K2S04 dosing when not dosing KNO3 - The Planted Tank Forum
 
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post #1 of 12 (permalink) Old 02-18-2009, 06:03 AM Thread Starter
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K2S04 dosing when not dosing KNO3

I don't dose KNO3 because my nitrates are 50ppm out of the tap, and I don't have enough growth to drive 'em down much. I dose K2SO4 at the same rate as KH2PO4 (amount varies by tank depending on lighting an CO2).

How much K comes from KNO3, and is my K2SO4 dosing making up for it, or should I dose more? I ask because I think I'm seeing some signs of K deficiency, but I'm not certain (old anubias and ludwigia leaves yellowing, getting holes, and decaying leaf edges).
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post #2 of 12 (permalink) Old 02-18-2009, 06:48 AM
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You need to dose more K2SO4. I dose about 1/4 tsp per 30 gallons along with 1/8tsp of KNO3. If you get high nitrates in tap water it need not be form KNO3 so you may not have any K content in the tap water. K2SO4 does not harm fish so even if you overdose it a bit it will not cause any problems.

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post #3 of 12 (permalink) Old 02-18-2009, 01:37 PM
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KH2PO4 adds little K, as its dosed in smaller portions to get a PO4 between 1-3, so as said about you will need to dose K2SO4.

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post #4 of 12 (permalink) Old 02-18-2009, 05:15 PM Thread Starter
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I know I have to dose K2SO4. What I'm looking for is specific guidance on how much K2SO4 to dose. For example, EI calls for 1/2 tsp KNO3 + 1/8 tsp KH2PO4 three times a week in a 40-60g tank. If I don't does the KNO3 because my nitrates are high, how much K2SO4 do I need to dose in order to make sure I'm adding enough K.
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post #5 of 12 (permalink) Old 02-18-2009, 06:30 PM
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I believe suggested levels for K is 20ppm for EI. However, you don't state the size of your tank, light intensity, plant load, co2 addition or fert schedule so no one can really tell you how much to add.


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post #6 of 12 (permalink) Old 02-18-2009, 07:10 PM Thread Starter
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I'll give you my tank info, but what I'm really looking for is a formula that tells you how much to supplement with K2S04 if you are not dosing with KNO3. For example, "for every tsp of KNO3 that you don't add, add 1/2 teaspoon of K2SO4".

Tank #1
  • 46g bowfront
  • 78w of T5H0
  • Daily Excel + DIY CO2
  • 1/8 tsp KH2PO4 3x weekly
  • 1/8 tsp K2SO4 3x weekly
  • 50% weekly water change
Tank #2:
  • 10g
  • 15w T8. Was running 2nd 15w for 4 hours a day, but had to temporarily move that light to another tank.
  • Daily Excel
  • About 1/4 recommend EI dosing of KH2PO4 + equal amount of K2SO4.
  • 50% weekly water change
Tank #3:
  • 6g
  • 8w over 6g + some indirect sunlight.
  • Daily Excel
  • About 1/4 recommend EI dosing of KH2PO4 + equal amount of K2SO4.
  • 50% weekly water change
#2 and #3 were getting about 1/4 EI, but I was seeing much worse signs of K deficiencies in these tanks than in Tank #1, plus GSA (phosphate deficiency), so I bumped up both K2S04 and KH2PO4 in these two tanks about a week ago. I pruned off all the affected leaves that looked K deficient, so it will be a while before I can see the results.

#1 did not have GSA, just what looked like K deficiency. So, I assume I should keep KH2PO4 the same increase K2S04. I was just wondering how much. Would 2x the amount KH2PO4 dosing I normally do be a good start. In other words, if I dose 1/8 tsp KH2PO4, then dose 1/4 tsp K2SO4?

[Edit: All 3 tanks have been planted for about 7 weeks]
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post #7 of 12 (permalink) Old 02-18-2009, 07:59 PM
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I doubt you are going to see any "for every tsp of KNO3 that you don't add, add 1/2 teaspoon of K2SO4" but for your larger tank about 6 grams should give you 20ppm K with your addition of kh2po4.

You should be able to take that amount and use it to figure out your needs for the other tanks.

There are also a bunch of calculators you can use to find out how much to add to your tanks.

http://www.csd.net/~cgadd/aqua/art_plant_aquacalc.htm

http://www.aquaticplantcentral.com/f...fertilator.php


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post #8 of 12 (permalink) Old 02-18-2009, 08:00 PM
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for your 46 gallon, 1/4 tsp of K2SO4 adds 4.12 ppm of K. I would think you would 1/8 - 1/4 tsp of K2SO4 inplace of KNO3.

A 1/4 tsp of KNO3 adds 2.89 ppm of K.

I used the fertilator on aquaticplantcentral to get those numbers.

John



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post #9 of 12 (permalink) Old 02-18-2009, 09:14 PM Thread Starter
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Thanks for the calc pointer. According to Chuck's calc, KNO3 and K2S04 both provide close to the same amount of K (K2S04 provides about 15% more). Assuming all of the K from KNO3 is needed, that would mean I should just replace the recommended KNO3 dosing with K2S04. So, I should increase my 1/8 tsp K2SO4 to 1/2 tsp. Otherwise I'm way under dosing K. My 1/8 tsp K2S04 + 1/8 tsp KH2PO4 gives me about half the K you would get with the EI recommended 1/2 tsp KNO3 + 1/8 tsp KH2PO4.

As for the 20ppm of K recommendation, that's only useful if I know how much K my plants use up each week. What's typical for low, medium, and high growth tanks? Regular EI dosing for my 46 gal would add about 30ppm of K per week.
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post #10 of 12 (permalink) Old 02-19-2009, 12:33 AM
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Quote:
Originally Posted by cjp999 View Post
As for the 20ppm of K recommendation, that's only useful if I know how much K my plants use up each week. What's typical for low, medium, and high growth tanks? Regular EI dosing for my 46 gal would add about 30ppm of K per week.
The 20ppm K is the target level you want to keep in your tank and since there is no easy and cheap way to test K accurately you add it each time assuming plants will use it. EI is designed to add that every time to provide unlimited nutrients for plant growth. Thats why its important and why I keep saying it.
If you have lower light and co2 you can cut stuff in half and go from there but there is no defined number for low, med, and high light tanks.
You are getting higher than 20 ppm because you are using the 40-60 gal dose. Its designed to allow the 60 to still get enough k and not OD smaller tanks.


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post #11 of 12 (permalink) Old 02-19-2009, 01:08 AM Thread Starter
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Quote:
Originally Posted by plakat View Post
The 20ppm K is the target level you want to keep in your tank and since there is no easy and cheap way to test K accurately you add it each time assuming plants will use it. EI is designed to add that every time to provide unlimited nutrients for plant growth. Thats why its important and why I keep saying it.
If you have lower light and co2 you can cut stuff in half and go from there but there is no defined number for low, med, and high light tanks.
You are getting higher than 20 ppm because you are using the 40-60 gal dose. Its designed to allow the 60 to still get enough k and not OD smaller tanks.
I fully understand all that. EI dosing levels are high enough so that you ensure ample nutrients for your tank, and the 50% water change will prevent them from climbing higher each week. With a 50% water change the most they could ever grow to is 2x the amount you add each week.

My point was that knowing the target is 20ppm does not tell you what EI dosing level to use. You also need to know the consumption rate. The EI tables have already taken this into account for KNO3, KH2PO4, and micros, but don't tell you how much K2SO4 is needed if you don't dose KNO3. You can however extrapolate how much K2SO4 is needed based on how much K you get from KNO3, and this is what I did in my previous post.

Given the 20ppm is the recommended target for K, and EI doses 30ppm a week, I would guess plant weekly consumption would be in the 10-20ppm range. Higher than that and levels stay too far below 20ppm. Lower than that and levels get too high.
  • At 20ppm weekly consumption rate (and 30ppm weekly dosing), you would eventually max out at 20ppm . By "max out" I mean the highest K level you would ever see. The lowest level would be 13ppm (before the 2nd dosing).
  • At a 10ppm consumption rate, you would max out at 25ppm, and bottom out at 20ppm
  • At a 5ppm consumption rate, you would max out at 50ppm, and bottom out at 32ppm
  • At a 0ppm consumption rate, you would max out at 60ppm, and bottom out at 37ppm.
Hope this makes sense. Anyway, the calculator gave me the answer I need so I think I'm set now. Thanks for the help.
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post #12 of 12 (permalink) Old 02-19-2009, 06:33 AM Thread Starter
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Grrr. Just realized it is K2SO4 that doesn't dissolve very well (I thought it was KH2PO4). Looks like I may need to dry dose, or get a much larger dispenser.
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