from the rules:
The player may place a Bamboo section on the irrigated plot of his choice, up to a limit of four sections per plot.
Fertilizer increases the growth of bamboo on its plot. Each time the bamboo grows, two sections are added instead of one (subject to the
usual 4 sections maximum).
Seems pretty clear to me.
Not quite as clear as it would be if the rain rule said to grow a bamboo section rather than place it - in some games, the verb being different would mean the bonus didn't apply...
I'm firmly in the "two sections" camp, but I can see an argument the other way.