Need help understanding phosphates - The Planted Tank Forum
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post #1 of 8 (permalink) Old 03-08-2018, 12:53 AM Thread Starter
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Need help understanding phosphates

Since my phosphates are always at 0, this week I've decided to increase the amount macros I'm dosing.

For the past 4 days, Every day in the morning I add

P - 0.42ppm
PO4 - 1.3ppm

NO3 - 7.5ppm
N - 1.5ppm
K - 5.3ppm

Then I test the water in the evening... About 8 hours after adding ferts. ....And my phosphates are holding at 0.

Also worth noting that my nitrates stay at 5 consistently between water changes.

My 20gal tank is getting towards what I would call heavily planted... At least on the high side of moderate planted. It's medium light with CO2

So, I have to assume my plants are using up all the available phosphates? Either that OR this is an expected result when testing 8 hours after adding.

Which leads to my main question... Should I be dosing even more? Is it beneficial to have a steady phosphate reading?




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post #2 of 8 (permalink) Old 03-08-2018, 01:46 PM
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What substrate are you using? Are you dosing iron the same time as you dose your macros?

Higher CEC substrates could very well 'absorb' phosphates, while iron could bind with phosphates and precipitate.
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post #3 of 8 (permalink) Old 03-08-2018, 01:48 PM
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Quote:
Originally Posted by madcrafted View Post
What substrate are you using? Are you dosing iron the same time as you dose your macros?

Higher CEC substrates could very well 'absorb' phosphates, while iron could bind with phosphates and precipitate.
Phosphate is not a cation and therefore cannot be found on the Cation Exchange
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post #4 of 8 (permalink) Old 03-08-2018, 02:04 PM
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Damn, just had flashbacks of failing AP chem. lol

Well it has to be a faulty test kit or iron. There's just no way plants are utilizing that much phosphorus with such nitrate levels and medium lighting.
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post #5 of 8 (permalink) Old 03-08-2018, 03:10 PM Thread Starter
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I am dosing iron every other day at 0.95%.

My light is an AI prime HD freshwater which is only a max PAR of like 88 at full intensity, but I run it under 50% even at peak times. There's a 2 hour ramp on either side of a 3 hour peak.... I'm only guessing that's medium lighting.


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post #6 of 8 (permalink) Old 03-08-2018, 03:42 PM
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It seems like you are dosing macros in sufficient proportions. How do your plants look? That's really all that matters. I just realized you said you are dosing this amount daily, correct? If that's the case, then maybe your plants are just guzzling it down. Your nitrates and most likely potassium levels would build up with daily dosing at those levels if phosphorus was deficient. Alot of folks dose at a 10:1 ratio of N to P but I prefer a ratio closer to 5:1 myself... which you are not too far off from.
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post #7 of 8 (permalink) Old 03-08-2018, 05:32 PM Thread Starter
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Well the plants are growing strong and looking pretty good.

But yeah it's daily as of now just to see if I could get a reading. Nitrates are not going over 5ppm at any point over the week. No idea about K though. .I don't have a kit for that.


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post #8 of 8 (permalink) Old 03-11-2018, 10:10 PM
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Phosphate is not a cation and therefore cannot be found on the Cation Exchange
Maybe not because of cation exchange,but the phosphate ions can indeed be adsorbed by the colloids in the clay.
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