My assumption would be that the root systems don't take up the extra minerals and nutrients any faster, but the plant grows faster, leading to the ratios being

My assumption would be that the root systems don't take up the extra minerals and nutrients any faster, but the plant grows faster, leading to the ratios being off. So if you took the two plants, grown in low-CO2 and high-CO2 conditions, and reduced them to constituent atoms, you might find that the two plants have the exact same amount of iron by mass, but the high-CO2 plant masses more overall, leading to less iron per gram of edible plant material. Or you end up with more iron concentrated in the leaves or stalks, and less in the grain, or ... Living organisms are complex balancing equations of thousands of chemical reactions running simultaneously, with the rates at which they run interacting as one reaction produces or consumes the input to another. Increasing the availability of CO2 causes some reactions to run faster, but what happens after that is really, really complex. If a reaction that was bound by the availability of CO2 previously becomes unbound, some processes will run faster, but if process A and process B compete for a common input X, and process A is running faster because of increased CO2 availability, process B may now run slower because X is being depleted faster than usual. Some problems like this may be solvable, but it's terribly complex and some of the solutions will have more side effects and some solutions will be really tricky to figure out. 60, 70% of agriculture is about maintaining a consistent environment for your plants to grow in, with the same amount of sunlight, the same amount of water, and the same concentration of soil nutrients as you have found in years past to produce the best results. The prospect of having to rediscover what the new optimal values for all of the parameters you currently juggle, whether you are growing tomatoes, grapes or corn, is ... interesting.