The myth of N an P trapped on the sand bed

sixty_reefer

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On a different thread we where discussing ways to test for improving testing in our hobby more in particular nitrogen and phosphorus, those kind of testing that only large labs can do at the moment.
During the discussion I suggested testing the sand bed, this is where I believe most nutrients get trapped, it is only logical to start from there to try and find alternative testing, I suggested doing similar testing like they do in farming, where organic nitrogen and phosphorus get tested before being able to be released into plants etc…

I’m here to share on what I’ve stumbled with while suggesting this alternative. It seems that a different route was chosen and I’d like to explore this one further.

test 1 on nitrates on my sand bed.

5E86D558-B0FB-44BC-8976-7864FF264AD7.jpeg


test 1:
I’ve tested the water in my system on the vial to the left, middle vial was sand from my tank and Ro water and vial to the right was only to determine nitrates in my ro.

conclusions: the small amounts of sand that you see on the middle vial released 10ppm of nitrates.

although you may be thinking that my test kits could be unreliable in freshwater (my kit can be used in salt and fresh) I decided that I needed to know more and executed test 2 and 3 in a different manner.

test 2:

F5C99CB1-58EC-422A-A9C2-C29C4B0EA9ED.jpeg


test 3:

C5F94965-146A-40A3-9EAE-C0FC67E439CA.jpeg


test 2 and test 3 were all made with tank water leaving no chances that the test kit is not reading well, my idea was to determine the tank water residual and subtract it from the sand + tank water residual.
My surprise is that as more sand I add, the more the concentration of nitrates or phosphates increase, making it fairly difficult to determine how much phosphates and nitrates are really trapped in my sand bed if I was to add more water to my tank water the result would be the opposite and reduce the residual.
in addition test 1 and 3 have different residual due to different amounts of sand, wend testing with the same amount of sand residual is always consistent.

This brings out some thoughts to me that probably won’t make any sense to many.
My thoughts is, if residual nitrates and phosphates trapped on the sand bed are much higher than the residual on the water column would that be the reason for Cyanobacteria, dinoflagellates, diatoms and nuisance algae to cover this surfaces and feed on all those trapped nutrients?
does this explain why some folks get a spike in nutrients after disturbing the sand bed?

what’s your thoughts on this crazy observation?

is it possible that aragonite sand or rock being able to interfere with the test kit results?

will the test kit used just be testing nitrates or nitrates-nitrogen?
 
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sixty_reefer

sixty_reefer

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Was this from the top of a sand bed? How long has the tank been established for? I would be very interested in seeing more testing like this with a large sample size.
Yes, the sample was taken from the top of the sand bed and used in all testing from the same batch. The system is around 8-9 moths old and the residual in the water is 10ppm nitrates and 0.1 ppm phosphates stable.
 
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sixty_reefer

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Small video of the reaction happening inside the vial, it made some sort of reaction that am not used to see during normal water testing.

 

Randy Holmes-Farley

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FWIW, N and P may get trapped in pores between sand grains as organics break down, but nitrate has no affinity to bind to sand, while phosphate has a well studied affinity to bind to calcium carbonate surfaces.
 
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sixty_reefer

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FWIW, N and P may get trapped in pores between sand grains as organics break down, but nitrate has no affinity to bind to sand, while phosphate has a well studied affinity to bind to calcium carbonate surfaces.
Does that mean that the results are real? I was concern that the aragonite could somehow interfere with the residual on the samples, there was a lot of organics/detritus on the sample that settled in the sand bed overtime , does the reagent in the test kit got the ability to extract the nitrates from the organics in the sand bed represented in the samples?
 
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sixty_reefer

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If you put sand in a phosphate test, it will dissolve in the acid used.
That would explain the gas release wile doing it, couldn’t close the cap on the vessel due to the risk of blowing the cap of the vessel with pressure
 

Randy Holmes-Farley

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That would explain the gas release wile doing it, couldn’t close the cap on the vessel due to the risk of blowing the cap of the vessel with pressure

Yes, that could be dangerous.
 
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sixty_reefer

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I’m trying to understand what you did here. Did you have sand in the vial you were running the test in? Why?
the idea was to try and figure out how much ppm of nitrates and phosphates could potentially be trapped in the sand bed before they become available in the water column. And further investigation should be carried out to how nuisance organisms trap the nutrients in the sand bed following a bloom not allowing those nutrients to dissolve in the water column. I believe that in situations of depleted nutrients in the water there is still plenty of nutrients in the sand that don’t reach the pelagic bacteria.

Why not scoop up a vial of sand and tank water, stir, let settle, and then separate out the water and test that?

What brand of test kits are you using?
That seems logical, i’ll try that later on and see if I get similar results.
That’s actually what they do on soil testing, mix reagent 1 with soil agitate and then pass the liquid to another vial and add the colour reagent. It should make the test safer to execute as the acid won’t react with the sand.
from there I’d like to try and come up with a template that would tell us how many ppm of nitrates or phosphates are trapped per pound of sand.
Something like X ppm on the test kit will equal to Y ppm per pound of sand.
Am still not sure of the functionality of the results, and how we should interpret them.
One of the possible uses for this test could be a tool to determine import export as we are possible measuring nutrients before they become available in the water column giving us a idea of what a system is demanding for N and P.

I’m using JBL test kits
 
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sixty_reefer

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I’d just like to say that this preliminary finds wouldn’t be possible without @Dan_P input, not absolutely sure if he would like to carry on looking into.
 

Randy Holmes-Farley

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I would just caution that for phosphate, this is not a useful test if you put sand in the test where it dissolves.
 

Randy Holmes-Farley

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I would also add that there may be nitrite in the pores of the sand, so don’t ignore that impact on nitrate measurement.
 
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sixty_reefer

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I would also add that there may be nitrite in the pores of the sand, so don’t ignore that impact on nitrate measurement.
I’ve done the same procedure for nitrite and come to zero, I was hoping to do ammonia and leave it for 24 hours but it seems that I lost my ammonia test kit as I’ve not used it in a long time now.
 
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sixty_reefer

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I would just caution that for phosphate, this is not a useful test if you put sand in the test where it dissolves.
I was hoping to the test in a different way later on for both similar to the video below


The idea is to agitate the sand and water to allow organics separation from the sand in one vessel and move the agitated water to a different vessel and perform the test as normal without the sand interference.
 

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It's a thought provoking experiment. If it could be done in a proper lab with control samples and different age sand, water tests done on the water column to determine parameters before tests etc, it would be a worthy line of enquiry because it's often been a question I've seen come up as to how much nutrients does the sandbed trap, how it releases and whether vacuuming the sandbed is beneficial or not.

I'll be watching this one closely to see how you get on
 
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Could you explain in more detail what the aims are here?

If I understand correctly you are wanting to develop a more accurate phosphorus test?

We know that detritus collects in the sand if not vacuumed etc why is testing the sand going to help here?
I think the aim is to ascertain how much nutrients get trapped in the sand bed
 
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