How many more years can Canadian agriculture produce its rich, annual yield of food? How is the exploding world population going to be fed? The spider-web pattern of the storage bin, above, will soon be spread over a 35,000-ton answer to these questions. "This cone-shaped, 200-foot-diameter bin, at Esterhazy, 120 miles east of Regina, was not built to hold a harvest of golden grain. It is one of three giant storehouses built to contain the first sizeable crop of potash -- vital mineral fertilizer for all growing things -- that will shortly be mined half a mile below the Saskatchewan prairie. This deposit of potash -- biggest in the world -- will help Canadian fanners harvest larger yields from less acreage and end reliance on lower-quality imports. In addition, large exports will be made to the United States and other countries. First-year production from Esterhazy is expected to reach 420,000 tons. This will be trebled the following year, in striking contrast to present annual imports of 150,000 tons -- worth four million dollars. The Esterhazy mine shaft is one of several prairie probes that have battled through the surface formations, acting as mammoth, life-giving roots for world agriculture. The valuable potash they draw to the surface, made into chemical fertilizer, will boost the larders of the earth's hungry millions. And, for Esterhazy, Saskatchewan, the mine development since 1957 has meant changing from just another small, wheat-growing village on the prairie to: Esterhazy, the Potash Town of Canada and, some predict, potash capital of the world.