You are mistaken. For starters it is important to understand that most Americans, North or South, still lived in a rural setting and worked on a farm. Urban living and industrial work was becoming more and more common, especially in New England, but it was still in the minority on a national basis. The farms that supported the large cities, were almost entirely in the North as were most of the large cities. The lone exception is Baltimore, the only large Southern city. There were some Southern plantations that specialized in food production and contributed to the Northern cities, namely in Southern Maryland. However, Maryland remained in the Union forestalling any potential loss of food. Most Southern plantations in addition specialized in non edible cash crops. The two big exceptions are rice, most prominently grown in South Carolina but also North Carolina and Georgia. The second is sugar, grown primarily in Louisiana but also in Texas. Both of these are more luxury foodstuffs however.