The rate, as determined by the board, shall be such as will produce, after due allowance for delinquency, the amount determined as necessary to be raised by taxation on the secured roll. On or before September 1st of each year the board shall fix the rate, composed of the number of cents or fraction thereof for each one hundred dollars ($100) of assessed valuation of land exclusive of improvements and mineral rights, such as will produce, after due allowance for delinquency, the amount determined as necessary to be raised by taxation on the secured roll.