Top Dressing Moss with Calcium Carbonate

[Rick]

Ok here's some results from a trial I did with adding different amounts of CaribSea Cichlid Sand (aragonite sand, aragonite is almost pure calcium carbonate) to a fixed amount of NZ sphagnum moss in 100ml of RO water.

The amount of moss was the hardest to control. I compressed as much dry moss as I could pack into a 50mL measuring scoop, but after going into water and expanding, I could see that the 0.4gr exposure had more moss than the other exposures.

I was surprised that even the lowest dose of sand (only 0.2 grams) was more than enough to buffer the moss pH into optimal fertilizer uptake range, and actually put the alkalinity and conductivity (TDS) beyond what I would want to keep my plants in long term.

 

[cnycharles]

that might explain why some of my plants that I had made some media for a year ago (small/medium nz treated bark, sponge rock, charcoal and some other mixes with some aragonite sand) died. some was expected because they dried out too much, but I noticed that some big phals that were 'tall' with lots of stem roots, died off in the media and where the roots were growing through air down into the media. above the pot things looked 'okay'

I had lots of styrofoam peanuts around the outside of the media/plant in the big pots, so it's unlikely that everything got too wet down inside, especially since I often wasn't watering as much as needed for them. probably the roots burned because I put a fair amount of aragonite sand in the media and sometimes sprinkled it in; mix big wet/dry with too much ec....

my big, multi-growth delenatii that had been growing very well in a chc based media declined drastically in the high nz bark/aragonite based media (dele liking acidic and media being high ec and pH)

 

[gonewild]

I had lots of styrofoam peanuts around the outside of the media/

Styrofoam may be toxic because of residual manufacturing chemicals.

 

[gonewild]

Rick, in your test pots how did you get samples? Is there free water?
Was the sand mixed in or top dressed?

I wonder how the would relate if the moss was alive and growing?

 

[Rick]

Rick, in your test pots how did you get samples? Is there free water?
Was the sand mixed in or top dressed?

I wonder how the would relate if the moss was alive and growing?

Not pots, completely submerged/soaking in water. Actually about 1 inch of free water on top of the moss. (except for the 0.4 gr exposure that swelled almost to the water surface).

Not sure about the live moss thing, but that's really a whole separate question. Really just trying to look at typical potting mix ingredients and supplements.

We "accuse" sphagnum, and sometimes bark of being "strongly" acidic, and requireing buffering ammendments. I've always just done this by eyeball, and watching the plants. But if you just watch the plants, it may be too late by the time you think you have trouble.

Given how little aragonite it took to boost the pH, conductivity, and alkalinity, I now figure I was probably just getting enough alkalinity out of my diluted well water to keep the pH/alkalinity fine without any ammendment at all. But with ammendment, I have to water heavy with very low TDS water to flush out all the excess bicarbonate coming off the ammendment!!

 

[Ozpaph]

Thank-you. Very interesting. Will need to think about them for a while.

 

[Stone]

Shows that most people who think that many paphs need limestone to grow well probably dump huge amounts on their plants compared to what is actually needed to stabilize pH and provide some Ca. What is the mesh size of the sand? That's another very important consideration. The same amount (in weight) of a fine dust would give crazy results!

 

[Stone]

I wonder how the would relate if the moss was alive and growing?

Well you would kill the moss. I've noticed that live sphagnum turns very dark green and finally dies with very low additions of carbonate (and probably anything else leading to high TDS)

 

[ALToronto]

I can see how a very small amount of CaCO3 can raise the pH of pure water by 2 full points (100 times). But this relationship is not linear, and no matter how much aragonite sand is added to sphagnum, the pH will probably max out within a few decimal points of 8.

What does all of this mean for paphs that normally grow on limestone? They're effectively growing on CaO and CaCO3 with a bit of moss whose acidity is probably isn't doing anything to lower the pH and alkalinity. I get the argument that very little aragonite is necessary to raise pH to a reasonable level, but I don't see anything that indicates that an excessive amount of aragonite will do any harm, as long as it's used for limestone lithophytes (i.e. not delenatii)

 

[Rick]

What is the mesh size of the sand? That's another very important consideration. The same amount (in weight) of a fine dust would give crazy results!

Not dust, coarser than beach/play sand. Maybe 1/2mm grain size.

Yes, the higher the surface area to volume ratio the faster it dissolves, But this material is made for use in aquariums, so it doesn't go away very fast. I have some baskets that I added this stuff too over a year ago, and I still see it in the matrix.

 

[Rick]

What does all of this mean for paphs that normally grow on limestone? They're effectively growing on CaO and CaCO3 with a bit of moss whose acidity is probably isn't doing anything to lower the pH and alkalinity. I get the argument that very little aragonite is necessary to raise pH to a reasonable level, but I don't see anything that indicates that an excessive amount of aragonite will do any harm, as long as it's used for limestone lithophytes (i.e. not delenatii)

Actually if I were to use a coarse (like 3cm diameter) hard limestone gravel, or big solid boulder, it would not release near this amount of alkalinity this fast. I live in a karst limestone area, and the spring waters/creeks/streams in my area do not demonstrate TDS/alkalinity levels to this extent. I doubt that the limestone cliff dwellers in rainforest conditions (where it rains a lot more than it does in Tennessee) see any higher TDS/alkalinity levels. Deep wells are a whole different matter in karst. Folks do grow the calcarious paphs with tons of amendment, but run the racers edge on TDS accumulation, nitrogen management issues, and watering rates to ensure a low TDS buildup.

You've also identified another issue in that pH control is required for all potted plants, and lime addition to potting mixes has never been isolated to cliff dwellers. So how much lime are you going to add to the potting mix for your delenatii and callosum, or do you want them to go to 4.0?

 

[Rick]

Shoot just noticed a missing data point.

The alkalinity in the 0.2 gram exposure was 214 mg/L

So no aragonite = 10.2
0.2 gr aragonite = 214.0
1.0 gr aragonite = 336 mg/L

Also note the final pH in the 0.2 and 1.0 gram systems are very close (6.7 vs 6.8) but conductivity and alkalinity are about 150% greater at 1gr than for 0.2gr

Yes no matter how much sand you add, the pH will not go much greater than 8.0 su, but you will be burning roots from TDS, and the plant will not be able to utilize nitrate as a nitrogen source.

Also for reference, I took the 50ml of compressed moss out of the 100ml bottle it was in, and uncompressed. It filled a standard 2" (deep style) seedling pot to what I consider a normal density for potting up plants. So the useable amount of aragonite sand to use in a 2" pot of moss would be about 1/20th of a tsp of sand.

 

[abax]

Please translate this information to a non-chemist. I occasionally use sharp
builder's sand in baskets and now I'm a tad confused.

 

[Rick]

Please translate this information to a non-chemist. I occasionally use sharp
builder's sand in baskets and now I'm a tad confused.

The Cichlid Sand is a pH buffering sand made for use in African Cichlid and Salt water aquariums. Aragonite is one of the two pure forms of calcium carbonate. The other is calcite. Same basic chemical but different crystal forms.

But the Cichlid Sand is equivalent (kinda/sorta) to the oyster shell or dolomite/lime that everyone likes to use for pH support (or "supplying calcium") in orchid media that is based on acidic bark or sphagnum moss. Your builders sand is generally some type of fairly inert silica sand. Once its well rinsed, it doesn't really do anything chemistry wise, but helps drainage and water flow in the pot.

 

[Ozpaph]

OK had some time to think.

Rick, great experiment. I think the data applies to a static system. Could you consider the point that the initial high tds and pH changes might occur because there is a lot of freely soluble salt/buffer in the 'sand'?
What happens if you change the water and keep the same sphagnum and remaining 'sand' and add more water? I suspect the tds and buffering capacity fall off and reach a much lower steady state.
From the plant growing perspective I would expect early changes to the run through water 'content' but as one continues to water the plant much of the initial effect fades but there would be a persistent low level buffering effect on pH and calcium supply in the root zone. So in my mind I'm aiming for a 'low and slow' dissolution and influence from the 'sand' over the life cycle of my potting mix.
The question I ask is what is the best dosing period of adding the 'sand' to maintain an appropriate pH and supply supplemental calcium in the root zone.

Thanks for your thoughts.

 

[Stone]

I don't see anything that indicates that an excessive amount of aragonite will do any harm, as long as it's used for limestone lithophytes (i.e. not delenatii)[/

I would go as far as to say that no paphs need to be treated any differently with regard to pH or Ca need. As far as harm, you can have Fe becomming less available over pH 6.5. That usually the first thing to go wrong.

 

[Stone]

The question I ask is what is the best dosing period of adding the 'sand' to maintain an appropriate pH and supply supplemental calcium in the root zone.

The only way is to know your p/mix pH and the only way know that is to check and make the adjustments.
The standard is to add a 50/50 ground limestone/dolomite at 1-2 gms/L of mix if pH is just above 5 and 2-3 gms if its below 5. pH will vary according to fertilizers used so you should check every 3 to 6 months if growth is not what it should be. If growth is good, you don't need to do anything.

 

[Ozpaph]

The only way is to know your p/mix pH and the only way know that is to check and make the adjustments.
The standard is to add a 50/50 ground limestone/dolomite at 1-2 gms/L of mix if pH is just above 5 and 2-3 gms if its below 5. pH will vary according to fertilizers used so you should check every 3 to 6 months if growth is not what it should be. If growth is good, you don't need to do anything.

that sounds scientific

 

[ALToronto]

So how much lime are you going to add to the potting mix for your delenatii and callosum, or do you want them to go to 4.0?

My little delenatii seedling is still in the grower's 2" pot with bark chips; I bought it a month ago and haven't done anything with it yet. I think I will put it in lava rock or LECA (but not s/h; I don't have much success with that), just so that the medium pH doesn't degrade.

My limestone-dwelling paphs will be going on a special mount. I just got a big bag of hemp fibres, and I will be making a very lightweight and porous 'wall' with hemp fibres held together with a bit of my low-pH concrete. More fun experiments ahead.

 

[Rick]

OK had some time to think.

Rick, great experiment. I think the data applies to a static system. Could you consider the point that the initial high tds and pH changes might occur because there is a lot of freely soluble salt/buffer in the 'sand'?
What happens if you change the water and keep the same sphagnum and remaining 'sand' and add more water? I suspect the tds and buffering capacity fall off and reach a much lower steady state.

Yes this is a static system and you are bringing up the right questions. If you look at the higher sand dose rates, you can see that 5X the sand does not add 5 times the TDS and alkalinity. So that doesn't indicate a quick release of readily soluble salt, but dependent on what the acid from the moss is able to dissolve from the pool of sand. If this was something soluble like calcium or sodium chloride you would get a linear increase in conductivity with increasing dose. But seeing that all parameters level off its apparent that the acids from moss also leach out to some equilibrium of buffer too.

But changing the water will knock the TDS/alkalinity back down. In your pots, there is always some water held up in the moss, to release acid, which in contact with sand will release Ca and bicarbonte. Knowing that balance between water exchange/flush, and maintenance of good pH TDS is the tricky part for sure. At what point in your pot is the condition closer to static or dynamic? Mounted plants with no media is totally dynamic. The more water retentive the potting material, the more acidic.......all gets you closer to this all out static test. Monitoring TDS/conductivty is probably the fastest/easiest way to watch this in the GH. Spot checks with pH testing would be awesome too.

All these numbers would end up being site specific if you maybe use Chilean moss with oyster shell or dolomitic lime. But the aragonite is a fairly uniform material, and now we have some numbers to show how chemically active it is (rather than just speculating).

What I plan to do on the next trial, is to try different dilutions of my well water (maybe with a daily water exchange) to determine a minimal alkalinity input to offset moss acidiity without a pot ammendment.