A fertilizer supply shock is imminent for US farmers as CF Industries Holdings, Inc. warned Thursday that rail shipments of crop nutrients would be reduced to top agricultural states, which couldn’t come at the worst time as the Northern Hemisphere spring planting season is underway. The world’s largest fertilizer company said Union Pacific had hit it with railroad-mandated shipping reductions that would impact nitrogen fertilizers such as urea and urea ammonium nitrate shipments to Iowa, Illinois, Kansas, Nebraska, Texas, and California. Union Pacific told CF Industries without advance notice to reduce the volume of private cars on its railroad immediately....