"Fact"Check.org tries to argue that CO2 is dangerous to plants

FactCheck.org : Santorum’s Science

Santorum’s “tell that to a plant” crack begs the question — how dangerous can carbon dioxide be? Too much is definitely a bad thing. Exposure to high levels of CO2 can cause “headaches, dizziness, restlessness, a tingling or pins or needles feeling, difficulty breathing, sweating, tiredness, increased heart rate, elevated blood pressure, coma, asphyxia to convulsions,” warns the Wisconsin Department of Health, “and even frostbite if exposed to dry ice,” which is solid CO2. Poor air circulation in buildings and high carbon dioxide in soil seeping into basements can lead to high levels of the gas.

Plants do, in fact, absorb CO2. But even plants might not like too much of it. A 2008 study conducted at the University of Illinois found that instead of increasing organic matter in soil, higher carbon dioxide levels actually led to less organic matter. Increased CO2 also may limit plants’ ability to cool the air. A 2010 article in Science Daily said that a study by researchers at the Carnegie Institution for Science found that carbon dioxide’s effect on vegetation was causing some of the earth’s warming.

Maximizing Your Plant Growth With CO2

Plant growth requires a tremendous amount of carbon dioxide (CO2). At the center of every plant cell is an atom of carbon, which the plant has absorbed from the surrounding atmosphere. When all other growth influencing factors are kept in their ideal ranges, CO2 becomes the limiting factor. This means as you increase the CO2, you will also increase growth rates and yields.

...The ideal level of CO2 supplementation to maximize plant growth in a well maintained garden is generally 1500 ppm