The manufacturer, importer or agent of any liming material shall pay annually to the director of agriculture on or before December 31 a license fee of $20.00 for each liming material he offers for sale in this state. Whenever the manufacturer or importer shall have paid this license fee his agents shall not be required to do so. All vendors of liming materials shall keep on file subject to inspection by any authorized agent of the director of agriculture for a period of 1 year all invoices, freight bills, truckers' receipts, way bills and similar shipping data pertaining to liming materials that would establish date and origin of the shipment.