The Cal-Mag of my dreams?

[MikeInOz]

'Calcium must be balanced with Magnesium, and the ideal Ca-to-Mg ratio is 4:1.' Copied from another post and below

from the picture I see CA 1.2% and MN 3.4 % is that backwards the ideal ratio ?

Yes! But 4 to 1 is just a guide. You can probably get away with 2 to 1 or 3 or infinity to 1 for limestone growers.
As far as I can tell the extra Mg in that product is added as balls of what looks like Magnesium carbonate. They are slow to dissolve but just to be sure I always add coarse Gypsum and/or limestone to the mix or top dress after potting. The plants seem to respond very well to that. Extra Ca can also be added as ground superphosphate while mixing up. This also adds S and P of course. By the time I add the gypsum/limestone the ratio of Ca to Mg is probably more like 10 to 1. I only add the limestone to the limestone species. The others just get Gypsum. About 1/2 gram per 10cm pot. Things like Epithelantha and Ariocarpus get gypsum and a large amount of limestone ''sand'' (<>1mm particles). As much as a teaspoon for a 10cm pot and they are growing well with that. So much for the ''Ca restricts growth'' meme!

[Steve Johnson]

Here's a side-by-side guaranteed analysis of the General Hydroponics/Terra Aquatica FloraMicro and FloraBloom:

FloraMicro-FloraBloom.jpg (83.55 KiB) Viewed 2800 times

I'll definitely be making the switch from Dyna-Gro to both of these ferts when the next growing season begins here in SoCal. Plenty of Ca and Mg too, so I don't think I'll need the TPS CalMag anymore. Maybe I do? Let's ask Mike...

You can probably get away with 2 to 1 or 3 or infinity to 1 for limestone growers.

5% Ca in the FloraMicro and 1.5% Mg in the FloraBloom vs. 4% Ca and 1% Mg in the Dyna-Gro 7-7-7 and TPS CalMag -- sounds like the FloraMicro and FloraBloom should be better and quite sufficient on their own. Could I go even higher if I do add some of the TPS? Mike, I freely admit that I don't know my cacti as well as I should, so take a look at this list:

Ariocarpus fissuratus
Astrophytum asterias
A. capricorne
A. 'capristigma'
A. myriostigma var. quadricostatum
Aztekium ritteri
Cephalocereus senilis
Copiapoa hypogaea
C. laui
C. tenuissima
Coryphantha hesteri
C. retusa
Cumarinia odorata
Cumulopuntia rossiana var. fuauxiana
Discocactus buenekeri
Echinocereus rigidissimus var. rubispinus
E. viridiflorus ssp. canus
Epithelantha bokei
E. micromeris
E. micromeris var. dickisoniae
Eriosyce napina ssp. glabrescens
E. napina ssp. lembckei var. duripulpa
E. odieri var. odieri
E. senilis
Espostoa lanata
Frailea castanea
F. grahliana
Geohintonia mexicana
Gymnocactus ysabelae
Gymnocalycium baldianum
G. ochoterenae ssp. vatteri
G. stellatum
G. stenopleurum
Mammillaria crucigera
M. deherdtiana var. dodsonii
M. grahamii
M. guelzowiana
M. matudae
M. perezdelarosae ssp. perezdelarosae
M. spinossissima
M. theresae
Melocactus matanzanus
Obregonia denegrii
Parodia subterranea
Pelecyphora aselliformis
P. strobiliformis
Pilosocereus azureus
Puna clavarioides
Rebutia heliosa
R. pygmaea
Stenocactus lloydii
S. zacatecasensis
Sulcorebutia callichroma var. longispina
S. heinzii
S. rauschii
Tephrocactus articulatus var. inermis
T. articulatus var. papyracanthus
Turbinicarpus jauernigii
T. klinkerianus
T. polaskii
T. pseudomacrochele
T. pseudopectinatus
T. valdezianus
Wigginsia vorwerkiana

I live in a 2nd floor apartment, so not bad for 12 square feet of growing space! Anyway, a lot of these cacti have shown substantial growing improvement after 2 years of Potassium sulfate supplementation and a year of adding the TPS CalMag. 3.5 ml each of the FloraMicro and FloraBloom per gallon of water will replace 1/2 tsp. 7-7-7 and 1/4 tsp. CalMag per gallon. If I continue adding 1/4 tsp. of the TPS CalMag to the dilute FloraMicro-FloraBloom watering solution, would that be a problem?

[ohugal]

@jerry: It's a bit annoying, the Terra Aquatica website doesn't seem to have elaborate data sheets on their products. Here is a link: https://www.terraaquatica.com/mineral-f ... s/tripart/. Terra Aquatica is the same as General Hydrophonics, but GE has a more informative website: https://generalhydroponics.com/products ... loramicro/. I will just compare the labels on the bottles to both websites to be sure. The website I get it from: https://www.stadstuinshop.nl/nl/terra-a ... eding.html (only in dutch).
Is there a way to measure the hardness of water?

Guessing from the post of Steve, the GH/TA FloraMicro/FloraBloom are very good basic setup. The data sheet of the GH/TA CalMag supplement mentions not to add it if you use GH/TA FloraMicro (soft water version). FloraMicro already contains enough calcium.

@Mike: When adding additional lime, gypsum or both, is the process repeated each year? What are some common lime lovers and/or gypsum enthousiasts? As Steve I would also like to know if it's needed in the case of the GH/TA TriPart series.

[jerrytheplater]

@jerry: It's a bit annoying, the Terra Aquatica website doesn't seem to have elaborate data sheets on their products. Looks like Steve posted them above. Same product.

Is there a way to measure the hardness of water? First, here is a site where you can read about the different types of hardness, since you want to delve deeper and learn. https://www.hach.com/parameters/hardness

Yes there are many ways to test for hardness. Aquarium shops will have the cheaper test kits. Depends on how much you want to spend and what results you want. The aquarium hobby in the US uses the terms General Hardness or GH and Carbonate Hardness or KH. https://www.bigalspets.com/ca/api-gh-kh ... water.html General Hardness is measuring your Calcium and Magnesium. Carbonate Hardness is measuring the carbonate levels in your water-close to Alkalinity, but not exact.

I have a Hach test kit for Hardness which will allow me to test both Calcium and Total Hardness-Magnesium is calculated by subtraction. https://www.hach.com/p-hardness-calcium ... or/2448600

You can also test for Alkalinity. This will tell you how strongly your water is buffered, primarily with Carbonates in most parts of the US. https://www.hach.com/parameters/alkalin ... alkalinity

See the red above for my comments.

[MikeInOz]

@Mike: When adding additional lime, gypsum or both, is the process repeated each year? What are some common lime lovers and/or gypsum enthousiasts? As Steve I would also like to know if it's needed in the case of the GH/TA TriPart series.

A lot of the Texas and Northern Mexico plants grow in soils with both limestone and gypsum so I give them both. Things like arios, epithelanthas, many thelos, all astros, all coryphanthas and escobarias many mamms ( you have to check) pelecyohora, etc. No need to add limestone more than once. Gypsum is much more soluble so I add a bit at the start of each growing season. If you can use particles of gypsum which are say 2 - 3 mm in size they will probably last 2 years or more. Limestone will basically last forever.
All the other things like notos, rebutias, lobivia etc get a little gypsum but no limestone as they like more acid conditions. If you regularly add trace elements (especially if they are chelated ) there is not so much need to worry about the pH. (but I still try to keep it around 5.5 - 6). I have only seen severe chlorosis in 2 cacti. An echinposis and a Neobuxbaumia. If it gets too bad there is no saving the plant. So most things seem to tolerate a fairly wide range. Those products you mention seem to be for hydroponics and have nitrate as the major N source. I believe I read that cacti prefer ammonium to nitrate so I'm not sure if they are the best option?? The Ca would be from the CaNo3

[MikeInOz]

I'll definitely be making the switch from Dyna-Gro to both of these ferts when the next growing season begins here in SoCal. Plenty of Ca and Mg too, so I don't think I'll need the TPS CalMag anymore. Maybe I do? Let's ask Mike...

You can probably get away with 2 to 1 or 3 or infinity to 1 for limestone growers.

5% Ca in the FloraMicro and 1.5% Mg in the FloraBloom vs. 4% Ca and 1% Mg in the Dyna-Gro 7-7-7 and TPS CalMag -- sounds like the FloraMicro and FloraBloom should be better and quite sufficient on their own. Could I go even higher if I do add some of the TPS? Mike, I freely admit that I don't know my cacti as well as I should, so take a look at this list:

Steve, the ratios seem ok to me (with more K) but see my comments about nitrate v ammonium in my reply to ohugal.
The list below is a mixed bag. I have put an LG (both limestone and gypsum) or L (limestone) or G (gypsum only) G+ (double the gypsum) next to the ones I know and what I give them. I don't think giving gypsum ( calcium sulphate ) would hurt any cactus even epiphytes.
All would receive the same amount of N P and K. We have to standardize the culture because if we try to replicate the habitat for every species insanity would soon follow.
(Note! This is what I do - it doesn't mean doing something else won't work and just as important, or even more important is the environment - water light intensity and air movement for the various species)
You will need to research the habitats of others.

Ariocarpus fissuratus LG
Astrophytum asterias LG
A. capricorne LG
A. 'capristigma' LG
A. myriostigma var. quadricostatum LG
Aztekium ritteri LG
Cephalocereus senilis LG
Copiapoa hypogaea G
C. laui G
C. tenuissima G
Coryphantha hesteri (escobaria?) LG
C. retusa G
Cumarinia odorata
Cumulopuntia rossiana var. fuauxiana
Discocactus buenekeri G
Echinocereus rigidissimus var. rubispinus LG
E. viridiflorus ssp. canus G
Epithelantha bokei L or LG
E. micromeris L or LG
E. micromeris var. dickisoniae L or LG
Eriosyce napina ssp. glabrescens G
E. napina ssp. lembckei var. duripulpa G
E. odieri var. odieri G
E. senilis G
Espostoa lanata G
Frailea castanea G
F. grahliana G
Geohintonia mexicana G+
Gymnocactus ysabelae LG
Gymnocalycium baldianum G
G. ochoterenae ssp. vatteri G
G. stellatum G
G. stenopleurum G
Mammillaria crucigera G+
M. deherdtiana var. dodsonii G
M. grahamii G
M. guelzowiana G
M. matudae G
M. perezdelarosae ssp. perezdelarosae G
M. spinossissima G
M. theresae L or LG
Melocactus matanzanus G
Obregonia denegrii G
Parodia subterranea G
Pelecyphora aselliformis LG
P. strobiliformis LG
Pilosocereus azureus G
Puna clavarioides
Rebutia heliosa G
R. pygmaea G
Stenocactus lloydii
S. zacatecasensis
Sulcorebutia callichroma var. longispina G
S. heinzii G
S. rauschii G
Tephrocactus articulatus var. inermis
T. articulatus var. papyracanthus
Turbinicarpus jauernigii LG
T. klinkerianus LG
T. polaskii LG
T. pseudomacrochele LG
T. pseudopectinatus LG
T. valdezianus LG
Wigginsia vorwerkiana G

I live in a 2nd floor apartment, so not bad for 12 square feet of growing space! Anyway, a lot of these cacti have shown substantial growing improvement after 2 years of Potassium sulfate supplementation and a year of adding the TPS CalMag. 3.5 ml each of the FloraMicro and FloraBloom per gallon of water will replace 1/2 tsp. 7-7-7 and 1/4 tsp. CalMag per gallon. If I continue adding 1/4 tsp. of the TPS CalMag to the dilute FloraMicro-FloraBloom watering solution, would that be a problem?

No problem. But I also see that those Flora products contain carbonates. Hopefully they are super fine because they are likely to be in solid form.
Do they have an acid added? If not you will need to check the pH of each batch of solution you make up and bring it down (or up) to pH 6 or a bit lower.

[jerrytheplater]

I live in a 2nd floor apartment, so not bad for 12 square feet of growing space! Anyway, a lot of these cacti have shown substantial growing improvement after 2 years of Potassium sulfate supplementation and a year of adding the TPS CalMag. 3.5 ml each of the FloraMicro and FloraBloom per gallon of water will replace 1/2 tsp. 7-7-7 and 1/4 tsp. CalMag per gallon. If I continue adding 1/4 tsp. of the TPS CalMag to the dilute FloraMicro-FloraBloom watering solution, would that be a problem?

No problem. But I also see that those Flora products contain carbonates. Hopefully they are super fine because they are likely to be in solid form.
Do they have an acid added? If not you will need to check the pH of each batch of solution you make up and bring it down (or up) to pH 6 or a bit lower.

Mike, I have a quart each of the three General Hydroponics fertilizers that must be about 15 years old. I just noticed the order of the Nutrients the fertilizer is composed of is different on mine compared to what is being sold now. Same chemicals, different order. I'm going to assume the order listed is in order from greatest to least, like in food ingredients. If anything were going to settle out, it would have in mine. No settling visible.

SDS for each found on the GH website after some hard searching.

Regular Flora Micro pH listed as 5.8. I tested mine with a freshly calibrated pH probe tonight and got 6.3. Reasonable since it appears the formulation has changed over the years. Same nutrient analysis though. I don't have any modern product to test.

Flora Bloom pH listed as 3.5. Tested at 3.5.

Why do I note the pH? It is in reference to your concern about carbonates in the Flora Bloom fertilizer. At that low pH of 3.5, no carbonates or even bicarbonates can exist. The Phosphoric acid in the Flora Bloom has neutralized the carbonate to CO2 and most/all has gassed off. It has also raised the pH of the Phosphoric acid from 1.5 to 3.5. I can't say for 100% certain as I don't work for GH, but my bet is they used carbonates as pH adjusters AND a source of the needed nutrients. The pH of the Flora Micro is higher (5.8 ), and it is possible some of the Calcium carbonate will exist as bicarbonate, but not very much. Most would be Carbonic acid/CO2. Again, pH adjust of the fertilizer solution.

[Steve Johnson]

Mike and Jerry, thank you so much for your responses -- they're quite helpful. I have a few things to unpack there, so I'll post them either tomorrow night or over the weekend. The start of the 2023 growing season here in SoCal is at least 3 months away (feels like it'll be forever to get there!), and this gives me plenty of time to prepare.

[ohugal]

Jerry, thank you for the links! I've read the information and understand the subject matter a bit better now. I am planning to place an order for some light and moisture measuring equipment at Pronova (a pretty good shop of agricultural measuring equipment in Germany). I see they have tests for both carbonate and general hardness (in packs of 100 or 6 strips). What is not yet clear is what parameters I am striving for and how to interpret the results. Let's see.

MikeInOz, thank you for the list of plants! Very helpful! There is one thing I don't fully understand. It seems you are worried about the carbonates in the GH/TA TriPart fertilizers, but calcium carbonate is main ingredient of agricultural lime... Or is about the sollubility?
To be clear, you add (additional) calcium and magnesium through limestone and gypsum? I just read magnesium is mainly present in dolomite containing limestone.

Steve, I see from older posts (2014) you are quite invested in the matter. A never ending quest it seems. With the right calcium and magnesium levels in the TriPart products, there is no need for the TPS CalMag supplement in the future, right? I will be testing the mixture in a month for pH, carbonate and general hardness (and magnesium by comparing the results of the last two). I will post my findings here.

[Steve Johnson]

Steve, I see from older posts (2014) you are quite invested in the matter. A never ending quest it seems.

Yes indeed! I understand why casual growers may prefer to keep their practices as simple as possible, but I stopped being casual about it once I joined the forum 11 years ago. The shared learning experience here is wonderful -- IMO the most informative online cactus and succulent resource, and way better than what we'll get elsewhere.

With the right calcium and magnesium levels in the TriPart products, there is no need for the TPS CalMag supplement in the future, right?

Well, yes and no. Going through the collection list I posted -- if was adding gypsum and/or limestone in the right amounts for the right species as indicated by Mike, I wouldn't need the TPS CalMag when I start using the GH ferts. Call me lazy (and not at all confident about being able to follow Mike's lead), but it's easier if I stay away from the gypsum/limestone thing and continue supplementing the GH ferts with TPS CalMag. Unless I misread him, he said that I wouldn't be overdoing the Ca and Mg. The question is -- if continuing with the additional TPS supplement won't hurt, is there any particular benefit? Let's go back to a salient comment in this post:

(Note! This is what I do - it doesn't mean doing something else won't work and just as important, or even more important is the environment - water light intensity and air movement for the various species)
You will need to research the habitats of others.

My investment in the matter stops short of that, and with a number of things vying for the limited free time I have, I may err on the side of caution and simply stick with the Ca and Mg my cacti will be getting from the FloraMicro and FloraBloom ferts. Good news is that I don't have to make an immediate decision on this one way or the other.

[MikeInOz]

I live in a 2nd floor apartment, so not bad for 12 square feet of growing space! Anyway, a lot of these cacti have shown substantial growing improvement after 2 years of Potassium sulfate supplementation and a year of adding the TPS CalMag. 3.5 ml each of the FloraMicro and FloraBloom per gallon of water will replace 1/2 tsp. 7-7-7 and 1/4 tsp. CalMag per gallon. If I continue adding 1/4 tsp. of the TPS CalMag to the dilute FloraMicro-FloraBloom watering solution, would that be a problem?

No problem. But I also see that those Flora products contain carbonates. Hopefully they are super fine because they are likely to be in solid form.
Do they have an acid added? If not you will need to check the pH of each batch of solution you make up and bring it down (or up) to pH 6 or a bit lower.

Mike, I have a quart each of the three General Hydroponics fertilizers that must be about 15 years old. I just noticed the order of the Nutrients the fertilizer is composed of is different on mine compared to what is being sold now. Same chemicals, different order. I'm going to assume the order listed is in order from greatest to least, like in food ingredients. If anything were going to settle out, it would have in mine. No settling visible.

SDS for each found on the GH website after some hard searching.

Regular Flora Micro pH listed as 5.8. I tested mine with a freshly calibrated pH probe tonight and got 6.3. Reasonable since it appears the formulation has changed over the years. Same nutrient analysis though. I don't have any modern product to test.

Flora Bloom pH listed as 3.5. Tested at 3.5.

Why do I note the pH? It is in reference to your concern about carbonates in the Flora Bloom fertilizer. At that low pH of 3.5, no carbonates or even bicarbonates can exist. The Phosphoric acid in the Flora Bloom has neutralized the carbonate to CO2 and most/all has gassed off. It has also raised the pH of the Phosphoric acid from 1.5 to 3.5. I can't say for 100% certain as I don't work for GH, but my bet is they used carbonates as pH adjusters AND a source of the needed nutrients. The pH of the Flora Micro is higher (5.8 ), and it is possible some of the Calcium carbonate will exist as bicarbonate, but not very much. Most would be Carbonic acid/CO2. Again, pH adjust of the fertilizer solution.

Thanks Jerry. Sounds all good.

[MikeInOz]

MikeInOz, thank you for the list of plants! Very helpful! There is one thing I don't fully understand. It seems you are worried about the carbonates in the GH/TA TriPart fertilizers, but calcium carbonate is main ingredient of agricultural lime... Or is about the sollubility?
To be clear, you add (additional) calcium and magnesium through limestone and gypsum? I just read magnesium is mainly present in dolomite containing limestone.

Only a problem if there is no acidification (by whatever means) possible and the pH starts to drift up.
Here is my example. I start with an acidic mix. Around 5.5 or so. The limestone cacti get solid limestone ''sand'' and gypsum added. Because the limestone will probably bring the pH up to neutral and because it is only very slightly soluble, and I use rainwater with no carbonates present and my fertilizer contains mainly ammonium and or urea N (which is converted to ammonium and acidifies the mix) the pH of the soil in which the limestone is should not drift too far between alkaline and acid and it will ensure a never-ending supply of Ca. Borg said it all those years ago - use limestone gravel and rainwater.
So that seems how to grow the limestone cacti... A more or less acidic to neutral mix but always with Ca. and carbonates available.
Exactly how it works in nature. That's why it is wise to use limestone as a solid in your mix but acidify the water if you don't use rain water. If you don't acidify your water that is towards the hard side, (but not too hard), you should not use too much nitrates and use only ammonium fertilizers. Ammonium sulphate is the most acidifying fertilizer.
Failure to do so will retard the growth of your cacti sooner or later. If it is very hard you should probably not add any limestone. A pH test of your potting mix should be done every 6 months to check what is actually going on in the soil.

[Steve Johnson]

This thread is expanding well beyond what I started, but I'm fascinated by the discussions here, so I hope y'all won't mind if we continue on the topic of water quality.

Not long after I joined the forum in November 2011, I discovered an article on acidification written by Elton Roberts and Malcolm Burleigh. (I'm sure that a good number of our members are familiar with his work.) Then as I started acidifying my tap water in early 2012, I wanted to get a bead on the water quality here in Los Angeles. The most recent water quality report I could find was the 2010 report from the Southern California Metropolitan Water District, and here's some data from that report:

MWD_alkalinity_and_Calcium2011.jpg (37 KiB) Viewed 2644 times

MWD_hardness_and_pH2011.jpg (32.19 KiB) Viewed 2644 times

Thanks to what I learned from Jerry, I know the difference between temporary and permanent hardness, so let's dig a little deeper here. Per the Hach website:

"The amount of carbonate vs. non-carbonate hardness can be found by measuring alkalinity. If the alkalinity is equal to or greater than the hardness, all of the hardness is carbonate. Any excess hardness is non-carbonate hardness." When I started using a Milwaukee Instruments pH 600 calibratable pen-style meter, I tested my tap water and got a pH of 7.9. Extrapolating from the data in the MWD alkalinity and hardness tables, we can see that there is (or at least was) a combination of temporary and permanent hardness in my tap water.

Not sure exactly when it happened or why, but the water coming out of my tap has been a steady pH of 7.0 ever since I tested it in 2020. And when I ran the test, I was horrified to find out that I had been over-acidifying. No serious harm done, so I made darn sure that I wouldn't make that mistake again -- pH of my acidified water no lower than 5.0, periodic testing to verify, and that's it! As soon as I start using the GH ferts, I'll be very careful about A. testing the watering solution before I acidify, and B. adding the right amount of 5% white vinegar as the acidifier so that I get a final pH in the 5.0-5.5 range before I water the cacti.

After downloading the 2022 MWD water quality report a few moments ago, I pulled out the relevant data:

MWD_relevant_data2022.jpg (54.42 KiB) Viewed 2644 times

This brings up 3 questions, and the answers should be able to guide me on what to do when I start using the GH ferts:

• Does the 7.0 pH coming out of my tap mean that the hardness of my water is entirely permanent?

• If so, can my cacti take up the Ca and Mg in the acidified tap water?

• If the answer to question #2 is "yes", would the plants still benefit from adding the TPS CalMag to the GH ferts?

Just a reminder on 2 things -- first, the amount of Potassium sulfate stock solution going into the FloraMicro-FloraBloom watering solution will be the same as it was with the Dyna-Gro 7-7-7 watering solution. And second, I grow 64 of my 68 cacti in a soil-less 60% pumice and 40% granite gravel mix. I don't know if either of these facts would affect the answer to question #3, but I wanted to bring them up just in case.

On the matter of acidification, we need to be aware of a phenomenon called "pH rebound". I tested this with my soil-less mix in 2014 (bearing in mind that the pH of my tap water was 7.9):

• 1/2 tsp. of the 7-7-7 and 1 tbsp. of vinegar in a gallon of watering solution, pH tested at 5.5.

• I saturated a potful of the pumice and granite gravel mix with plenty of runoff water for testing, pH tested at 6.1, so I knew that the mix is slightly alkaline. (That surprised me -- I thought it would be pH-neutral.)

• Testing the runoff water, it rebounded up to 7.0 after 6 days.

Is it possible that I should acidify rainwater too? I'll leave it as a rhetorical question for now, although we may get an answer with something else I'd like to test -- the final pH of the dilute FloraMicro and FloraBloom in a gallon of rainwater. With plenty of prime Grade A "sky juice" already stored, I'll have the opportunity in spring.

[ohugal]

It's like I'm back in school.

Because we're adding so many products to our water, is the order important? Would be good to know before I start testing my watering solution and eventually the run-off of my soil mix, also 60% pumice : 40% gravel mix. Although I add diatomaceous earth (powder form). I would be testing after each step and the order is now:
1) tap water (1l)
2) acidify (viewtopic.php?t=43525&start=45)
3) fertilizer (0,92 ml GHE FloraMicro + 0,92 ml GHE FloraBloom)
4) potassium sulfate (38,8 ml potassium sulfate stock solution (1,84 gr/l))

I also got some data from the water company regarding our tap water:


pH : 7,1 (viewtopic.php?t=43525&start=45)
general hardness : 15°F = 8,4°D = 10,5°E = 150 mg CaCO3 (calcium carbonate) per liter = 60 mg calcium per liter

Calcium = 270 mg/l
Magnesium = 50 mg/l

(1°F = 0,56°D = 0,7°E = 10 mg CaCO3 (calciumcarbonaat) per liter) = 4 mg calcium per liter)

How do I interpret this? I didn't exactly excel at math and chemistry back in the day.

[Steve Johnson]

It's like I'm back in school.

Right there with you -- and sometimes I feel like I'll never graduate!

Would be good to know before I start testing my watering solution and eventually the run-off of my soil mix, also 60% pumice : 40% gravel mix. Although I add diatomaceous earth (powder form). I would be testing after each step and the order is now:
1) tap water (1l)
2) acidify (viewtopic.php?t=43525&start=45)
3) fertilizer (0,92 ml GHE FloraMicro + 0,92 ml GHE FloraBloom)
4) potassium sulfate (38,8 ml potassium sulfate stock solution (1,84 gr/l))

Wrong order. It should be:
1) Tap water.
2) Fertilizer and Potassium sulfate stock solution in the amounts you just specified.
3) Diatomaceous earth in whatever amount you're using. By the way, why are you using it?
4) Test the pH of the runoff water (that's the "slurry test" I described in my acidification how-to thread).
5) Add a small amount of the acidifier you'll go with (either 5% white vinegar or citric acid, take your pick), then test the final pH. Your target
pH range should be 5.0-5.5. If it's above 5.5, add a smaller amount in increments until you hit the target. If it goes below 5.0, dump out the watering solution and start all over again.
Once you determine the correct amount of acidifier that hits your target, write it down and tape it to your watering can so you don't forget. Since the pH of tap water may tend to fluctuate, I highly recommend that you test your watering solution every month or two in case you'll need to adjust the amount of acidifier. If you ever change your mix (or use different mixes for different species), you'll need to run the "slurry test" again. If you don't, you're set. I realize all this is "labor-intensive", but as I said earlier, those of us who aren't so casual about optimizing the growing quality of our cacti over the years won't mind taking the time to do it properly.

I didn't exactly excel at math and chemistry back in the day.

I'm right there with you too. Hopefully someone with better math and chemistry skills (think that'll be Jerry) will interpret the water quality data you and I have posted.