# DTPA or HEEDTA - which is better for plants ?



## kekon (Aug 1, 2005)

As i like to roll my own micro fertilizers i've always wanted to get hold some more information about chelators. As far as i know many fertilizers use DTPA and HEEDTA chelators (HEEDTA is like "something between" EDTA and DTPA)
But the question is why some manufacturers use both DTPA and HEEDTA chelators instead of just one of them ? (for example Tropica uses both HEEDTA and DTPA). There are also oher fertilizers that use both chelators, for example "Kramerdrak" fertilizer produced by "Drak" in Germany.
Some say that the stronger the chelator the more energy plant must use to release metal ions from the chelator and, as a result, strong chelator is worse assimilable by plants. Some time ago i found information that HEEDTA is better assimilable by plants especially in aquatic environment. Also, HEEDTA is practically non-toxic.


----------



## Seattle_Aquarist (Mar 7, 2008)

Hi kekon,

DTPA would likely be a better choice of a chelator than HEDTA. DTPA maintains 100% stability of the iron up to a PH of about 7.0 and then starts to drop off to about 50% stability as the PH rises to 8.0. HEDTA only maintains about 40% iron stability up to 8.0.


----------



## kekon (Aug 1, 2005)

It seems that some ferts (like Flourish and Pfertz) use non chelated microelements like copper, zinc, manganese. Only iron is chelated. Maybe simple non chelated nutrients (copper suplhate, zinc sulphate etc.) are easier for plants to absorb ?


----------



## JeffyFunk (Apr 6, 2006)

kekon said:


> It seems that some ferts (like Flourish and Pfertz) use non chelated microelements like copper, zinc, manganese. Only iron is chelated. Maybe simple non chelated nutrients (copper suplhate, zinc sulphate etc.) are easier for plants to absorb ?


Nonchelated metals are obviously cheaper for the companies to sell than chelated metals. Mn and Cu need to chelated too. Plantex CSM has Mg in a chelated form as well; why that element needs to be chelated, I don't know since people often recommend Epson salts for Mg (MgSO4*7H20). I've often wondered what effect the chelator has on the metal absorption rate but don't have the budget to test it out...


----------



## kekon (Aug 1, 2005)

I don't think the cost is a problem. The amounts of Cu, Zn, Mn are very small in fertlilizers intended for planted tanks. It seems to me that the use of plain salts (like CuSO4, MnSO4 etc.) is because they are better absorbed by plants.


----------



## Marcel G (May 29, 2012)

Why some elements need to be chelated? Because otherwise they will readily bound to other elements in the water to form insoluble precipitates. For example, Fe(2+) or Fe(3+) attracts OH(-) or PO4(2-) ions. So if you would have Fe(2+) or Fe(3+) in the water, in a matter of seconds (or minutes at most) they will form Fe(OH)3 or Fe2(PO4)3 precipitates.

There are different chelates available:
1) Fe-gluconate
2) Fe-EDTA
3) Fe-DTPA
4) Fe-EDDHA (or Fe-EDDHMA)
etc.

Fe-gluconate is not a chelate in a true sense; it is a complex. It is the only complex (I know of) which contains the iron in the form of Fe(2+). Other chelates uses Fe(3+). There is some discussion concerning the ease of uptake of Fe(2+) vs. Fe(3+). Propably, Fe(2+) is uptaken more easily than Fe(3+). But plants are for sure able to utilize Fe(2+) as well as Fe(3+) if it is in some complexed/chelated form.

Each chelate/complex has its stability. Fe-gluconate won't last more than 1 day. Other (stronger) chelates may last much longer in the solution. All chelates/complexes degrade. The degradation rate depends not only on pH, but also on light intensity, temperature and bacterial activity. The higher the light and temperature in aquarium, the faster the degradation. That's also the reason, why it is advisable to supply chelated microelements on a daily rather than weekly base (the chelates probably won't last for a whole week => at least some weaker ones).

Also, as long as the iron is in the chelate, it is protected against other anions in the solution => like OH(-), PO4(2-), HCO3(-), CO3(2-) etc. So as long as the iron is protected (chelated) you won't see any precipitates in your tank. The precipitates form only when the iron gets out of the chelate protection (i.e. when it is released from the chelate). But plants are able to utilize iron even from the chelate (they can strip it from the chelate on the plasmatic membrane, or they can devour the whole chelate and strip the iron inside the cell).

PS: Seachem uses just Fe-gluconate complex in their fertilizers, which is stable no more than 1 day (then the complex breaks down and the iron Fe(2+) will be oxidized to Fe(3+) form, and forms a precipitates). Without chelate/organic-complex the Fe(2+) or Fe(3+) won't stay in this form in our tanks with high level of dissolve oxygen for a long time (seconds or minutes at most).

I hope, this information helps.


----------



## kekon (Aug 1, 2005)

Thanks, ardjuna
This information helps a lot. I know there is debate about Fe++ vs Fe+++ between aquascapers but from what I see most iron chelators contain Fe+++ form.


----------



## Yo-han (Oct 15, 2010)

Really good reply from Marcel! But since we keep temperature stable (low or high) and light etc. is what it is. It mostly depends on your tanks pH if you want to select the best option. I use Fe-gluconate, DTPA and EDTA for my tank which has a pH of 6.6-6.8 during the day and 7.8 during the night. Never seen any iron problems! I've done it without the gluconate, not much difference except for my Blyxa, which did waaay better right after I added the gluconate to the mix.


----------



## Marcel G (May 29, 2012)

I did a test with using different iron chelates in my 4 test tanks where I can maintain same conditions.

Here are the results:








Here's the link to full-resolution picture (35.7 MB):
http://www.prirodni-akvarium.cz/img/experimenty/test_fe_srovnani_3d_rostliny.png
_The photograph was taken on the third day after the addition of micronutrients (the second day was the same)._

What's on the picture:
1) 0.9 ppm Fe-gluconate
2) 0.9 ppm Fe-EDTA
3) 0.9 ppm Fe-DTPA
4) 0.75 ppm Fe-DTPA + 0.15 ppm Fe-EDDHMA
5) 0.13 ppm (Fe-gluconate + Fe-EDTA + Fe-DTPA + Fe-EDDHMA) = 1/7 of the full dose

Tank's parameters:
- Temperature 82-84°F (28-29°C)
- pH 6.35 to 6.45, bicarbonates 61 ppm HCO3, carbonate alkalinity 2.8°dKH (1 mM/L) => 35-45 ppm CO2
- total alkalinity 3.6°dKH
- nutrient content: 90 ppm NO3, 60 ppm K, 9 ppm PO4, 25 ppm Ca, 9 ppm Mg, 0.9 ppm Fe (+ other trace elements)
- total hardness 5.6°dGH (1 mM/L)
- source water = RO/DI water (3 µS/cm)
- conductivity after adding minerals: ~250 µS/cm
- light intensity: 100 µmol PAR (bottom), 200 µmol PAR (water surface)

Note: The only tank where the iron did not precipitated was the tank no.3 (Fe-DTPA). I was surprised that even in the tank with Fe-DTPA and Fe-EDDHMA combination of chelates the iron precipitated the next day after the addition of micronutrients. Most precipitates are in the first tank (Fe-gluconate). The higher the pH, temperature, irradiation (light intensity) and Fe, PO4(-2) or OH(-) concentration, the faster the reaction. Here I use quite high temperature (82-84°C), very high iron levels (0.9 ppm) and very high PO4 levels (9 ppm), so under normal conditions the results may be different.


----------



## Yo-han (Oct 15, 2010)

Is the growth in case 5 boosted or was there more erectus to begin with?


----------



## Marcel G (May 29, 2012)

Yo-han said:


> Is the growth in case 5 boosted or was there more erectus to begin with?


The _Pogostemon erectus_ in the 5th tank is another experiment, so it's there just for a comparison.

PS: After 5 days the Fe-DTPA chelate was still OK, and the iron did not precipitated.

Four days increase of _Pogostemon_ in the four tanks:
Tank #1 (Fe-gluconate): 1.5 cm (initial height of _Pogostemon_: 15.7 cm, final height of _Pogostemon_: 17.2 cm)
Tank #2 (Fe-EDTA): 1.0 cm (initial height of _Pogostemon_: 15.9 cm, final height of _Pogostemon_: 16.9 cm)
Tank #3 (Fe-DTPA): 1.9 cm (initial height of _Pogostemon_: 14.3 cm, final height of _Pogostemon_: 16.2 cm)
Tank #4 (Fe-DTPA + Fe-EDDHMA): 1.8 cm (initial height of _Pogostemon_: 16.9 cm, final height of _Pogostemon_: 18.7 cm)

I had to end this test for now, but later I'll repeat and revise it, as 4 days are too short period for any serious conclusions.


----------



## kekon (Aug 1, 2005)

_Here I use quite high temperature (82-84°C),_

I'm somewhat confused...
Are you sure that you used such high temperature in the tank ? It's hard to believe that plant can survive under such conditions.
From the pictures shown it looks the the plant in the picture no. 3 (where the DTPA was used) looks rather poor whereas the plant tin the pic. no. 5 is very bushy and has strong colors (combination of Fe-gluconate + Fe-EDTA + Fe-DTPA + Fe-EDDHMA).
When i added 0.5 ppm of DTPA into my tank the water turned yellow and all the plants showed iron toxicity so with 0.9 ppm it must have been even worse... But i didn't try 0.9 ppm Fe.


----------



## Marcel G (May 29, 2012)

I did not meant 82-84°*C* but 82-84°*F* (28-29°C) ... it was a typo!
The _Pogostemon erectus_ in the fifth tank was from another experiment, so it was much higher than the other four samples in tanks #1-4. The samples in the tank #1 to #4 were just fresh, while the sample in tank #5 was 30 days old. As to the toxicity issues, maybe with some other plant species 0.9 ppm Fe may cause some issues, but with my _Pogostemon erectus_ and _Ludwigia sp. 'Red'_ there was no visible issues during my test.


----------



## kekon (Aug 1, 2005)

So, what conclusions can we draw from this experiment ? Which chelator is best for aquatic plants ?


----------



## niko (Jan 28, 2004)

None is better than the other. The use of chelated iron depends on how the entire system runs. 

It makes the most sense to add iron in different chelated forms. But that will only makes sense if the Ca:Mg ratio is correct, the N and P are what is necessary, organics are in check, traces are added without questions if they are enough or not. 

Other factors - pollutants like S, K, and Cl which are normally added with conventional fertilizers and accumulate.


----------



## Marcel G (May 29, 2012)

kekon said:


> So, what conclusions can we draw from this experiment ? Which chelator is best for aquatic plants ?


From my little experiment it seems that all the tested chelates (Fe-gluconate, Fe-EDTA, Fe-DTPA and Fe-EDDHMA) are stable for at least ~1 day. So if you would dose the micronutrients on daily basis (i.e. each day), you can be quite sure that the iron will be available for plants. The problem may arise when you try to dose your micronutrients on weekly basis (i.e. once a week). In this case virtually all Fe-chelates (except Fe-DTPA) will break down, and all the iron will oxidize and precipitate (i.e. becomes unavailable to plants by the second day at the latest). So if you want (for whatever reason) do dose micronutrients just once a week, the best choice is definitely Fe-DTPA which seems to be the most stable Fe-chelate available. Even the Fe-EDDHMA is not as stable as Fe-DTPA. I was told that Fe-EDDHMA consists of two isomers (ortho-ortho and ortho-para), and while ortho-ortho is very stable chelate, the ortho-para is not even called a chelate, because it's very unstable. So the overall stability of the Fe-EDDHMA chelate depends on the ratio of these two components. Unfortunately, in most cases the ortho-para component may be included in the chelate by up to 50%, which is the reason why this chelate may be worse than Fe-DTPA. Beside this, the Fe-EDDHMA is more susceptible to calcium substitution (under high concentration of calcium the iron in the chelate may be replaced by calcium, which again will lead to iron precipitation). And lastly, the Fe-EDDHMA causes strong discoloration of water, so in higher amounts (> 0.1 ppm) it's just unfit for use => under only 0.15 ppm Fe-EDDHMA the light intensity decreased in my tank by approx. 20% (measured by PAR meter).

So according to my opinion, best chelate for use in our tanks is definitely Fe-DTPA.
But if you are able to dose micronutrients on daily basis, then you can probably use whatever chalete. Fe-gluconate is supposedly better uptaken by plants as it contains iron in the more "digestible" form of Fe(+2) while other chelates use Fe(+3) form.

_PS: To be honest, I don't understand niko's arguments._


----------



## kekon (Aug 1, 2005)

Thanks, Ardjuna !
Your posts dispelled all my doubts. The only question is why some manufacturers use both (HEEDTA + DTPA) in their products instead of just DTPA ? Maybe the reason is the cost ? Seems strange....


----------



## Marcel G (May 29, 2012)

kekon said:


> why some manufacturers use both (HEEDTA + DTPA) in their products instead of just DTPA ?


I don't know. Just a little note: HEEDTA is not the same as EDDHMA. I tested EDDHMA, not HEEDTA chelate. Fe-HEEDTA may be better choice under some circumstances (see this article). Actually, I don't have HEEDTA available right now, which is the reason I did not use it in my little comparative test. Also, my test was just a "minitest" so take it with caution. I don't have enough time right now to do some "big" testing, as all my attention is now focused on my growth experiment which is finally moving from the preparatory stage into the first sharp test.


----------

