ceg4048 said:
Hi,
"Root feeder plants" is another chapter in the book of plant keeping mythology.
This is the part of your post that I was disagreeing with, not that swords and cryptocoryne plants feed from the water column. You yourself have already stated that you were not claimning that sword don't feed from the roots. Which was the basis of my disagreement. Root feeding plants is not a myth, the myth is rather that these plants feed only from the roots.
As to cutting back the roots, cut the roots of a cryptocorne or sword plant far enough (more so in the case of crytocoryne plants) and you will get SOME die back, even in the presence of high levels of nutrients in the water column. They will however grow back faster in these conditions. You can not prove that the plants are mainly feeding via their stems and leaves from the water column, as you have already agreed nutrients pass both ways from substrate to water column.
As to gaseous transfer across the leaf membrane, this is achieved using stomata, the size of which could indeed limit the transfer of gasses to CO2 and O2, this is the very nature of osmosis as "diffusion across a SELECTIVELY permeable membrane". Reverse osmosis systems are a perfect example of this, where nothing larger than a water molecule is able to cross the membrane. However plants do indeed take in nutrients via the leaves, in both aquatic plants and terrestrial ones.
I am also quite sure that your planted tanks have a plant substrate in them, which will be providing nutrients to the roots, along with those provided by diffusion and currents through your substrated, which will in part account for some of your success with these plants.
We can however agree that the idea that plants feed exclusively, or preferentially, by the roots IS a myth. However, I have found that these types of plants do seem to do better with some form of substrate fertilisation. I would also suggest that in cases where the substrate is composed solely of an intert, nutrient poor, aggregate that substrate fertilisers are even more useful, in effect helping to turn your substrate into more of a planted tank substrate.
Incidentally, I would not say that laterite is useless. It's just another form of clay, as are most of the planted tank substrates. I would however say that it does best when combined with other substrates (peat etc). When I first started keeping a planted tank there were not the choices that there are today, and the choices that were available were even more expensive than they are now. As such I chose to use laterite, combined with peat, activated charcoal and a 1-2mm silica grit (rounded granuals), and to enrich this further using root tabs and balls. Ever since I have seen phenominal growth in ALL plants, including so called slow growers. I have even succesfully grown plants that require high light in a low light setup. I had to stop growing larger swords, purely because they just got too big too quickly! I would not however ever suggest or advocate the use of 'root' fertilisers in isolation, I quite agree that water column dosing is as, if not more so, important. If 'root' fertilisation was completely pointless, then there would be absolutely no market for, or need to use, plant substrates or even DIY ones such as Akadama. I just wish that I had had these choices when I set up my planted aquarium.
Ade