I'm reading through Diana Walstad's book and so far it's fantastic. It has plenty references to specific biochemical processes, yet casual enough for a layperson to get the general concepts and have enough material to follow up on to learn.
I also visited Ouroboros Farm, and I love it there. I'd like to do some volunteering there.
Right....but what I'm after is if the plant root act like a nitrogen magnet to actually pull the nitrogen from the water or if they just happen to use nitrogen from water it takes in?
Lets say you have 100 gallon of aquaculture dirty water and your plants uptake 10 gallons(10%)
Will your nitrogen be reduced by 10% (just "cleaning"water it uptakes from the 10 gallons of water or like 80%(if they can somehow selectively uptake the nitrogen from the 100 gallon system)?
In other words, there is a water current passing by the roots. Do the roots just indiscriminately pull in water from its surroundings and then start to process whatever it happens to find in that water or does it somehow selectively target nitrogen from that water and decide to uptake only nitrogen from that water at a faster rate then just the water that is removed from the system?
If I may reiterate your question to make sure I understand: does a plant take in water from its roots, after
which it processes desired compounds from the water that is now contained within its cell walls, or does it take nutrients directly from the water?
With the caveat that I'm still learning, I believe the answer is that while both are possible (Diana mentions intake of water that is then transpired through the leaves I'm guessing), for aquatic plants it will mainly absorb nutrients directly from the water, just because it's already dissolved in water. And plants will intake some water for water's sake, and this is probably drastically less pronounced in submerged plants vs. terrestrial or even emersed.
To use your 100 gallon example, I think fairly little water volume will be actually used by the plants, especially when compared to evaporation. Assuming no nutrient deficiencies in the water and/or ambient air, the plants will take as much as it can get, and according to Diane Walstad, often much more than it needs. The ratio will depend on the plant type and the nutrient profile, since certain nutrients are more readily taken in than others. So back to your example, the plants will take way more of the nutrients available than the % of water volume it will use. In other words and for purposes of this example, nutrient uptake is not
limited by the water volume the plant needs for itself. I'm sure a relationship exists, but they are probably not so linearly related.
Also, keep in mind that nutrient uptake is not only via the roots, but leaves as well (as in the case of submerged plants). And depending on the plant and nutrient, the leaves or roots can be the preferred method of transport.
Another cool example that I believe indicates direct uptake (and now I'm just showing off what I learned from reading Walstad
), is that plants can affect the water parameters directly around its roots (e.g. increase/decrease pH), so as to make certain elements more readily accepted/effective. If water uptake was a prerequisite to nutrient uptake, I imagine that the above mechanism wouldn't need to occur around the roots, if at all.
There are many others on here with deep knowledge and can provide a much more nuanced perspective, and maybe even call me out on any terrible representation of facts, lol.