you guys are confusing yourselves with ppm scales, use EC.
Very much agree with this.
EC is a better measure of ionic strength--which is extremely important in nutrient mediums.
As Quantum9 said, the plants do not "eat" nutrients.
They absorb them selectively according to equilibrium (if absorbed through osmosis) or according to concentrations (if transported by proteins).
There is a bit more selectivity to uptake than I think you're initially letting on--much of uptake is controlled by proteins which transport one specific molecule. They will do this faster/slower depending on several factors--the most important of which is the concentration of the nutrients in the medium, which is only very slightly more important than temperature.
I slightly disagree about the 4% figure in terms of the nutrients.
Let's look at phosphate as a great example here:
So we know that cell growth proliferates by mitosis (eukaryotic cell division). In order for this to occur a full copy of the DNA of each cell must be completed before two daughter cells can be formed.
C. sativa has been estimated (after incomplete genome sequencing) to have somewhere near 400 million base pairs.
Each base pair will be flanked by 2 molecules of phosphorous on either side (as the phosphorous provides the backbone for DNA).
So thats 800 million molecules of P required for each and every new cell (and the reality is that this is much higher--because new ATP/GTP must be formed for each new cell, as well as phospholipids (and other molecules of which P is a structural element).
Let's just break this down for the DNA and forget all of that, though.
Each mole of phosphate contains 1 mole of phosphorous. 1 Mole of phosphate weighs 94.97 grams.
That amount contains 6.022x10^23 molecules of phosphorous (and 4 times as many of oxygen).
If we do the math we find that this breaks down to about 10billion cells worth of phosphorous (for DNA needs only)
A 6 foot plant is probably into the area of trillions of cells (anywhere from 5-40 trillion cells). Some might not need new DNA--but they will all need additional phosphorus inputs.
Similar arguments can be made for, at the very least, nitrogen [and sulfur, albeit less ubiquitously].
Even though its not as ubiquitous structurally, sulfur is
extremely important for catalysis, structural synthesis, and enzyme function.
Nitrogen is a component of every single amino acid, and every single base pair in DNA. It is an extremely important molecule--and makes up much more than 4% of a plants needs. Literally something like 80% of the molecules in the plant contain nitrogen. Essentially all of the macromolecules (polymers) in the plant are made up of monomers which contain at least one atom of nitrogen. This includes every single protein, which make up the bulk of the plant--and represent the most important thing for us to "feed" (protein synthesis).
Now it is true, the relative amounts are lower than C, H, and O--because these make up essentially everything. While the 4% figure may be close to the mark, I think it isn't really informative in the way it should be.
It puts less emphasis on these molecules/elements when in fact they need the most attention (
because CHO are so ubiquitous).
