Question of the Day Current thinking on the Ca:Mg:K ratio?

[Art]

We know very little​

We know that we know very little about our plants when it comes to some things. For example, it is not possible to determine the exact amount of nutrients that our plants need to grow well due to the enormous amount of variables involved in our aquariums that may impact this.

We don't limit to control algae​

We've moved on from trying to limit a particular nutrient in the hopes that this would limit algae more than our plants. We've proven that this is not a good way to limit algae. For me, it was @plantbrain who popularized focusing on growing healthy plants and not worrying so much about nutrients. At least in the USA.

We think consistency and availability are important​

We also seem to have concluded that we do need to have most of the nutrients plants need available to them in the aquarium. We provide a complete macro and micro fertilizer and job done. We really don't need to know how much the plants are drawing from the available pool. We just care that there is enough so that they can pull what they need. So, consistency and availability of nutrients is important.

We think ratios don't matter??​

We seem to have discarded nutrient ratios as not relevant for planted aquariums. There are plenty of very successful hobbyists growing great plants that don't worry about the ratios.

However...

A study has shown that, at least for certain aquatic plants, they need much more potassium than they do magnesium (8:1). Also, that more magnesium than calcium causes growth problems. For Lemna minor and Elodea it appears that a Ca:Mg ratio of 5.6:1 is ideal.

Based on the above, should we be making sure that our fertilizer contains roughly five times more calcium than magnesium? And, eight times more potassium than magnesium?

Would really love your thoughts on this.

 

[Count Krunk]

I guess a better question, what ratio are people running right now? Either on purpose or just due to their dosing.

Test and tell people! Haha

 

[Burr740]

Based on the above, should we be making sure that our fertilizer contains roughly five times more calcium than magnesium? And, eight times more potassium than magnesium?​

Ive tried out various ratios of all these together rather extensively, using a wide range of plants and conditions. Including the easiest low tech plants to the most sensitive plants in the hobby, various substrates from fresh soils, old soils and inert blasting sand, and KH ranging from 0 to about 6.

But I have not done this with methods controlled enough to hold up to even the lowest scientific standards. All I can confidently say is what my experience has been. In some cases people like Vin and Gregg did the same things along with me, and got the same results. But still my current level of understanding remains strictly anecdotal

Having said all that, this is what I think based on personal observation

These three nutrients are strongly related to each other in how the level of one affects the availability/use of the other two. Especially Ca and K

We dont need as much Mg as we think. 4-5 ppm is enough even with Ca and K at 40+. Higher Mg, even up to 2:1 Ca:Mg, doesnt seem cause noticeable issues, it just isnt needed

I used to think NO3:K was the most significant factor. My tanks have always needed K to be at least equal to or higher than NO3, higher is better. But I suspect its more the relationship between Ca and K, where K needs to be at least roughly equal to Ca. Mulders chart indicates K has a strong relationship with both, while NO3 and Ca do not directly affect each other. In most of our tanks Ca is the highest nutrient present, in some tanks its NO3. I dont think that matters. It's probably whichever one is the highest that K should be equal to (Ca) or slightly greater than (NO3)

K is the last nutrient we want to have limited. Its best to keep it among the highest nutrients in the tank

K being too low can cause numerous issues that show up way before the textbook pinholes appear. Most notably pale tops and a drastic reduction in size. This is especially prelevant in green stem plants, which are usually the first ones to act up when K is too low

 

[Burr740]

We think ratios don't matter??​

Im not sure there's an exact precise ratio of anything that will work best for all tanks. But I do believe having ratios in general within in a certain range is absolutely important. If ratios didnt matter we'd all be dosing 40/40/40 and talking about something else right now

 

[SkaleyAquatics]

I guess a better question, what ratio are people running right now? Either on purpose or just due to their dosing.

Test and tell people! Haha

Currently running Ca:Mg:K at 50:10:40. But After reading @Burr740 response I am wondering if I need to increase potassium.

I have seen decreased size in some plants. Either that or go drop down on my Ca and Mg, the only reason I increased to 50:10 was because of curling in what I believed was Hygro. Siamensis 53b but I do not have that problem with any plants besides H. salicafolia, even still after the 50:10 ratios. The first plant that comes to mind in small size is myriophyllum tetradrum 'green' its nothing like the roirama I have. The other plant is Ludwiga Inclinata 'Meta' Plant was doing great and now is extremely small in size and I have not changed anything in my setups in months. I however, do not want to drastically change anything as both of those plants are in farm tanks while plants in the main tank are doing great currently but if I am seeing issues in farm tanks then how long before I start seeing issues in the main tank? Something I will have to consider.

Im not sure there's an exact precise ratio of anything that will work best for all tanks. But I do believe having ratios in general within in a certain range is absolutely important. If ratios didnt matter we'd all be dosing 40/40/40 and talking about something else right now

I firmly believe that we at least need a baseline on somewhere to start.

 

[Burr740]

Currently running Ca:Mg:K at 50:10:40. But After reading @Burr740 response I am wondering if I need to increase potassium.

I have seen decreased size in some plants. Either that or go drop down on my Ca and Mg, the only reason I increased to 50:10 was because of curling in what I believed was Hygro. Siamensis 53b but I do not have that problem with any plants besides H. salicafolia, even still after the 50:10 ratios. The first plant that comes to mind in small size is myriophyllum tetradrum 'green' its nothing like the roirama I have. The other plant is Ludwiga Inclinata 'Meta' Plant was doing great and now is extremely small in size and I have not changed anything in my setups in months. I however, do not want to drastically change anything as both of those plants are in farm tanks while plants in the main tank are doing great currently but if I am seeing issues in farm tanks then how long before I start seeing issues in the main tank? Something I will have to consider.

53B is a lot more touchy than other hygros and is especially picky about K. Pure speculation from way over here, but what you describe sounds like it needs more K, or the ability to better use whats there. 40 ppm should be plenty

Can you control Ca or are you stuck with 50 from tap water? 35-40 should be plenty for any plant in any type of tank. And you can drop Mg down to about 5 or 7, but its probably the Ca. Adjusting those two down a little should increase the availability of the existing K

All this of course assumes your co2 is good and NO3 isnt limited. But Im confident you know what good co2 looks like, so unless youre running low no3 on purpose it just about has to be K related imho

 

[SkaleyAquatics]

Can you control Ca or are you stuck with 50 from tap water? 35-40 should be plenty for any plant in any type of tank. And you can drop Mg down to about 5 or 7, but its probably the Ca

I can adjust it down. I just finished off my storage water for the week and after reading this had decided to adjust to Ca to 35 and Mg to 7.

All this of course assumes your co2 is good and NO3 isnt limited. But Im confident you know what good co2 looks like, so unless youre running low no3 on purpose it just about has to be K related imho

Definitely know what good co2 looks like. No3 is pumped into 30ppm between water changes with half coming after a water change then the rest in 2 doses on tuesday and thursday. I have not checked No3 levels with a test kit but would be curious what they are.

 

[Burr740]

I can adjust it down. I just finished off my storage water for the week and after reading this had decided to adjust to Ca to 35 and Mg to 7.

Be sure to give an update somewhere if you notice any difference. Im personally very curious and this kind of info from somebody that actually knows what theyre doing is great for the hobby in general

 

[cjday115]

I see a lot of talk regarding Potassium (K+) as possibly the most important nutrient in all this talk of ratios, but isn't potassium in notoriously poor amounts in the majority of water systems that don't originate from near areas of volcanic activity? One would think that sodium also plays a significant role as a 1+ cation, and it's relatively abundant, yet we rarely if ever dose sodium, except as a byproduct from dissolving whatever anion is ionically bound in the salt. Sodium plays a critical role in osmoregulation. While Potassium is an important micronutrient for biological processes besides plants, could it not also be that in our tanks that often utilize RO water, that the excess potassium is compensating for the lack of sodium?

My local waters according to the USGS have ~3-4ppm Potassium and ~37-38ppm Sodium, which when taken together is suspiciously close to the 40ppm Potassium that @Burr740 has observed to be good for growth. Although this might be due to plant life sequestering potassium out of the water and inside the cells and interstitial fluid.

I could be totally wrong, and I'm open to correction. I just figured I'd contribute my thoughts to the discussion. Obviously our tanks are very different environments than the Great outdoors, but it leaves me wondering..

 

[Count Krunk]

Not sure where you're located but you have to think of that. Certainly none of these plants live where I'm at, rural VA. It gets much too cold.

Also, how many of these plants are coming from the wild vs being grown in facilities? Assuming they're being grown, mostly. The plants have likely adapted to aquarium life, nutrients wise.

In reefing, corals tht are aquacultured or fragstock from a parent coral that was aquacultured or has been in an aquarium for many years do better than recently collected wild specimens. They are adapted to what tanks can provide, which is certainly lesser than what the oceans can.

 

[cjday115]

Not sure where you're located but you have to think of that. Certainly none of these plants live where I'm at, rural VA. It gets much too cold.

Also, how many of these plants are coming from the wild vs being grown in facilities? Assuming they're being grown, mostly. The plants have likely adapted to aquarium life, nutrients wise.

I'm willing to bet that it is better for the plants to use potassium over sodium as high sodium inhibits many enzymatic processes, as well as sodium more easily passing through ion channels as it is a smaller ion than potassium. My musing is more related to potassium being utilized in such higher concentrations than natural possibly because there is a lack of other 1+ cations like sodium in our aquariums, when in nature the smaller atom is more prevalent.

In other words: Do plants need high potassium if there is sufficient sodium present? How much of that potassium is actually providing benefit beyond osmoregulation? It's more a question of optimal levels vs "scraping by" on my part.

Also as to where I am, I'm in West PA, and many plants we commonly see in aquaria originate and/or thrive in the temperate zones such as ours, for example, a few Ludwigia species are native to my region such as L. palustris. Also Rotala ramosoir and some Eriocaulons.

 

[Count Krunk]

Hmm i thought these were mainly tropical. I haven't seen any in the local water ways haha.

 

[Anonymous_One]

While Potassium is an important micronutrient for biological processes besides plants, could it not also be that in our tanks that often utilize RO water, that the excess potassium is compensating for the lack of sodium? ...

Unfortunately, I can't find the scientific study that said this right now (I'll try to find it when I have more time), but sodium (Na) and potassium (K) are supposedly mutually substitutable to some extent. So, when potassium is deficient, sodium can substitute (replace) it to some extent. But sodium is not (unlike potassium) an essential nutrient. Anyway, in that study, the researchers found in the few aquatic plants tested that even though they were able to do without potassium (and replace it with sodium), their overall condition suffered greatly. In other words, if you replace potassium with sodium, the plants are likely to be in significantly worse shape. But the opposite was not true. For reference, I'll add the normal concentrations of potassium and sodium in the Amazon (source: https://doi.org/10.1029/2008GL034619):

• groundwater springs: 1.0 ppm Na, 1.5 ppm K, 0.5 ppm Ca, 0.6 ppm Mg (~77 ppm CO2)
• 1st order streams (mountains): 2.4 ppm Na, 2.7 ppm K, 2.1 ppm Ca, 1.4 ppm Mg (~9 ppm CO2)
• 2nd order streams (valleys): 4.5 ppm Na, 4.8 ppm K, 4.4 ppm Ca, 3.0 ppm Mg (~9 ppm CO2)

 

[cjday115]

Thanks @Marcel G, this answers my questions on the matter to a degree.

I would be interested in testing to see where a general cutoff point would be, but for all our purposes, it doesn't matter.

 

[Anonymous_One]

If I may add something, @cjday115, I personally do not believe that there are any aquatic plants that require potassium concentrations higher than a few ppm. If someone is having trouble growing aquarium plants and fixes it after a significant increase in potassium concentration (say to 40 ppm), then I suspect the problem lies elsewhere. In other words, the potassium problem is a placeholder, with the root cause of the problem likely to be something else. The reason I think this is because of the generally low potassium concentrations in natural waters, where the light intensity is several times greater (500-1500 µmol PAR underwater) ... although we need to take into account the nutrients in the sediment also (but most plants seem to prefer to take up potassium from the water column). Some aquarists do use CO2 concentrations many times higher (i.e. 30-70 ppm in planted tanks vs. 5-10 ppm in nature), but I wouldn't say that elevated CO2 concentrations can create such a disproportionately high demand for potassium (e.g. 40 ppm). It would be good to test this, of course, but I don't think that would be entirely easy in our conditions.

Another important guideline is that plants contain essential nutrients in a certain proportion, and that large variations in external nutrient concentrations (= in water column or sediment) are reflected only by relatively small variations in their tissue (or dry matter). Assuming that a plant requires an external nitrogen concentration of e.g. 30 ppm NO3, this corresponds to approximately 5 ppm K. There is simply no point in adding more potassium because the plant does not need (utilize) it. It may accumulate it, but it won't do it any good. [The only two cases where a plant may actually need such a high concentration of potassium (e.g. 40 ppm) is if it needs it for osmoregulation (with a significantly skewed internal charge balance), or if the nutrient ratios in the external environment are somehow significantly distorted, causing some antagonisms.] Of course, the same is true with other nutrients. Increasing the CO2 concentration from, say, 10 ppm to 70 ppm is, in my opinion, not capable of pushing the nitrogen requirement to more than the 30 ppm NO3. Some weedy aquatic plants may be able to use a little more, but the difference between that 30 ppm NO3 and say 100 ppm NO3 is likely to be minimal. I.e., increasing the nitrogen concentration from 30 ppm NO3 to 100 ppm NO3 will, in my opinion, only result in a minimal increase in photosynthetic rate (say, only 10%) ... in this kind of plants. [For other plants, such high concentrations can already lead to their decline.]

 

[Burr740]

If I may add something, @cjday115, I personally do not believe that there are any aquatic plants that require potassium concentrations higher than a few ppm. If someone is having trouble growing aquarium plants and fixes it after a significant increase in potassium concentration (say to 40 ppm), then I suspect the problem lies elsewhere. In other words, the potassium problem is a placeholder, with the root cause of the problem likely to be something else. The reason I think this is because of the generally low potassium concentrations in natural waters, where the light intensity is several times greater (500-1500 µmol PAR underwater) ... although we need to take into account the nutrients in the sediment also (but most plants seem to prefer to take up potassium from the water column). Some aquarists do use CO2 concentrations many times higher (i.e. 30-70 ppm in planted tanks vs. 5-10 ppm in nature), but I wouldn't say that elevated CO2 concentrations can create such a disproportionately high demand for potassium (e.g. 40 ppm). It would be good to test this, of course, but I don't think that would be entirely easy in our conditions.

Another important guideline is that plants contain essential nutrients in a certain proportion, and that large variations in external nutrient concentrations (= in water column or sediment) are reflected only by relatively small variations in their tissue (or dry matter). Assuming that a plant requires an external nitrogen concentration of e.g. 30 ppm NO3, this corresponds to approximately 5 ppm K. There is simply no point in adding more potassium because the plant does not need (utilize) it. It may accumulate it, but it won't do it any good. [The only two cases where a plant may actually need such a high concentration of potassium (e.g. 40 ppm) is if it needs it for osmoregulation (with a significantly skewed internal charge balance), or if the nutrient ratios in the external environment are somehow significantly distorted, causing some antagonisms.] Of course, the same is true with other nutrients. Increasing the CO2 concentration from, say, 10 ppm to 70 ppm is, in my opinion, not capable of pushing the nitrogen requirement to more than the 30 ppm NO3. Some weedy aquatic plants may be able to use a little more, but the difference between that 30 ppm NO3 and say 100 ppm NO3 is likely to be minimal. I.e., increasing the nitrogen concentration from 30 ppm NO3 to 100 ppm NO3 will, in my opinion, only result in a minimal increase in photosynthetic rate (say, only 10%) ... in this kind of plants. [For other plants, such high concentrations can already lead to their decline.]

I agree with a lot of this. But I dont think we can relate what nutrients exist in their natural environment to what our tanks need to have

NO3/K/CO2 doesnt exist in a vacuum. Mulders Chart is absolutely relevant. Notice how much Ca is in those amazon lakes, its extremely low too. Mg on the other hand is relatively high in comparison to K and Ca. Id be curious about PO4, B and Cu

I dont think any co2 injected tank can do well with only a ppm or two Ca. Do you? The lowest I can recall ever with good results is about 15. Even Dennis' highly successful low dosing method (more specifically low NO3) runs 20-30 ppm Ca. Idk exactly, but Ive seen him recommend 40 minimum for ramosior florida.

It sounds wrong because the plants in our tanks arent burning through 30 ppm of Ca in a week (or K or Mg or even PO4) Not even close. I had icp tests done a few years ago, t post water change, mid week and end of week, on 3 different tanks. Ca barely moved, K only dropped 3-5 ppm. Both were front loaded post water change (all at once for the entire week)

But for whatever reasons, and admittedly I dont know what the reasons are, beyond a relative balance of all nutrients (ionic balance certainly plays a role). But for whatever reasons, our tanks show better results when certain levels of these nutrients are present. And its fundamentally unrelated to what the plants actually uptake and use, or whats in their natural environment

 

[Anonymous_One]

@Burr740, I only mentioned it to show that even such low concentrations are sufficient for [at least some] plants in nature. So these plants demonstrably do not need higher concentrations of these nutrients. That other plants may need or benefit from higher concentrations I do not dispute. At the same time, I recognize that what plants actually consume (uptake) may not be the same as what they need to have in the external environment (e.g. water column). In other words, the amount that plants consume is certainly not the same as the external concentration. So, for example, at an external concentration of 5 ppm K (in water column), plants will only consume 1 ppm K, whereas at an external concentration of 20 ppm K they may uptake the 5 ppm (which is the optimal amount for them). But what is the relationship between external concentration and real consumption I don't know myself. But I would be cautious in saying that plants require e.g. 40 ppm K. Plants realistically require at most 5 ppm K. If we have to add more to cover their need, then I would wonder (think, ask, seek) why. Maybe it really is necessary in certain circumstances, but maybe it isn't. Few aquarists have tried testing the same plant at different potassium concentrations (and different ratios of other nutrients, for example ... or different pH/HCO3 etc.) to be able to draw that conclusion. But anyway, it's good to discuss it.

 

[Burr740]

@Burr740, I only mentioned it to show that even such low concentrations are sufficient for [at least some] plants in nature. So these plants demonstrably do not need higher concentrations of these nutrients. That other plants may need or benefit from higher concentrations I do not dispute. At the same time, I recognize that what plants actually consume (uptake) may not be the same as what they need to have in the external environment (e.g. water column). In other words, the amount that plants consume is certainly not the same as the external concentration. So, for example, at an external concentration of 5 ppm K (in water column), plants will only consume 1 ppm K, whereas at an external concentration of 20 ppm K they may uptake the 5 ppm (which is the optimal amount for them). But what is the relationship between external concentration and real consumption I don't know myself. But I would be cautious in saying that plants require e.g. 40 ppm K. Plants realistically require at most 5 ppm K. If we have to add more to cover their need, then I would wonder (think, ask, seek) why. Maybe it really is necessary in certain circumstances, but maybe it isn't. Few aquarists have tried testing the same plant at different potassium concentrations (and different ratios of other nutrients, for example ... or different pH/HCO3 etc.) to be able to draw that conclusion. But anyway, it's good to discuss it.

My main point was having 5 ppm K may work in nature where Ca is 2 or 3 ppm, but when Ca is 30 in our tanks the plants cant "get" that 5 ppm, it has to be closer to Ca (and relative to other antagonistic nutrients) Both Mulders Chart and ionic balance principals also suggest this is logical

 

[macek.g]

So, well, my experiments in my Dutch tank are somewhat overturning some theories here, or general assumptions.

For about 3 years I have been running a tank where I supply negligible amounts of K, it is about 0.1-0.2 K per week, Mg similarly per week
However, I change RO water
Ca -20 25, Mg 6-7 ,K-1 (and I also very often exclude K from this)

It has been almost 3 years where my Ca-Mg-K supply looks more or less like this
When I read Dennis Wong's article a few years ago about higher Ca -40-50 ppm for Rotala Ramosior Florida, I started to laugh a little.
It even seems to me that Rotala Sunset has higher requirements in this respect (higher Ca)
We are omitting one quite important fact, the higher the Ca, the greater the micro protection and this is shown by all kinds of graphs, not only Mulders, Ca-Micro antagonism is common.
Well, no matter how you look at it, we have different experiences.
As for K, I agree with Marcel, Maq
K is taken up by plants preferentially in reserve, for natural reasons
Of course, there are exceptions, as Burr mentioned, K deficiencies are very quickly visible on plants of the Hygrophilla species (polysperma, Siamensis, corymbosa, etc.)
Just as the exceptions in the case of Na, plants of the Valisneria type, which are C4 photosynthetic

 

[Burr740]

So, well, my experiments in my Dutch tank are somewhat overturning some theories here, or general assumptions.

For about 3 years I have been running a tank where I supply negligible amounts of K, it is about 0.1-0.2 K per week, Mg similarly per week
However, I change RO water
Ca -20 25, Mg 6-7 ,K-1 (and I also very often exclude K from this)

It has been almost 3 years where my Ca-Mg-K supply looks more or less like this
When I read Dennis Wong's article a few years ago about higher Ca -40-50 ppm for Rotala Ramosior Florida, I started to laugh a little.
It even seems to me that Rotala Sunset has higher requirements in this respect (higher Ca)
We are omitting one quite important fact, the higher the Ca, the greater the micro protection and this is shown by all kinds of graphs, not only Mulders, Ca-Micro antagonism is common.
Well, no matter how you look at it, we have different experiences.
As for K, I agree with Marcel, Maq
K is taken up by plants preferentially in reserve, for natural reasons
Of course, there are exceptions, as Burr mentioned, K deficiencies are very quickly visible on plants of the Hygrophilla species (polysperma, Siamensis, corymbosa, etc.)
Just as the exceptions in the case of Na, plants of the Valisneria type, which are C4 photosynthetic

Chris Hendy (think he's on here) also has a smoking tank and runs K drastically lower than no3, but as low as you describe. His routine always baffled me

Outliers seem to exist in this hobby no mater what the subject is. But Id be hard pressed to believe 1 ppm K would be enough for any tank with plants much less a co2 injected one