@urlyman Er, this sounds suspicious to me.
There's a thousand-year-old agricultural trick, crop rotation. Considering that we now know how it works, particularly about the nitrogen-fixing ability of legumes, and that last year's nutrients won't be depleted from soil in a single year, wouldn't many farmers be able to compensate for fertiliser deficit lasting for one or two years?
=> My hunch is, the pressure would be on (some) farmers for the first year, and indirectly, on agricultural social security systems. We could see an upwards forcing on two- and three-year staple food commodity futures right away, which can leak into present because it would make profitable to hoard and hold food, but "the food futures are up" is a common pattern, and most governments have both the experience, the tools, and the financial capability of handling it for a year or two. I'd predict the systemic manifestations of this chain of events to first lead to increased political instability in the countries whose governments are already marginal, not a substantial sharp increase in global food prices.