I'm thinking he's talking about running out of phosphate [0]. Phosphate is a critical plant nutrient and mining it has been essential in increasing yields in the last century - it's literally sustained the population growth. The known supplies are running low, and it washes out of the soil on an annual basis so needs to be replenished to keep up yields — or we need to find other sustainable sources. The 50 years correlates with other (very) rough estimates of remaining supplies available to use for the current agricultural paradigm.
The overarching problem with the statement is soil is so different everywhere. And managed differently everywhere. You can't just make a blanket statement. If I was going to make a blanket statement I would be more concerned with erosion than nutrition because soil is an ecosystem that can be repaired. But if it is gone, then that's a different story.
[0] https://www.discovermagazine.com/planet-earth/phosphorus-is-...