Comment

Comment by Vlad Jovanovic on July 4, 2012 at 12:09am

No problemo Bob...

Let's start with chelating and what that means. An easy way to grasp it would be to (in your head) replace the word "chelating" with "binding", since that's basically all that that is. Binding. Soluable binding. And by binding (to an extent) making 'inactive' the bound metal ion. No removing of electrons, no transitioning of valency...Just binding.

Iron is a "transitional metal", so it will want to 'transition' between forms (valency...Fe2+ being di-valent, called ferrous...Fe3+ tri-valent called ferric ).  depending on environmental conditions. It will (in part because of this easy transitioning) form complexes with just about any frigginn thing it comes into contact with. This can be 'good', or this can be bad. It just depends on what you are wanting to accomplish...Let's just stick to an AP RSG filter2.0  scenario, and not any industrial or commercial proccess'...

So, say you create the proper environmental conditions for ferric iron (Fe3+) to 'transition' to ferrous iron (Fe2+), these conditions would entail an anoxic environment with a carbon food source for the 'iron transitioning bacteria' to do their work (iron cycle)...It is these conditions that will give you the Fe2+ not any "chelating". (Remember 'chelating' just means 'binding'

So now you have plant usable Fe2+. That Fe2+ is gonna want to transition back to Fe3+ as soon as it hits an oxygenated environment (like your AP water). This may only take seconds (depending on pH and temps)...Which would pretty much suck.

Now here is where the chelating would come in...once you have the iron in Fe2+ form you want to keep it that way for as long as possible. Organic acids would play the role of the 'chelating agent'. Iron will of coarse play the role of 'metal ion you want to bind'. This will keep it (well, in this case slow it down at least) from transitioning and forming other complexes and precipitates...That is the role of the chelating agent and the point of forming chelated complexes....To keep the metal ion soluable and non-reactive.

As far as CEC...don't look at it so much as "the rocks have a greater CEC than the roots in the water, but rather the rocks have a CEC, the water doesn't.

Remember that stupid Dolly Parten movie, or those stupid game shows where someone is standing in a glass bell contraption with air and Dollars blowing by them...and they have to grab as many dollars as they can, as the money whizzes by, in a given amount of time..? Well that machine is your DWC, you trying to grab dollars are your plants roots. Now imagine that that same amount of Dollars are lying calmly stacked on a table...it would be much easier to pick up as many as you felt you needed, yes? Well, that would be your media bed and it's CEC is the table I suppose...The ions are there, where your plants need them...and they have all the time in the world.

When pickings were slim, DWC suffered first,even though the media bed fared much better, but when you added the iron...there was now enough to go around, in solution, CEC be damned...

Yes, the roots around the rocks probably had/have better access to the iron than the roots in the water.

Comment by Bob Campbell on July 3, 2012 at 6:54pm

@vlad -

I  am trying very hard to describe this ion exchange correctly and appreciate your patience. I've spent a great deal of time studying how this all works together.

Please tell me if I state any of this incorrectly.

Iron  can be chelated by other chemical reactions, but also with the help of bacteria and amino acids.  The iron is chelated by removing two or three electrons from the iron creating FE2+ and FE3+ .

Then depending on the Cation Exchange Capacity this chelated iron is taken up by the plants.  CEC 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 of the chelated iron.  (The cations).

By 'robbing' I mean that the rocks have a much greater CEC than the roots in the raft water, and end up attracting, and holding onto the major portion of the available iron cations.  This allows the roots in the rocks better access to the iron chelate than the roots in the water.

Comment by Vlad Jovanovic on July 3, 2012 at 1:11pm

Bob, once again...don't look at a medias cation exchange capacity (CEC) as "robbing your plants of anything...key word being "Exchange"...

most all media and soil (with the possible exception of perlite and some types of sand) have a CEC rating. The in a battle between who gets the cation, the plants rhizosphere will always win out...

I used the CEC bit, to explain why, within the same (and very new) AP system, a DWC raft will show deficiencies before a media bed will even though they share the same water source.

Comment by Bob Campbell on July 3, 2012 at 1:01pm

@Jon - It was Vlad that suggested the rock may be attracting the Iron Cations. I'm quite certain it was an iron deficiency because it cleared up immediately after using a product that actually has iron chelate in it.  The first product I used had nearly imperceptible levels of FE2+.  Don't use the stuff they make for aquariums.

It's probably not due to lack of DO.  I have 4 air stones under the raft.

 

The water looks like it's boiling!

Comment by Jon Parr on July 3, 2012 at 12:28pm

Bob, I would imagine pumice to behave like perlite, which most users wind up having a hard time with. It floats, and the variable size lets it end up everywhere in the system, including pump and pump shaft. Not a huge problem, just take care to contain it, after all, it's most common use is as an abrasive.

I read through you site, very interesting that you feel the gravel sucked up the ferrous iron. Other than the visual difference of raft produce to media produce, is there any other reason you believe this? I'm not trying to be pessimistic, just making sure I understand. Media holds the gunk and the bioslime, and naturally more mineral available to the plants. I doubt the gravel you are using will be any more or less iron robbing than other medias, just my hunch. Run your iron theory by Vlad, and see if he agrees. He knows his iron chemistry.

I'm also not sure if I agree that iron caused the yellowing and chlorosis your basil exhibits looks like over-all deficiency, which may be caused by low DO under your raft (low DO causes roots to be poor at uptaking all nutrients, even if they are present in available form). Iron shows yellowing only in the newest growth, nitrogen in the oldest growth, and your basil pic was yellow forms stem to stern.

Comment by Bob Campbell on July 2, 2012 at 10:10pm

I just posted this in Media Based Systems but could not find a way to link directly to my post.

II think I can find the time to pick up half a yard tomorrow.  I'll let you know what I think ASAP.

Robert C. Rowe (Bob)  suggested pumice so I checked a local landscape supply here in Chico, and found this.  The handful below may need a little screening, but the price is right and the description says it's pH neutral! 

I'd suggest looking around at your local soil suppliers

Pumice(3/8 x 1/16) Pumice is an excellent soil conditioner, as it is highly porous giving it excellent water and air holding properties.

Advantages of Using Pumice:

• Excellent conditioner for soils that need increased aeration and drainage.
• Loosens the density of heavy soils, letting in the air and water plants need.
• Increases water retention in light and sandy soils.
• Reduces crusting, cracking, flooding and shrinking & swelling of clay soils.
• Holds moisture in the soil, reducing watering requirements by as much as 35%, but pumice will not compact or become soggy.
• Is inorganic, so it will not decompose or compact over time, meaning it functions continuously and can be recycled and reused.
• Does not attract or host fungi, nematodes, or insects.
• PH neutral
• These advantages can be realized with as little as 10% addition of pumice to the soil or growing medium.

$28.50 ½ yd. - $55.00 per yd.

Comment by joe on July 1, 2012 at 11:54am

I use the lava rock, scoria. It is quite rough on the hands. However, I never dig in it with my hands. I always use a trowel. For planting, I take two hand trowels facing each other and push them into the gravel. This creates a cavity where I insert the plant, then I remove the trowels and the trench will mostly back itself.

The gravel you have is not affecting pH? In my experiments I found Pea gravel to buffer the pH upwards. pH is really important for iron availability. I try to keep my system pH below 7, which for me means adding a lot of HCl because my tap water is 8.2 pH.

The biggest issue in AP with iron though, in my opinion, is not simply pH, but a deficiency of iron in the media. In soil there is usually abundant iron and the problem of iron deficiency in plants is high pH. In AP, there is not much iron, even if it is highly available. So it must be added. I use Iron Sulfate.

I have heard a lot of good things about expanded shale. $80/yd isn't too bad, if some of you want smaller quantities maybe you could combine your orders and get a yd.

Comment by Bob Campbell on July 1, 2012 at 9:14am

@Fishy - I'm glad you are willing to experiment with Expanded Shale.  Since my system is  currently filled with large plants; I'm not motivated to make the long haul to Stockton and back right now, but after I pull all the tomatoes out in the fall I will want to replace the gravel with something lighter, and less abrasive to my hands and finger nails.  I'm not a girly guy, but man that gravel is really hard on the fingers.  I have figured out that adding pea gravel helps, but I'm guessing that expanded shale will be much better.

I have encountered another problem with gravel in my outdoor system where I have both gravel and deep water culture tanks sharing the same water.  The iron cations are attracted to the gravel, and literally sucked out of the water which caused the plants in the raft to turn yellow. 

The basil plant on the left is from the raft, the plant on the right is in the gravel.

It has taken quite a bit of iron chelate to saturate the gravel.
I want to find out if expanded shale will also interfere with the Cation Exchange Capacity (CEC) like the gravel or if the FE2+ will remain more available to the plants in the deep water culture.

I have recently made several blog posts about iron deficiency that may be of interest.  ChicoAquaponic.  I wish I had taken chemistry in school.  Understanding the intricate relationships of bio chemistry has become very interesting to me.

I also expect expanded shale offers more surface area for the bacteria which is really important to me because my system is out of balance, and should have more area devoted to grow beds. 

Comment by Fishy McFisherson on June 30, 2012 at 11:04pm

I'll be taking a serious look at buying media in the next month. I have the info from the first time you posted the expanded shale retailer in my bookmarks. I'll end up needing slightly over a yard, and after shoveling a yard and a half of gravel in the last week, half the weight sure sounds worth the cost difference....

Comment by Bob Campbell on June 30, 2012 at 4:12pm

Here's a report from my contact in Tucson Arizona who bought some expanded shale from a retailer who is repackaging bulk into bags.  Do the math... $89/yard and they resell it for $20/60lbs

$20 for 60lbs. It's very light even the bigger chunks, though the dust is very fine and sandy requiring lots of washing.

I've been trying to get my local landscape retailer and hydroponic stores up here in Chico to carry it but nobody seems to see the value.

  Oh well, someday they will remember this long haired crusader preaching the attributes of expanded shale and wonder why they waited so long.

Maybe now that Hydroton is off the market they will take notice.

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