of state, county, and municipal taxes, merchandise held by merchants and the raw material, merchandise, finished products, tools, machinery and appliances used or kept on hand by manufacturers shall constitute a class separate and distinct by itself. All cities in this state having a population of over three hundred thousand inhabitants are authorized to levy for local purposes a less ad valorem rate of taxation than that levied by them on real estate or other property for the same purpose, and such reductions may from time to time be arranged to apply on both or either the tax rate for payments of valid indebtedness or the tax rate for city purposes. All such cities, for city and local purposes, are hereby authorized to license, tax and regulate the occupation of merchants and manufacturers, and may graduate the amount of annual license imposed upon a merchant or manufacturer in proportion to the sales made by such merchant or manufacturer during the year next preceding any fixed date; provided, however, that no such license, tax, or regulation shall apply to grain and other agricultural crops in an unmanufactured condition, as defined in section 137.010, which are subject to assessment, valuation, and taxation under subsection 3 of section 137.115.