This is a solved problem since the invention of the bucket and pointy stick, when “fertiliser” meant “manure”.
> With CO2 being 50% higher that means you'd already have to fertilize 50% more to get the same Nitrogen to CO2 ratio as preindustrial levels.
Why do you want to match the air rather than the plant’s actual growth? Plants are limited by a lot of different factors, so while CO2 helps a bit, doubling it doesn’t double the nitrogen use all by itself.
> a farmer didn't follow the fertilization limits
While this is a real issue, there are many solutions, and worrying about CO2 altering crop yields isn’t one of them.
An unrelated issue. That’s important. I have no reason to expect it to get worse. What runs off the fields is necessarily not used by crops.
> And you can bet farmers are already maxing that out as much as they can on an industrial scale.
I doubt that.
All industries, agriculture included, are incentivised to maximise profits. Optimising profit sure isn’t environmentally friendly, but it also doesn’t mean treating fertiliser as a magic potion.
A productivity optimised farm looks like this: https://farmingaquaponics.com/aeroponics-vs-hydroponics/
That’s pointlessly expensive right now for most crops, so those are fairly limited. Likewise, the economically optimal quantity of fertiliser used by farms is different from the optimal quantity for the health of people living down-river — but that’s just as true regardless of if CO2 is higher or lower than we consider desirable for the general global climate.