Like history, there where various specific regions that gained a notable level of expertise. Wim gave the best talk I've seen on this topic at the AGA conference some years ago in Dallas, TX.
He detailed tank after tank fully plants and well scaped by anyone's standards, of dutch style fully planted tanks............in the 1940-1950's range. Really nice tanks, no CO2 etc.
The first reference I've ever found for CO2 for planted aquariums dates to 1963 using yeast DIY.
Other tanks certainly existed prior, but less is known.
Much of what was learned was based more on simple gardener approaches, taking from Terrarium and other horticultural fields. Other groups came at the aquarium from more a fish perspective and dabbled with plants. A few hit on a few good things and got lucky.
They had less notion as to why, they speculated.
In 1933, Hoagland developed this solution for growing plants hydroponically in solution:
(Source: Epstein 1972)
KNO3: 1M 6.0mls N 224ppm
Ca(NO3)2*4H2O 1M 4.0mls K 235ppm
NH4H2PO4 1M 2.0mls Ca 160ppm
MgSO4*7H2O 1M 1.0mls P 62ppm
(Hoagland and Snyder, 1933)
KNO3 1M 5.0mls N 210 ppm
Ca(NO3)2*4H20 1M 5.0mls K 235 ppm
MgSO4*7H20 1M 2.0mls Ca 200 ppm
KH2PO4 1M 1.0mls P 31 ppm
S 64 ppm
Mg 48 ppm
In the 1960's, more quantitative use with aquatic plants from Gerloff suggested some different changes, (modified Hoagland's, some NH4 etc)
You can see that the concentrations are very rich indeed.
This has long been an issue for these Know Nothing's claiming that excess NO3, K+, Mg, PO4 stunt plants............yet this is the standard reference solution for non limiting nutrient media(for the macro's at least), this is also about the normal ppm's for hydroponic solutions used to grow commercial aquatic plants at Tropica and other nurseries such as FAN.
Now if EI which is 10-8x less concentrated causes that, why would growers use it, researchers use it and why is used for fertigation for terrestrial crops also? Most(90% of the contained pots used for ornamental plants) use fertigation for growing their smaller plants raised for sale(12 Billion $ per year in CA alone).
What do excess ppm's look like in these systems? Salt stress mostly.
This details out most of the basic references. Dupla use to run around telling everyone that they discovered and made the first Fe chelated dosing for aquatic plants. Apparently well beaten by a decade it seems(they claimed in the mid late 1970's).
Paul and Gerloff suggested a 1/5 th full stength for aquatic submersed plants.
Let me see here:
N is in the mid 40's ppm
P is about 12 ppm
Ca is about 30ppm-'s
K+ is about the same as N, 40'sppm
Seems EI is pretty lite, about 10X less.
This % is used in many research vats growing plants, eg Hydrilla here :
Plants for toxicity assessment - Google Book Search
What we do not know , is the concentrations for soils etc. They are used often.
Paul and Kevin had long referenced and the young(at the time) Paul KROMBHOLZ offer a lot of input as well to PMDD.
So that had the greatest influence on the development of most methods involving water column dosing in the hobby(PMDD) and brought the method to the hobby in a cheaper, DIY simple approach that put everyone on the same playing field for use of fertilizers.
I had to dig for much of these references over time as I looked more and more into it.
The history is interesting and should always be directed to offering credit where it is due. It adds support.
So what did I do?
Not much really, more arguing for it than the work and cleverness etc.
I took PMDD, add more, mostly by consensus, was more liberal and dosed PO4, added more light and thus more CO2(eyeballed it pretty much till things look like they should). I wanted that same pearling I had right after the water change and keep it there all week long.
Not just for 1 day.
So I added more etc.
Some plants started doing much better, other folks had troubles at lower ppm's.........with those same plants.
So I got 95% or more from PMDD, I just added PO4 more, which was not even my discovery, rather Steve Dixon who at the time was really into testing for NO3 and PO4 using lamotte/Hach test kits(they seemed like a very expensive choice to me, but wise today). I simply had the tank with high PO4 and did not know it. Not that I would have cared either
I just grew the plants well and watched them.
I did the old water change routine I'd always done.
Then started adding KNO3/Traces/KH2PO4/K2SO4.
I needed more CO2 to do that I figured.
Steve had taken a plant Physiology class at some point and liked it, had a good understanding. I'd just started taking college classes then.
We both discussed the ranges of ppm's used for aquariums.
The list of levels and parameters came from that(1996-1997).
Noting few people bothers to test, I took a leap of faith and suggested no test kits and replace it with known % water changes and dosing regularly.
Water changes are easy.
Some folks did not like the idea of not using to test kits like they had done for decades, so they tested still and did the water changes as well. Some/most where more conservative initially. Then newbies started making mistakes and had no algae as well even when over dosed.