Its not fertiliser instead of hydrogen, hydrogen is a required ingredient for the haber-bosch process. Currently we get the hydrogen from fossil fuels because it is cheaper.

grow crops

In reality that does not happen on its own. Once you figure in the use of land, water and energy to grow crops it's the by far most inefficient way.

Even without the additional costs in farming and just assuming all those plants happily grow on their own, plants are still losing... "Photosynthesis turns roughly 3 percent of incoming sunlight into organic compounds [...] "Artificial photosynthesis"—comprising a PV cell that provides the electricity to split water into hydrogen and oxygen—turns roughly 10 percent of incoming sunlight into usable hydrogen[...]"

Is that as efficient to produce from excess energy? Especially energy that might be highly variable. Hydrogen production can be turned on/off instantaneously and only need a fairly small amount of buffer space for it if you are turning it into something else.