CO2 is good!

I often hear the argument, from farmers, that the increase in carbon dioxide in the atmosphere is a good thing, because plants grow better with extra CO2 to utilize. They cite glasshouses that pump extra CO2 in to boost production.
Seems a straightforward argument for relaxing about the production of at least one of the 'greenhouse' gases.
However, as described in this email from a scientist here in NZ, the idea is not all it seems.

"There was a German study about CO2 and faster plant growth. Apparently the limiting factors for growth are the uptake of nutrients. So they reported an increased growth rate for a while, but then depletion of the soils and growth slows to again come into balance with the soil nutrient and mineral levels. So - CO2 is a little like steroids, and as long as you are using lignite to to make more fertilizers and diesel to dig up and transport more minerals, then yes, you can use all the bonus CO2.

It does actually feel like the age of stupid sometimes."

What concerns me is the effect of increased CO2, especially when levels become very high, on plant communities outside of the managed farming 'sphere' where extra nutrients aren't matching the 'steroid boost' that CO2 gives.