Are These Rocks Nutrient Theives?

This is one of the most important technical posts I have made.  First I will describe the situation leading to the what caused these plants to show different states of Iron or Magnesium deficiencies.  Please bear with me or skip to the last update of 2012/07/10

Beginning Wednesday 13 June 2012 I began foliar feeding chelated iron (FE2+).


On Monday 19, June I measured the FE and FE2+ in the water. 
Both were zero.    This amazed me because when I started the system I added 120ml of MircobeLift Chelated Iron to the system.





In the above photos you can clearly see a difference between the plant from the raft on the left and the plant in the gravel on the right.  Both share the same water.
Sunday 17, June - pH=6.4, Ammonia=0.25, N2=0, N3=0.5, PO4=10, GH=7, KH=1, FE=0, FE=0.0 mg/L, FE2+=0.0 mg/L
Sunday  after measuring the iron I added another 60ml MircobeLift Chelated Iron.   60ml is the recommended dosage according to the instructions on the bottle.
Monday 18, June - pH=6.4, Ammonia=0.5, FE=0, FE=0.0 mg/L, FE2+=0.0 mg/L
On Monday I added another 60ml and finally got a reading of FE=0.0 mg/L and FE2+=0.5 mg/L .
Just a side note about the Hagen Iron Test.  I don't like the test tube because it has a round bottom, and the cheap stand does not provide a place to hold it.

Here are photos taken the morning of Tuesday 19, June.

Clearly some improvement.  Not that I'm impatient; it will take time, but overall the deficiency is still evident.  About a week ago the plants in the gravel were somewhat yellow and pale when they were transplantded from my indoor grow bed, and given a large dose of MirobeLift Iron.  They suffered a little from the transplant, but soon recovered.  When the plants in the raft continued to pale I became more concerned, and  took these steps to correct the problem.


Tuesday 19, June pH=6.4, Ammonia=.25, FE=0, FE, FE2+=0

It seems that the problem has not yet been resolved. 
Tuesday morning I added another 60ml MircobeLift Chelated Iron
I'm nearly out of MircobeLift Chelated Iron so I hope to find the time today to buy some MaxiCrop Chelated Iron.  I think it's available at Lowes or Home Depot.

UPDATE: The  MircobeLift Chelated Iron was found to be an inappropriate product for this system.  I have documented the difference between this and another product IN THIS LINK.
MircobeLift Chelated Iron is suited to aquariums not aquaponics where vegetation is more important than the color of the water

UPDATE - 2012/07/10:

Here is what I've learned with the help of  Vlad Jovanovic.
Much of what follows was taken directly from Vlad's conversations with me.  I have changed some of the wording to put it into this context, and hope I have not misstated any of what he told me.
Without Vlad's patients and kindness I would never have been able to post this topic.

A lower pH is required for FE2+ uptake by the plants because the organic acid (the chelating agent) keeps the FE2+ bound in that ferrous state long enough for the plants to utilize it. Otherwise the ferrous iron will begin to precipitate out of solution and stick to the rocks and walls of the tank.

But the water was shared with and flowed from the raft into the gravel so why then did the raft exhibit the worst symptoms of FE2+ deficiency?

CEC or Cation Exchange Capacity, refers to the quantity of negative charges in soil existing on the surfaces of clay and organic matter. The negative charges attract positively charged ions, or cations, hence the name ‘Cation Exchange Capacity’.

The rocks have a higher CEC than the water which most likely has a CEC near zero.  This may have attracted the FE2+ through the opposite change and caused the FE2+ to accumulate on the surfaces of the rock, .and since there was not enough of it in solution to go around CEC played a role in helping the media bed plants to exhibit less dramatic signs of iron deficiency.

The cause of the Fe2+ becoming unavailable is essentially in a word...oxygen. Without any sort of binding agent (or 'chelating agent', remember those terms are interchangable...) the 'un-bound' Fe2+ will revert to plant un-usable Fe3+...in a matter of seconds. That is why it is important (again, in the homemade variety) to 'chelate' the Fe2+ with organic acids i.e tannins from Oak leaves or whatever. Otherwise the 'window of opportunity' for your plants to use them is small (seconds/minutes as opposed to hours/days). 

Temperatures and pH play a role in this scenario too, helping to either delay, or speed up the conversion of Fe2+ to Fe3+...but oxygen is the 'culprit'. The tannins and lowish pH help 'protect' the effects Fe2+ from oxygen (could be one easy way of looking at it). Remember iron is a transitional metal, so just converting rust (Fe3+) to Fe2+ without Oxygen with an RSG (Really Smart Guy) filter isn't enough. You have to keep it that way long enough for your plants to use it.

But in the end the CEC wasn't the root cause of all the plants showing an iron deficiency.  Without it the plants may have looked pale and yellow, but the root of the problem was not enough Fe2+ in solution. (Due to the bunk product I was using at first).

There may or may not have been a Magnesium deficiency as well. (Vlad says probably,... but we'll never know since I added the Epsom salt and lowered the pH around the same time as switching Fe products)...pH is real important to bio-availability of many plant essential elements.

Remember Chelation does not involve a transfer of electrons, but instead it is the ability to bind the iron compound in the ferrous state. The ferrous and ferric transition happens when iron compounds change states of oxidation. It is a low Oxidation Reducing Potential (ORP) that will tend to unbind the ferrous FE2+ and allow the compound to transition into a ferric FE3+ state because of the availability of oxygen. 

Whereas a high ORP with organic acids will act as the chelating agent binding, and keeping the iron soluble and non-reactive with the oxygen.

The raft is recovering nicely 2012/07/08

I hope this helps you to understand the way pH, temperature, cations and the CEC, Oxidation Reducing Potential (ORP) and chelation work to provide and available nutrients.

 I will continue to update this post if any more information comes to light.   

Here is a good article from Nate Storie at Vertical Food Blog
http://verticalfoodblog.com/iron-in-aquaponics/