Joined: Oct. 2006
Ichthyic: I agree that it is surprising that it has not been done before but I suspect that they are probably correct. Some root competition studies (between plants of two species, or plants of the same species) were done in the 50s and 60s but not much since that I know of (I have been out of the area for a while so I could be out of date here). I was slightly involved in root studies in the 60s and 70s and a major problem was getting the soil off the roots. Even a small amount can have a large effect on the dry weight and any macroscopic organic matter is almost impossible to remove. We used to wade into a local stream and use that as our running water source (drains quickly get plugged by soil). I would hate to have to separate two interwoven root systems, although I suppose these days there could be biochemical methods for estimating the amounts of each.
Another problem is that some roots are very sensitive to light and even a few seconds exposure can check their growth so clear pots or glass-walled pits are limited in their usefulness.
Roots are altogether frustrating to deal with and publications on root growth and morphology generally come slowly so very little has been done in recent years, really since the rise of 'publish or perish'. Most research on roots focuses on nutrient uptake, including VAMs (vesicular-arbuscular mycorrhiza - fungi that live in close association with roots and are important in nutrient uptake).
All sweeping statements are wrong.