I've read some of his material, and have tried to find some independent sources regarding their demographics, agriculture, and imports. Those sources (via naive online search, filtered by bias) seem to line up pretty well: China's population is rapidly aging and their pyramid has inverted, China subsists on grains and pork but they must import corn to feed the hogs and struggle with outbreaks of ASF. Extreme weather (drought, rain) is ravaging their harvest. The war in Ukraine and the Russian sanctions have pushed up global fertilizer costs to which China -- the top producer -- has responded by implementing strict quotas on phosphate exports, a strange choice.
"As the top-producing country, China puts out 90 million MT annually for 30 percent of global supply." -- https://investingnews.com/phosphate-outlook-2022/
So, I'll give you that Peter Zeihan might be trying to sell his books, but it's not like there's zero corroborating sources.