# The real affects of Fe/PO4



## dennis (Mar 1, 2004)

Receint posts have brought up an interesting question and one I have mulled over in the past, namely the chemical reactions associated with mixing PO4 and a Fe trace solution into one dosing container. Many people have reported both positive and negative experiences. I seem to have noticed that people who report bad effects, ie precipitation from the water colum by the PO4 and Fe binding into a fairly insoluable compound, also tend to use Seachem products while those with positive experiences tend to use products with a stronger chelator attached to the Fe trace.

Jeff Ludwig's reply to my speculations in another thread is quoted below. I am hoping that he and others with a strong chemistry background can help work this out.



JLudwig said:


> As for the iron/phosphate comment, the chemistry is very complicated, I'm plowing through some of this right now. The reaction rate will depend very strongly on the exact chelating agent used and the pH of the fertilizer solution. Lower pH values should stabilize the fert solution significantly (due to chelating agent equilibrium and also the phosphate should be mostly protonated and less reactive). If you plan on doing this, be sure to mix with RO or distilled water, use DPTA as a chelate (TMG/CSM *not* iron gluconate) and add a few drops of hydrochloric acid for good measure.


Also, at the same time, please post your experiences associated with the combination of PO4 and Fe solutions in one dosing container. The more experiences we have to look at, the better an answer we can find.


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## JLudwig (Feb 16, 2004)

Dennis,

Right now I'm in the middle of fabricating some special tanks to setup for testing, I hope to get some real quantitative data on this. Iron phosphate precipitation is a real issue, whether it makes a difference to plants is a completely different issue. At first the idea was to do experiments using floating plants, for example Frogbit, as this removes CO2 limitations. The problem with doing this, is that floating plants must be able to use oxidized iron iron in nature, they are most likely very efficient iron scavengers.

So instead, in order to understand this effect, I think we should look at the most sensitive species, stuff like Tonina sp., Eustralis stellata (non-broad leaf variety), etc. There seems to be a feeling that KH can affect the most sensitive of plants, I'm starting to wonder if it isn't a simple pH issue: as the overall tank KH drops, so does pH and it would tend to hinder iron phosphate precipitation. This would also explain why ADA substrates (which lower pH) seemingly grow plants like gangbusters, at least in the initial few months. And why the folks in Singapore seem to get colors and grow plants like some of us cannot (well, me at least). I wonder if iron delivery is _the issue_ when it comes to the most sensitive of species, especially those that seem to do better in lower KHs.

I'm going to make a special "test tank", roughly 20L is size with 8 watertight compartments. I'm going to explore chelation effects (try gluconate iron, DPTA iron, no iron, and unchelated ferric sulphate) and also see if KH (and therefore pH) are affecting iron uptake takes. I hope to get numbers by putting a known quantity of plants into the chamber at the start of the experiment (5 2" sections of rotundifolia for example), then going on dry weight measurements. I can also use this technique to see the effect of concentration on growth.

In the mean time, I'm dropping my show tank's KH via RO. I'm dosing N/P/K on change day, leave a day off, then dosing iron daily for 5, one day off. I'm adding roughly the same concentration of Fe and PO4, this way I don't build an excess of either and the plants are gauranteed to get a shot to each one unreacted for a brief window.

I dont think the issue is really that big though, I've seen in person plenty of successful setups that dose Fe+phosphate at the same time. None of them use iron gluconate though.

Jeff


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## plantbrain (Jan 23, 2004)

Jeff, having many years of experiences with both DTPA chelator and gluconate complex, I use and prefer TMG.

Erik and I spoke at length about this(he has super soft tap, mine was moderate hard), I have high PO4 in the tap, he has none.

The pH optimum is better for the DTPA at higher pH's(lamda max is about 7.5). 

Marine systems should use EDTA.

Softwater systems can likely use either TMG or Flourish.

Still, think about this for a moment: would you rather have a source that is present longer and most of the time but a little harder to remove from the binding agent, or would you rather have something less available over time and easier to remove from the binding agent?

Hard question to answer, but given the time and frequencies I've used both, I do prefer TMG.

I think to generalize will be very difficult, too many different tap waters, too many different CO2 levels, lighting levels etc.

I have not noticed any color differences with ADA soil though.
Same as Flourite etc.

Regards, 
Tom Barr


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## Laith (Sep 4, 2004)

Interesting. 

But just to be clear, are we saying that a chelator other than gluconate is better in harder water, irrespective of the issue of mixing Fe and PO4?

Or that in harder water, PO4 is more likely to precipitate with gluconate chelated Fe than with DTPA chelated Fe?


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## Laith (Sep 4, 2004)

Jeff, did you ever get this experiment off the ground? If so, any feedback yet?

Also, anyone have any thoughts re the question in my previous post?


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## JLudwig (Feb 16, 2004)

Laith said:


> Jeff, did you ever get this experiment off the ground? If so, any feedback yet?


Sorry Laith, I have not, once again things in grad school have picked up, I haven't really been able to read the forums/work on the hobby too much  Maybe in a few months... As for the chelating agent, I would need to do a literature search to look at where the equilibrium lies, but it is possible that every target pH (and therefore KH) would have an ideal chelating agent. The effects may be competing, thats why I can't easily see what's going to happen. The same mechanism that makes the iron availabile to plants also exposes it to reaction, so a more reactive iron might be more bioavailable, its unclear which effect would win out. That would depend on the plant's ability to strip the iron from the chelate which is the big question mark I think. You want the iron bound strong enough to prevent oxidiation/precipitation but weak enough that the plant can use it.

I know the info is out there, probably some books on hydroponics would be of a lot of use, but at least here in the US that may put you on a watchlist so I'm reluctant to go googling for the info  (just kidding, I hope.)

Jeff


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## jude_uc (Feb 7, 2006)

At one point (before I read the warnings) I was using a neutral regulator which is almost entirely phosphate. During that period, I noticed that any flourish iron I added almost instantly formed a precipitate. However, I also occasionally used other iron products which were chelated with EDTA or DTPA. Those products never produced a precipitate. As a validation for these observations, after realizing what was in the neutral regulator, I did several water changes. Somewhere around the time when my test kit read less than 5 ppm phosphate, I was able to use flourish iron without instantaneous precipitation.

-Adam


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## Edward (May 25, 2004)

Hi
Three methods I have experimented with: 
*1st* KH by CO3, pH about 6
*2nd* KH by baking soda, pH about 6
*3rd* zero KH, pH bellow or equal 6

The plant health and growth was as follows:
*1st* worst
*2nd* great
*3rd* great

Dosing daily NO3, PO4, K, Mg and Trace Elements at the same time.

Here we can see that even though the lower pH helps and KH by baking soda doesn't hurt, the CO3 is what many plants don't enjoy.

Edward


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