What was done
The authors review additional data relative to some concepts they introduced in a paper four years earlier (Lloyd and Farquhar, 1996), wherein they concluded that plant growth enhancements due to atmospheric CO2 enrichment were "nearly as often as not greater under low nutrient conditions."

What was learned
In addition to the data presented in their earlier paper, the authors evaluated the data assembled in the paper of Poorter (1998) and came to essentially the same conclusion: "the most frequent observation is that there is actually no significant difference in the growth responses of low- vs. high-nutrient plants and quite often low-nutrient plants have greater responses."

What it means
There has long been a "widely held assumption," in the words of Lloyd and Farquhar, that plants growing on soils of sub-optimal nutrient concentration will not respond to atmospheric CO2 enrichment as strongly as plants growing on soils of optimum fertility.  This assumption has many times been stated as fact by climate alarmists attempting to play down the significance of the aerial fertilization effect of atmospheric CO2 enrichment.  With this publication by two of the world's preeminent investigators of the phenomenon, however, we can perhaps hope to see that campaign of misinformation come to a screeching halt.  (But we wouldn't advise holding your breath until it does.)

Poorter, H.  1998.  Do slow-growing species and nutrient-stressed plants respond relatively strongly to elevated CO2?  Global Change Biology 4: 693-697.