There is appropriated for each fiscal year beginning with the fiscal year ending June 30, 1936 an amount equal to 30 per centum of the gross receipts from duties collected under the customs laws during the period January 1 to December 31, both inclusive, preceding the beginning of each such fiscal year. Such sums shall be maintained in a separate fund and shall be used by the Secretary of Agriculture only to (1) encourage the exportation of agricultural commodities and products thereof by the payment of benefits in connection with the exportation thereof or of indemnities for losses incurred in connection with such exportation or by payments to producers in connection with the production of that part of any agricultural commodity required for domestic consumption; (2) encourage the domestic consumption of such commodities or products by diverting them, by the payment of benefits or indemnities or by other means, from the normal channels of trade and commerce or by increasing their utilization through benefits, indemnities, donations or by other means, among persons in low income groups as determined by the Secretary of Agriculture; and (3) reestablish farmers’ purchasing power by making payments in connection with the normal production of any agricultural commodity for domestic consumption. Determinations by the Secretary as to what constitutes diversion and what constitutes normal channels of trade and commerce and what constitutes normal production for domestic consumption shall be final. The sums appropriated under this section shall be expended for such one or more of the above-specified purposes, and at such times, in such manner, and in such amounts as the Secretary of Agriculture finds will effectuate substantial accomplishment of any one or more of the purposes of this section. Notwithstanding any other provision of this section, the amount that may be devoted, during any fiscal year after June 30, 1939, to any one agricultural commodity or the products thereof in such fiscal year, shall not exceed 25 per centum of the funds available under this section for such fiscal year. The sums appropriated under this section shall be devoted principally to perishable nonbasic agricultural commodities (other than those receiving price support under section 1446 of this title) and their products. The sums appropriated under this section shall, notwithstanding the provisions of any other law, continue to remain available for the purposes of this section until expended; but any excess of the amount remaining unexpended at the end of any fiscal year over $500,000,000 shall, in the same manner as though it had been appropriated for the service of such fiscal year, be subject to the provisions of section 3690 1