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PPS-Pro in a garden pond

2K views 3 replies 1 participant last post by  Darkbluesky 
#1 · (Edited)
Hello,

I am not sure if someone else has tried this before, but I am dosing PPS-Pro in a garden pond to try to make plants healthy and control/get rid of the algae there (fuzz algae mainly).

I have CO2 injection which gives around 15 mg/l (between 15-20 mg/l) during day (7:30 am to 20:30). I stop it by night; I can't afford to waste it, I have to keep CO2 in 3100 liters with several cascades and fountain, which I want to keep running.

The direct sunlight hours, now, are 7, from 11:30 to 18:30 more or less. But it is different from an aquarium: the sun moves, changes intensity, color temperature and trees and other things cast moving shadows around. The depth is around 65 cm.

I began with PPS-Pro several weeks ago (I aim for water column values of No3 = 10, PO4=1), to my surprise it seems to work. I have to add 1 mg/l No3 daily, as PPS-Pro says, but PO4 I have a daily consumption of 0.5 mg/l, and not 0.1. But I have another problem with the micros:

I use Seachem Flourish plus Seachem Iron (ferrous gluconate, I still have 2Kg of Fe gluconate...). The problem is the cloudy water (thus precipitating and losing PO4 and Fe as insoluble ferrous phosphate) when I dose them together (or with 1 hour of separation, it's the same).

The problem is how can I dose iron, as I don't want to wait until PO4 is 0, as I think this could lead to problems (I would have oscillating PO4 concentration in the water column, from 1 to 0 then to 1, etc), which I think would not be convenient.

EDIT: My pH when CO2 is running (normally) is steady 7.1-7.2 (w/o CO2 it raises up to 8-8.3 at evening, but this is no more real as I have CO2 all day).

How can I dose iron by keeping 1 mg/l of PO4 in water column?

Thank you very much for any help !
 
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#2 · (Edited)
I have done a test yesterday and today.

Yesterday:

7:00 h:
I added PO4 to have 1 mg/l
I had NO3 = 12 mg/l
I did NOT add Fe

14:00 h:
I had 0.5 mg/l PO4

Today:

7:00 h:
I had 0.45 mg/l of PO4. I have NOT added PO4 neither NO3.
I added about 0.08-0.09 mg/l of Seachem Iron (ferrous gluconate)

14:00 h:
I had 0.3 mg/l of PO4.
I had 0 mg/l of Fe
-----------------------------------------------

That means that the PO4 has been reduced in 0.15 mg/l during today morning. That means 0.15*3100 liters = 465 mg of PO4 reduced, which is equal to 0.465 g/94.97 g/mol = 0.00490 mol of PO4.

We could guess that the Fe has combined as FePO4 and precipitated. If that was true, then all the Fe should have been used. Let's see:

To form FePO4 we need the same amount of Fe than PO4. So if 0.00490 mol of PO4 has precipitated (following our hypothesis), the same amount of Fe should have been used, 0.00490 mol of Fe, which is 0.00490 mol * 55.85 g/mol =0.274 g of Fe, which corresponds to a concentration of 274 mg / 3100 liters = 0.088 mg/l of Fe, which is inline with the Fe concentration (0.08-0.09) added today morning.

That means that all the Fe added corresponds to the PO4 'disappeared'. It seems to confirm that the iron has been completely wasted with the PO4 present in water.

Note: the Fe calculated concentration could be 0.11 mg/l if the PO4 reduction was considered 0.2 mg/l instead of 0.15, it is somewhat hard to identify exactly by using the color tests.

Of course it could be that the PO4 reduction is due to plants, and the Fe also, but it seems strange that PO4 consumption was different from yesterday (which was 0.5 vs 0.15/0.2 mg/l of today).

EDIT: Another possibility is that the Fe had precipitated as FeCO3, which is insoluble, the half-day 0.15-0.2 mg/l of PO4 reduction could be due then to plant consumption, below the 0.5 mg/l of 'yesterday' maybe because the PO4 concentration at dawn in water column was smaller than the 1 mg/l of yesterday's dawn... This would be an unlucky (and unlikely?) coincidence though. BTW, I have around 7-7.5 dKH of Alkalinity (KH test).

As you see this could simulate the alternate days dosing scenario, but then, are we throwing away our Fe dosing? Are we doing it for NOTHING (aside of keeping purchasing iron fertilizers)?
 
#3 · (Edited)
I am a bit surprised nobody has commented, maybe the subject is too evident and I miss something, there is not too much knowledge about it, or I wrote in the wrong part of the forum. Anyway, I have new data to add:

Due to this subject I stop fertilizing during last days until I could find a solution for my iron-hungry (mainly floating ones, so no root tabs solution is acceptable) plants and at the same time keep the PPS fertilizing (minimum PO4 conc. in water).

So this sunday I had:

8:00 am:

NO3 = 3 mg/l
PO4 = 0.35 mg/l
Fe = 0
KH = 8 dKH
GH =6.5 dGH

I added 0.2 mg/l of Seachem Iron (ferrous gluconate)

The evolution of the Fe concentration has been steady reducing 0.03 each hour aprox. No cloudy water during all morning.

Then at 15:30 h:

Fe = 0.015 mg/l (definitely >0)
PO4 = 0.3 mg/l

So, it seems that adding Fe (ferrous gluconate) when PO4 has lower concentration (this case 0.35 mg/l) do not produces precipitation as FePO4 (or maybe only marginal).

It is true that the NO3 had also a big difference between the previous test (12 mg/l) and yesterday's (3 mg/l), but afaik NO3 would form iron nitrate Fe(NO3)3 which is very soluble, and *I guess* (confirmation needed) it is detectable by iron test kits. I guess that if this was the case, the Fe would still be available to plants.

On the other hand a 0.2 mg/l Fe consumption would ask for 1.02 mg/l of PO4 to form FePO4 and precipitate, but the consumption of PO4 between 8:00 h and 15:30 h has been only of 0.35-0.30 = 0.05 mg/l. This discards FePO4 formation…

Due to this data and that the Fe has stayed in water column for a good 7 hours (reducing its concentration steady) I am pushed to believe that the iron has been uptaken by plants. The 0.05 mg/l of PO4 reduction would be probably plant uptake, otherwise if it is FePO4 formation then it would correspond to 0.01 mg/l of Fe. Nothing to worry about, IMO.

(As a side note related to Fe(NO3)3 formation hypothesis, just in case is useful to someone: the related NO3 usage to form Fe(NO3)3 by binding all the 0.2 mg/l of Fe as fe(NO3)3 is 0.66 mg/l of NO3 for my pond volume. This is too small for the NO3 test kit resolution).

It seems that below some PO4 concentrations PO4 do not interfere with Fe (at least in ferrous gluconate form) although I had read before that the minimum concentrations above which FePO4 would form (and precipitate) were much smaller (undetectable for test kits) than the ones discussed here.

In the previous tests I had 0.45 of PO4, so the frontier seems there between 0.35 and 0.45 of PO4. Someone could argue too that maybe adding more Fe (0.2 mg/l vs 0.09 mg/l) would guarantee to avoid precipitation, personally I don't see why (I would think just the contrary). Or maybe the higher NO3 concentrations could start some reaction that may end in some way helping Fe precipitation (NO3 extracts Fe from gluconate complex, becoming Fe2+ to Fe3+, and then it dissolves in water column, freeding Fe3+ to precipitate as ferric hydroxide, is this possible at all?) ? Other thing ? I dunno, I am not a chemist.

I can't really find an explanation why it worked well yesterday and not in the first test (PO4 concentrations were not SO different, and at some moment the 0.45 mg/l PO4 concentration should have reached the 0.35 mg/l which was the one I had on sunday...). I guess I could do more tests to see if these findings are consistent, although it is a pain. Are these results in line with your findings? What do you think about it?
 
#4 · (Edited)
Well I did some more tests:

Yesterday with 15 mg/l NO3 ('high' NO3) and 0.35 mg/l of PO4, I added 0.25 mg/l of Fe (ferrous gluconate). Its evolution has been:

15:30 Added Fe=0.25 mg/l
16:45 Fe=0.15 mg/l
17:30 Fe=0.1 mg/l
18:30 Fe=0.05 mg/l
19:30 Fe=0

No cloudy water

Seems then that the high NO3 is not influencing, at least not too much, the Fe in water column, I guess I have to think that it has been uptaken by plants...

Then today with 15 mg/l of NO3 ('high' NO3) and 0.55 mg/l (higher than 0.35 mg/l) of PO4 I added also 0.25 mg/l, and the result has been:

10:45 Fe=0.25 mg/l
11:45 Fe=0.05 mg/l
12:45 Fe=0, PO4=0.45 mg/l

No cloudy water

The PO4 amount 'used' (0.55-0.45=0.1) today do not fit with the Fe added (0.1 ppm of PO4 means only 0.06 ppm of Fe to make FePO4), so I have to think that Fe has been used by plants, although much faster than yesterday...at least, most of it.

I guess Fe uptake speed depends on several factors so it could not be compared directly, thus I could maybe safely asume that for fast Fe ppm reduction, if PO4 'used' do not match with Fe, then it is due to plant uptake.

I have no clue then why it it did not work the first time (see first post)... Maybe I could try to make a test with still bigger increase of PO4 concentration (~1 mg/l) and see if the Fe availability decreases along with relative PO4 consumption, or not.

Maybe I'll do it if I find the problem another time. Perhaps I'll need to increase PO4 in water column, as I am still fighting with carpet like green algae in rocks, and fuzz in leaves, and do not know what to do to get rid of them.
 
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