# [Wet Thumb Forum]-K2SO4 why not?



## Chousal (Feb 8, 2004)

I've been reading several posts in this forum, you all dont like to use K2SO4, I've looking for the answer, but i didnt find it.

Could you tell me if its use is not good and why?

Saludos...

Acuariofilia most not be costly.


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## Chousal (Feb 8, 2004)

I've been reading several posts in this forum, you all dont like to use K2SO4, I've looking for the answer, but i didnt find it.

Could you tell me if its use is not good and why?

Saludos...

Acuariofilia most not be costly.


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## gpodio (Feb 4, 2004)

I haven't been following all the posts lately however I use K2SO4 and can't see any reason why not to. I know there have been some threads about maintaining a lower K level than previously suggested however K2SO4 as a source of K is just as valid as other sources. Perhaps someone suggested than seeing there is K in KNO3 and your trace mix that additional potassium sources are unnecessary?

Regards
Giancarlo Podio


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## Roger Miller (Jun 19, 2004)

There is no problem fertilizing with K2SO4 unless you are also fertilizing with other potassium sources -- especially potassium nitrate.

A lot of people use potassiuim nitrate as a nitrogen source. Plants use potassium and nitrogen in a ratio of about 1:1 (K:N) and potassium nitrate supplies them in a ratio of nearly 3:1. That means that potassium from potassium nitrate alone exceeds any possible plant demand. Additional potassium from K2SO4 is just going to build up in the tank.

If your tank has a substantial source of nitrogen other than potassium nitrate (heavy fish feeding, for instance) then the potassium from potassium nitrate may not be as excess as it is with just potassium nitrate as a source.

The situation might be a little different if you have a nitrogen source in addition to potassium nitrate *and* that source provides more than 2/3 of the total nitrogen supply *and* that source provides no potassium. In that case potassium could be in short supply and it might make sense to use potassium sulfate to give your plants some extra potassium. Fish feeding does provide some potassium, so the source probably has to be something other than fish feeding.

Roger Miller

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_"The indispensible first step to getting the things you want out of life is this: Decide what you want" -- Ben Stein_


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## Planted Engineer (Jan 15, 2004)

Roger,

Could you just explain why KNO3 means K:N of 1:1 ??? There's one atom of K and one atom of N. so this is why I am confused.

Also based on which paper are you saying that the ratio should be 1:1 ?

PE.


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## imported_George (Mar 28, 2003)

Roger,

My question may just be an extension of the Planted Engineer's second question concerning the K:N ratio (1:1) and the answer may be obvious but it eludes me at the moment. If plants utilize K and N in approximately the same ratio, why is the recommended level of K 15-20 ppm and NO3 5-10 ppm? Does it have to do with maintaining a K ratio that is appropriate for macros/micros other than N?

George


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## Chousal (Feb 8, 2004)

Thank you very much for your answers

Ok, I like to provide the most part of N "naturaly", with a little high population, a good bacterial colony, and regular feeding.

I would like to add: if the tank has a good ilumination, plants would prefer 20-25 mg of NO3, this would explain the right proportion 1:1 K:N.... I have cheked this in my acuarium.

Saludos...

Acuariofilia most not be costly.


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## Roger Miller (Jun 19, 2004)

PE,

I don't think I understand your question.

The composition of an average plant has a K:N ratio that is - in very round numbers -- about 1:1. To be less round about it the ratio is actually titled more to nitrogen. The K:N ratio in the most common "average plant" composition that I've seen published is about 1:1.3. In the few analysis I've seen for aquatic plants the ratio is:

Elodea occidentalis critical concentration ... 1:2 (cited in Diana's book)
Cabomba sp. 1:1.2
Ruppia maritima minimum 1:1.6
Ruppia maritima maximum 1.6:1

And in algaes the ratios is:
"Standard" aquatic plant 1:3.6
Marine phytoplankton 1:4.4
Freshwater Algae turf ~1:4 to

It appears that the K:N ratio in algae is much lower than in plants.

These (except Diana's reference) are all available on line.

Cabomba: 
http://www.wb2020.qld.gov.au/icm/mrccc/publications/Cabomba%20Pilot%20Study_03_2000.pdf

Ruppia maritima:
http://www.npwrc.usgs.gov/resource/literatr/ruppia/ruppia.htm

"Standard" aquatic plant (I think actually referring to algae) and Average marine photoplankton:
http://soest.hawaii.edu/oceanography/faculty_html/yhli/yhli_Tables.html

Freshwater algae turf
http://molecol.ifas.ufl.edu/images/wilkie2.pdf

The average plant composition can be had from any number of sources. I used:
http://www.greenhouse.crc.org.au/crc/ecarbon/publications/nee/chapter10_nutrientuptake.pdf
and converted from moles to weight.

If someone (PE?) really wants to delve into the question then there are a lot of published analyses of aquatic plants that could be used. Getting them is a matter of getting to a library and digging them up. There aren't that many on line.

The K:N ratio in KNO3 is 1:1 on a molecular basis. The weight ratio is 39:14, or approximately 3:1.

George,

I can't possibly defend someone else's recommendation about "advised" K and NO3 concentrations. Tom Barr put this together along with a few other folks from the SFBAAPS. I'm pretty sure that the relative use rates of K and N were not considered when that advise was formulated. I'm also pretty sure there was a prevailing belief that there was no "down side" to overdosing K.

The advised values of 20 ppm K and 10 ppm NO3 give a K:N ratio of almost 9:1; 15 ppm K and 5 ppm NO3 gives a K:N ratio of almost 14:1. I have no idea why someone would use those numbers.

Roger Miller

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_"The indispensible first step to getting the things you want out of life is this: Decide what you want" -- Ben Stein_

[This message was edited by Roger Miller on Tue February 24 2004 at 07:38 PM.]


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## imported_George (Mar 28, 2003)

Thanks for your reply, Roger. However, I certainly was not expecting to read that response. I had seen similar "recommended levels" referenced so many times in this forum and others that I assumed they were generally ACCEPTED.....NO?? Perhaps this has been discussed in other threads and I just missed it. At this moment, I'm just confused...damn confused.

Regards,
George


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## Robert Hudson (Feb 5, 2004)

George, thats because many, many average hobbyists follow what Tom Barr puts forth without question. Tom is the most dominating influence all over the internet. Most of us, myself included, are not qualified enough to know if he is right or wrong. Roger on the other hand has enough of a scientific background to understand all the ramifications.

Don't be confused, find out what works for you.

Robert
King admin
www.aquabotanic.com


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## Planted Engineer (Jan 15, 2004)

Roger,

I am not a scientist - I'm only an engineer... so this will be a stupid question - I guess. Could it be that we still need to dose more potassium because for the postassium to penetrate the plant - there should be much more of it in the water column. Could it be that the plant N:K is different than the water column N







:K??? Could it be that you need potassium osmotic pressure in order to penetrate the plant???? I know you will ask where would the rest of the K go. It will go during water changes.

Another thought - since algae needs much less potassium relative to N. wouldn't it be true that plants will starve for potassium way before algae ---> Algae bloom??? maybe that were the numbers came from????

I am asking all of this since I have already bought 50 pounds K2SO4....









PE.


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## Roger Miller (Jun 19, 2004)

> quote:
> 
> Originally posted by Planted Engineer:
> Could it be that we still need to dose more potassium because for the postassium to penetrate the plant - there should be much more of it in the water column.


No. Plants actively import potassium, they don't depend on potassium penetrating into the plant.



> quote:
> 
> Could it be that the plant N:K is different than the water column N
> 
> ...


The plant N:K should be different from the N:K ratio or concentrations in the water. Please note that I have been talking about matching the rate of supply for potassium and nitrogen against the rate of use. That is not the same things as talking about the potassium or nitrogen concentration in water. In fact, very few of us have any clue what the potassium concentration in the water actually is.



> quote:
> 
> Could it be that you need potassium osmotic pressure in order to penetrate the plant????


No. The osmotic pressure gradient is actually from the plant out, and it is substantial.



> quote:
> 
> Another thought - since algae needs much less potassium relative to N. wouldn't it be true that plants will starve for potassium way before algae ---> Algae bloom??? maybe that were the numbers came from????


Even with 2-3 ppm of potassium in the water we provide enough potassium that algae will not "starve" for the lack of it. This is not where the numbers come from. I don't think there ever was a specific justification for the potassium dosing level. If you go back to the dosing levels that Tom originally wrote up for SFBAAPS, it prescribes 20-30 ppm of potassium, but gives no justification for it at all.



> quote:
> 
> I am asking all of this since I have already bought 50 pounds K2SO4....


Got a lawn?

Roger Miller

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_"The indispensible first step to getting the things you want out of life is this: Decide what you want" -- Ben Stein_


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## imported_George (Mar 28, 2003)

Rober H & Roger,

Personally, I have never tried to maintain the K in my tank at the 15-20 ppm level....I just could not go there. Initially, there was a question as to the K level already in the tap water. A water analysis from the source well indicated 58 ppm. I do not think that the K level in the tap water is that high, or if it's even present, but I did not want to push it. So I only dose to maintain 6-8 ppm per week and it does not appear that the K:Ca ratios are at levels that cause a Ca deficiency. I do not have a particular reason for picking 6-8 ppm K but it seems to work O.K. in a high tech, heavily plant tank. Roger, because I was thinking that "15-20 ppm K" was the recommended or appropriate level, your response just surprised me. I was not aware that these are elevated levels and do not reflect the actual requirements of plants.

Also, I was not aware that Tom Barr, et.al. was the initial or original source for the K dosing levels. Does this include the other recommended levels for NO3, PO4, iron, etc. that are frequently specified. I have a short article written by Tom (DFWAPC) that proposes the K level of 20-30 ppm. I just figured he was pushing the K dosing from a lower, *generally accepted*, level of 15-20 ppm to 20-30 ppm and the other nutrient levels that were presented were just a re-hash of commonly know facts. Then there are the numerous referrals to Chuck's "calculator" that presents a recommended target level of K at 20 ppm. But enough of this.

I certainly understand that there is not a "cook book" method or hardline nutrient range that guarentees success in this hobby.

Yes, Robert I agree...Roger never cesses to amaze me at the depth of his understanding. Because Roger and I had similar academic training...geology...I believe that a large part of his knowledge and talent must come from a strong personal interest in a variety of subjects and are, in large part, self-taught.

Regards,
George


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## Roger Miller (Jun 19, 2004)

George,

A few years ago Dave Heubert (a Botany Prof at the U. of Manitoba) was the resident plant scientist on APD. He recommended that potassium should be 5-10 ppm. It makes your 6-8 look pretty good.

Steve Dixon, Tom Barr and others in the SFBAAPS started dosing higher potassium and liked its' effects in their tanks. I think they described the perceived effect as "sparkle." To this date I think that "sparkle" may be the only reason anyone has given for using potassium levels over 10 ppm.

At that time I think Steve was recommending 25 ppm, and Tom gave the range as 20-30 ppm. Since then recommendations have popped up here and there for levels mostly around 20 ppm. As far as I've heard, none of those recommendations came with a rationale. All of them fall into the category of "internet lore" -- a classification closely akin to "urban legend" and "old wive's tale."

More recently I think Tom accepted 10-20 ppm K as reasonable. That was a response to problems that Ghazanfar and others reported with calcium deficiencies that they linked to potassium levels.

Roger Miller

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_"The indispensible first step to getting the things you want out of life is this: Decide what you want" -- Ben Stein_


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