On and after June 13, 1958 no conservation reserve contract shall be entered into which provides for (1) payments for conservation practices in excess of the average rate for comparable practices under the environmental quality incentives program established under subchapter A of chapter 4 of subtitle D of title XII of the Food Security Act of 1985 [16 U.S.C. 3839aa et seq.], or (2) annual rental payments in excess of 20 per cent of the value of the land placed under contract, such value to be determined without regard to physical improvements thereon or geographic location thereof. In determining the value of the land for this purpose, the county committee shall take into consideration the estimate of the landowner or operator as to the value of such land as well as his certificate as to the production history and productivity of such land.