Having lots of experience of the fertilizer traffic on this side of the Irish Sea, the polythene cover would be shrink wrapping which was applied to (A) keep the bags clean, also weather proof for outside storage and (B) hold the bags in stacks on the pallets. Because when the bags were being moved on belts, rollers and auto palletizers in the factories they got sprayed with a silicon lubricant. This meant that when the pallets of bags were moved any distance they promptly fell off the pallets. dont ask how I know but I have hauled 1000's of tons of fertilizer. In later years the bags were also glued together but thats another story! I cannot remember the bulk bags having any kind of shrink wrapping, the product was contained by an inner liner that the outer, think big builders bag, contained which took all the lifting strains. Here endeth!!!!!!!!!!