The soil erosion committee shall not, except as herein otherwise provided, make recommendation to the board for the purchase of the first machinery for said county, or part of the county, unless it has contracts in the form herein required, signed by the landowners, duly witnessed, and approved by the secretary of the committee, covering at least seventy-five hundred acres of land, which, however, shall justify the purchase of as many as three outfits of machinery; and thereafter the subsequent contracts made in said county, or part of the county, shall be allocated to the first machinery purchased therefor, until the whole amount reaches a total of not less than four thousand acres to each outfit. Should the committee thereafter obtain or have additional like contracts in said county, or part of the county, covering another four thousand acres or more, the same shall warrant the purchase of additional machinery by recommendation or election as herein provided; and this shall furnish the guide for all further purchases in said county or part of the county. In exceptional cases, where the board finds as a fact that the amount of work to be done upon the land is much greater than in ordinary cases, the board of supervisors may, with the unanimous consent of the soil erosion committee, reduce the amount of contract acreage necessary for the purchase of one outfit of machinery to not less than two thousand acres, and the amount of contract acreage to be allocated to one outfit of machinery to not less than three thousand acres, and the committee may then recommend purchases of machinery when those conditions are complied with, provided that not less than two outfits are owned or to be purchased by said county or part of the county under this article.