Nairobi, May 11 -- The disruption of the Strait of Hormuz reaches further than most headlines suggest. As crude oil pushed past $100 a barrel for the first time in four years, and urea prices rose by more than a quarter within days of the conflict in Iran, the immediate concern in most capitals was energy, security and inflation.

In Africa, the calculation is different. Nearly a third of the world's traded fertiliser passes through that narrow strip of water, and the sharpest agricultural consequences are projected to arrive in the second half of this year, when farmers across our continent will be making planting decisions for the season ahead.

Africa has lived through this script before. Over the past two decades, numerous external sh...