The world is already over-populated. The only reason we can sustain such a large population is due to artificial fertilisers massively improving agricultural productivity. It has been estimated that if all current agriculture sources were to make the transition to purely organic food, ie, no artificial fertilisers, then at least 2 billion people would starve.
A lot of fertilisers are phosphorus based, which we happen to be running out of, in terms of natural sources. Phosphorous is running low and farm yields are falling again. Russia and China hold the largest natural sources of phosphorous, and in the future, will hold the keys to global agriculture.