Southern leaders/wealthy people rarely invested a lot of money into industry and railroads because they could make more money investing in land and slaves. Northerners could make more money investing in industry and railroads. So that's what they did. What that meant is that the planter class in the South tended to be very wealthy, but all most all their wealth was in the form of land and slaves. They didn't have a lot of cash. They didn't tend to invest in businesses. In the North there were increasingly just more people as a whole. And the average productivity was higher, because they were working with better/more tools. There were fewer of the really great personal fortunes in the North at that time. But the total productive capacity was greater. The output of the slave laborer was high, and the produce was valuable, and often sold for high prices. Prices did tend to go up and down, however. But the rate of productivity was essentially stagnant, because those slaves couldn't get any better at their jobs.
Actually, the latter is inaccurate. A recent study showed that slave owners managed to increase productivity on plantations. Simply by making the slaves work harder. Which is, after all, not that hard to do, when your 'employees' have zero rights.