Partial pressure of atmospheric CO2 is not the limiting factor in growth for most plants. Increasing CO2 concentration will therefore not have a linear effect.

Partial pressure of atmospheric CO2 is not the limiting factor in growth for most plants. Increasing CO2 concentration will therefore not have a linear effect. CO2 is a gaseous fertilizer, providing carbon to plants. But anyone that has ever kept a garden, or even a potted plant, knows that plants also need water, plus nitrogen, phosphorous, and potassium (N-P-K). Hydroponics growers can add to that list [Mg++], [Ca++], [SO4--], [Fe++], [Mn++], [Zn++], [Cu++], [BO3---], [MoO4--], and [Cl-] ions, usually present in the soil that is not used in a hydroponics system. So increasing CO2 has diminishing returns, until further increase results in no further plant growth.