Alright.........look at the ppm's from the West side of Davis wells:
http://cityofdavis.org/pw/pdfs/SummaryofallWells.pdf
Low Ca, high Mg.
Here's my aquarium:
Now if I can grow these plants at 1/2 the levels in the report...............(I diluted with 50% RO for water changes), my Mg was around 25ppm........the Ca was -18-20ppm.
The tap I have now is about 15ppm Ca and 2-3ppm Mg.
I find little difference, the coloration for many plants was better in the harder GH water.
In Santa Barbara, the Tap was mostly Ca++ and very little Mg.
No RO blended there.
82 Ca++ and 30-35ppm for Mg++
KH was 11-12
No issues.
Maybe it has much much much more to do with CO2 management and assumptions and virtually nothing to do with ratios or low Ca/Mg levels...........?????????
Given ther lack of controls and testing, and the evidence that these ratios and ppm's have little real comparative argument otherwise...........it would seem it's a CO2 issue.
I've not seen any conclusive evidence that Ca and Mg play much role in crops and aquarium plants for that matter. It might be an issue when folks limit Mg, which seems to be far more common than Ca++ , but the ratios play little role.
There is no factual basis for most of the claims made regarding this.
CO2?
Lots and lots.........
I do not think any researchers would note much ecological pattern changes due to Ca/Mg ratios, or that above a limiting level....that plants can/cannot grow or anyone that's shown Ca++ limitation in aquatic plants.
We have massive weed infestations in Lake Tahoe, which is a hyper oligotrophic lake, they are not limited by the low ca and Mg........they grow just fine.
12 ppm and 2ppm for Ca and Mg. Such lakes are pure snow melt, so they have little Ca and Mg. In Rainforest, the recycling of Ca and Mg from decomposing plant material and from sediments is high, but gets used and bound rapidly.
Borneo :
http://upm.academia.edu/AhmadZahari...pective_Maliau_Basin_Borneos_mysterious_world
Much much higher.
You can poke around a look for Ca and Mg levels in other regions where plants might be.