Fertilizer supplies to Southern Hemisphere agricultural powerhouses like Brazil and Argentina are at risk as a fragile ceasefire between the US and Iran frays just as planting season nears. The flow of fertilizer ships entering the Persian Gulf — a region that accounts for about a third of global urea exports — has slowed to a trickle since late June as tensions began to rise again with a spate of Iranian attacks on vessels transiting the Strait of Hormuz. Just four empty vessels booked to load fertilizer cargoes have transited the waterway into the Gulf since June 30, tanker tracking data compiled by Kpler and Bloomberg show. Two of those ships have recently loaded cargo but are yet to depart.