Higher fertilizer prices are making the world’s food supply more expensive and less abundant, as farmers skimp on nutrients for their crops and get lower yields. While the ripples will be felt by grocery shoppers in wealthy countries, the squeeze on food supplies will land hardest on families in poorer countries. It could hardly come at a worse time: The U.N. Food and Agriculture Organization said last week that its world food-price index in March reached the highest level since it started in 1990. The fertilizer crunch threatens to further limit worldwide food supplies, already constrained by the disruption of crucial grain shipments from Ukraine and Russia. The loss of those affordable supplies of wheat, barley and other grains raises the prospect of food shortages and political instability in Middle Eastern, African and some Asian countries where millions rely on subsidized bread and cheap noodles.