Fertiliser...
Much of modern farming's productivity depends on synthetic fertilisers. In the absence of nitrogen fixation, there would have to be a lot more crop rotation with legumes, alfalfa etc, plus treatment and distribution of sewage slurry...
IIRC, much hard-rock tunnelling and mining uses an inexpensive, nitrogen-rich explosive slurry instead of solid dynamite. Working practices would have to be different...
Uh, check out guano mining and guano islands, which were considered a 'strategic resource' in the late 19th century, as their deposits, up to 150 feet thick, were a convenient source of nitrates...
There were nearly wars fought over access to such islands' vital resource, and USA hastily annexed assorted Pacific islands: Had synthetic nitrogen fixation NOT come along, those islands would have been scraped bare and a terrible shortage would follow...
Uh, one butterfly might be that the USSR's efforts to grow cotton across vast swathes of land near the Aral Sea might have failed before so much of the area became a polluted, desertified waste...