The rio Guapore is located in the Brazilian state of Matto Grasso, an area which you claim contains some of the best natural plant growth. It is a 'clear water' river, meaning it is low in sediment and dissolved minerals.
It's a very large state with elevational changes, I;'ve seen clear water loaded and virtually pure. I'd not assume much based on clear water.
And we really do not have the information to say much of anything about the sediments.
The soils formed on the surrounding banks (where a lot of our aquarium plants grow) are typically nutrient depleted as well.
Soil from and in streams and rivers can come from vast distances within the water shed. These tend to build up on the bends and turns on river banks and streams, these patchy regions have VERY different nutrient content than the scoured regions. These bend beds are where we often find aquatic plants.
Irrigation canals in CA are similar hydrologically, and get aquatic weeds where sediment builds up, even if the canals are lined and concrete, sediment will still build up over time.
If the soil itself were nutrient rich, it would have values comparable to the Yanayacu which is a 'white water' river. White water rivers are characterized by a large amount of inorganic matter which forms nutrient rich soils. Even so, the Yanayacu only had 0.77 degrees of Mg hardness which is approximately 13-15 ppm. The ec was 205; about 10x that of the Guapore. All of the measurements were gathered during the dry season.
Nutrients contained in the soil are not going to equal the water column, in most cases, the water column is much leaner. Wetlands sequester nutrients out of the water column and lock it up in the sediment. This is why river deltas are so fertile. When sediment is removed, disturbed, or weeds pull out all the nutrients, then die off, then we get problems.
So why is this data meaningless? In regards to Mg deficiency, I think the first article you linked is meaningless since it only reports the absorption of N and P.
I linked those because they specifically measured mineral content contained in the plants themselves, this is basic stuff for nutrients and plant growth............
This tells you regardless of what is present in the soil or water column, if the plants have adequate nutrients in their dry weight biomass.
This way you know what is getting into the plant, no matter the seasonality or pulses of nutrients, which you may or may not catch when measuring soil or water. Merely measuring the water or the soil alone does not tell you much, it's incomplete.
Most plants in natural waters get a large fraction if not the most from sediment, this is not to say they cannot get their entire nutrient sources from the water column, just that in natural systems, this is typically coming from sediments. They are opportunistic, they will get the nutrients in either location.
I do not think the data is entirely meaningless, but it is only during low water when nutrients are the lowest, it does not express nutrients over the entire season when pulsed flows from rain bring in large amounts of nutrients.
There's not much nutrients in the water, this is about all you can conclude, nothing more and nothing about relative growth rates, nor Mg++ in plant tissue or in the sediment.
This data cannot say much, without sediment pore water and tissue levels.
I would not dare myself.
Secondly, if tissue analyses were provided they would show how much of what nutrients a plant can absorb, not how much it requires to add new growth.
Actually this can be determined by critical concentrations in the tissues of the nutrient of interest. This can be compared to other natural systems which have higher levels, or, a comparative Mg++ gradient. This can be measured in growth chambers to determine the range of Mg++ is limiting and what is the critical concentration.
There is not enough data for many of the species for this.
Regarding requirement for new growth, the critical concentration is the point where 90% of the max relative growth rate or higher is achieved independent of other factors.
Many plants take in more than they "appear" to need, PO4 is an excellent example. Luxury uptake, however, these luxury uptake studies tend to all be 8 week range test, not entire seasonal changes throughout the year, the phenology is not address of the plant in question.
If we look at nutrient pulses in aquatic systems, the PO4 is often highest in the early spring, then lowest in mid late summer(dry season).........which is when the plant has the highest demand for PO4 for flowering.........thus this store PO4 now becomes critical, no longer a luxury
Farmers have long know this, but they can add ferts any any critical point(so can we), plants in nature have to nab what they can, when they can.
Tissue analysis helps, so does sediment pore water, and so does water column measure.........but you cannot say much without all of them and over time and seasons in an aquatic system. Each piece is part of the puzzle.
Alone, they are not that effective in making broad conclusions.
Perhaps you do not have the time, the funding , the idea to measure all these, but you cannot run around talking about stuff without support.
All you can say is when you happened to be there, there was not much in the water column.
That's about all.
I completely agree that aquariums aren't very similar to natural systems. But at the same time, the plants we have in our tanks descended from plants mostly growing in nutrient poor water and soil.
I seriously doubt we can say that. I've seen most of the plants on this list natural systems that go from ultra oligotrophic systems, to the very rich systems. "echindorus, limnophila, cabomba,ludwigia, eichornia, tonina, bacopa, and hygrophila."
With the exception of Tonina.........we have EVERY one of these pesky weed genera in CA. I've also seen these all in Florida. There are wide ranges of comparative nutrient gradients here and in South America where these each exist, and I showed folks this on the Plant Fest, very different systems.
The soils were not sampled at these locations, you can NOT make any interferences without that data.
I work in the Delta sampling nutrients and plants and sediments, I also work in Lake Tahoe sampling curlyleaf pondweed which is in a very very different habitat than the Sacramento River delta. Aquatic plants grow very well and have spread in both locations
. In the warmer regions of CA, the other plant genera grow just fine. In Florida, year round. I do not think we can say much based on solely the water column readings from one low nutrient sampling date, the assumptions that nothing is in the soil and without any tissue data. Well, you can try, but the work has simply not been done to make a convincing argument for support.
In the aquarium, we can simply dose more Mg and see.
What level in the water column would be considered non limiting for say 300-400 species of plants? 1ppm? 10ppm? Should it be dosed in small amount daily, or are we fine dosing 5 ppm every 3-7 days?
Can we use inert sand?
You bet we can.
Can we do hydroponic culture?
So we can see about the water column, and there are a few tricks for sediment Mg++ as well. Basically take out the ADA AS or MTS and use a fine mesh nylon bag and squeeze it and remove the pore water and save this, then measure that for Mg.
It would be surprising to fine Mg limitations for aquatic plants, here in CA, we are N limited or light limited, sometimes temp limited. Many places are PO4 limited, some are CO2 limited. Take a look at Iron and the environmental levels relative to tissue levels. They are massively different(about 10^15 times so). It's tempting to say a lot more than we can about aquatic plants based on such data, but it really is not well supported at all. Yuo can talk about the low/absent Mg in the water column, but you cannot talk about plant demand/limitation, growth etc, seasonality, luxury uptake etc etc.
There's not enough information to draw any conclusion there.
In general, you add more ferts, you get more aquatic weeds.
We can control limiting factors in our tanks and this separates a tank full of healthy plants from one full of unhealthy plants. But what this data shows better than anything else I've come across is that most of us are in no way providing too little of any nutrient. It's clearly more important to make sure you provide a small amount of all nutrients in a manner that ensures rapid uptake than focussing on absolute values of one or two elements.
The data does not say a thing about relative rates of growth or uptake.
Nada, zilich.............says nothing about health of status, which is your question above.
Why do we need to make small frequent dosing amounts? Why can we not dose say 10ppm and do this once a week? Is this bad or does it produce bad or negative growth, or is this just wishful fanciful thinking?
How frequent and how low should it be?
What happens if we add MORE than this?
How would test these questions?
I've had the luxury of having a wide range of Mg++ ppm and GH's in my tap water, and within the two clubs here in the Bay/Sacramento region. Most here would argue more GH, Mg included, is better for plants. This is not merely me saying it, but a wide comparative group that meets and sees one another's tanks.
SF and Sac have VERY low Mg, and Davis, Ca has crazy high Mg.
Marin, something in between. We have to add more in Sac/SF.