As far as I know, the yields are more than sufficient to support the growers. If the yields are less, and the input costs are insignificant, the farmers still make more money. They have enough acreage in Brazil, I guarantee you that, to find as much as they need.
Let me just add one thing. The reason they can get away with 50 kilos of fertilizer.... It has been discovered that five bacteria live inside the sugar cane, and these bacteria are now known to be involved in producing nitrogen, releasing phosphorous, and all those beneficial impacts that this plant needs to survive in nature.
My son one day asked me, when we were walking in a meadow, “Hey dad, who fertilizes these plants?”--they were about six feet tall--and I said, “Nobody.” He says, “Well how come they grow so big?” So to some extent we have to go back to ecology and see what supports plants out there, because we've gone away from it in a long way.