INTERNATIONAL FERTILIZER INDUSTRY ASSOCIATION. PHOSPHORICACID, TRIPLE SUPERPHOSPHATE AND AMMONIUM PHOSPHATE PLANTS1988 SURVEY. AUGUST 1988, 39PP.BRITISH SULPHUR CORPORATION. WORLD FERTILIZER ATLAS, EIGHTHED. APRIL 1987, 121PP.PHOSPHORUS & POTASSIUM, NO. 187, P. 7 (SEPT-OCT 1993).