A petition for exclusion of land from a district shall be approved only if the exclusion of the land does not reduce the remaining area of the district to less than an area of 20,000 acres and not less than $1 million assessed valuation of taxable property according to the most recent assessment roll prepared by the county assessor. [Amended by 1955 c.227 § 10; 1971 c.727 § 107]