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I need help.  The raft flows into the ebb & flow gravel bed sharing the same water,  But the plants in the raft are yellow where as in the gravel they are green.   These basil plants were planted at the same time from the same pack. 

Here is my water chemistry.  I don't believe it can be the chemistry because the water is shared, but I don't want to leave any information out.

pH = 6.8. 

Ammonia = 0

Nitrate = 1.0, 

Nitrates = 2.0

Phosphates = 10 

GH = 7

KH=3

Here are the two basil plants side by side.

I'm wondering if this could be caused by the light coming through the IBC and vacant holes in the raft.  I will correct that ASAP, but in the meantime; is there anything else that pops out for you?

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I forgot to mention there is a lot of aeration in the raft from 4 air stones.

have you tried maxicrop with iron?  how old is the system?  what are you feeding your fish?

are you filtering the water prior to the dwc?  what do the roots on the raft plants look like?

@Keith - I added 1/2 bottle of Micolift Chelated Iron several weeks ago. That's about twice the amount recommended.  I have an Iron test kit on order and it should arrive any day.   Currently it's pee ponics until I get the nitrites down.  The water is filtered through an evaorative cooler pad, but since there are no fish yet it's just passing water.     Get the pun?

I have a ton of aeration going on and the roots are clean and white, but not as long and full as I would expect

funny!

i'm not familiar with micolift but it sounds like you have the chelated iron bit covered..

i would suspect that the media beds are keeping the nutrients closer to the plants (the nitrification process is occuring mainly in the media where all the surface area is)

i also would guess that as the system matures, the dwc will be ok

is there any chance you could have the water to to the media bed, then to the dwc?

 

Hi Bob, 

Almost any type of media (with the possible exception of maybe perlite and some types of sand) has something called a CEC (Cation Exchange Capacity). People will often test their soil's CEC before planting or sowing a new cultivar in order to get a better idea of how much of what amendments to add. Hydroton, gravel, expanded shale all certainly have a certain CEC.

Cations are just ions with a positive charge (like plant usable potassium, calcium, iron, magnesium, zinc etc...)...and a medias cation exchange capacity is that medias ability to "hold onto" cations...This seems to make those nutrients more plant available physically (because they are there where the plants need them...around the roots..."clinging" to the media). This would appear to be why lots of deficiencies will first show up in NFT or DWC raft, and not in a media bed even though all those 3 components may share the same water...A water filled raft as a hydroponic sub-system in AP, so it seems, is going to show deficiencies first, before a media bed will... because its ability to "hold a store of" certain nutrients (it's CEC) is much less than any type of media.

Hope the above makes sense...I kinda suck at explaining stuff sometimes...

@Vlad -

That sounds like a reasonable explanation.  Last week over the course of several days, I dropped the pH from 8.2 to 6.8.  I believe this can cause the nutrients to precipitate of the solution, as you have suggested.  So maybe they are sticking to the media thereby making that grow bed less susceptible to insufficient or locked out nutrients

If that is the case I will have to figure out how to make them available. It seems my parameters are all within an ideal range so that may be a challenge, but I have an iron test kit on the way, and I'm hoping that may give me a some clue.


The reason I dropped the pH was because both tanks were beginning to show yellow leaves.  The gravel bed has recovered but the raft has continued to get even worse.  This really does seem to indicate that you are correct.


What do you think my best course of action is?  I'm kinda leaning toward wait and see, but it's so distressing to look at those poor plants..

I think that by dropping the pH you will have solved your iron deficiency...Iron deficiency it seems is almost always pH related...too much above neutral will start to cause iron lock out...I've done this on purpose in a small system (kept the pH between 8-8.5 and watched the iron deficiency start to set in. I foliar fed an EDDHA iron celate and got to watch the plants respond well...then let them go deficient again (pH), then dropped the pH and the deficiency went away without foliar feeding.

Of course this comes with the understanding that you have at least 2-3ppm iron in the system, which is enough to stave off deficiencies in most plants...usually chelated iron is plant available almost regardless of pH (within reason of course). I'm not familiar with Micolift, though I don't see why it'd be any different.

Now, I believe most nutrients are more apt to precipitate out of solution at a high pH rather then a low one, so my thinking is kinda backwards from yours on that one...You may have "freed some up" causing them to go back into solution when you dropped the pH...IDK...At any rate, our plants will thank you for the drop in pH.

I'd just wait and see now that your pH is lower (below 7)... Or foliar feed some iron. Plants seem to respond/get better quicker that way...

I wouldn't bother with a test kit (well, it would depend on the cost/hassle). Put an oregano plant in the raft and use it as your "canary in the mine" as far as when to add iron. It should start to show an iron deficiency sooner than most other plants giving you time to react. See if that works for you...maybe you could compare it with what your iron test kit says...might be interesting info to have/share...

Don't confuse nutrient precipitates with cation exchange capacity...though your 'kinda' on the right track...I guess you could look at it as a matter of scale. CEC is on a "smaller" level (meaning tiny elemental ionic level)...precipitates are "larger" usually comprised of different elements that have formed complexes with one another. Like calcium phosphate, the white-ish-grey muck that's in everybody's hydroponic (media) systems after a while...zinc and iron will often stick to the calcium phosphate as well, adding to the muck. You can run your hand through it, touch it, see it...ions (cations or anions) you can not. They "cling" to the media small and invisible. (These are simplifications, but you get the idea)...

Anyways...hope the plants make a full comeback soon for you.

Oh yeah...I'm thinking your raft system should "catch-up" as your system gets older...nutrient water gets richer and pH is kept "in range"...

Thank you Vlad, you are a world class expert, and I feel confident saying we all appreciate your input. 

I will foliar feed with chelated iron in order to save the plants, and wait to see what happens.

Of course I will also document the water chemistry tests and results so that others can learn from this experiment. 

I think it will be very interesting when I am able to measure the iron.  I hope this test knows the difference between available iron and locked out iron.  We will see...

To present all of the details seemed like too much information, but it now appears recently lowering the pH was the most important of all information I could have shared.  Kudos to you for you hitting upon a very likely cause even though I left that out!  

I guess should also tell you I added about 100 grams of humic ore, 1/2 cup of dried seaweed and some fish flour last week.  I don't think that was enough in a 500 gallon system to make much different, but now I'm feeling like I should not hold any information back.

When I get this system stable I think I would like to do more experimenting with the humic ore.  Have you ever used it in aquaponics?


  

Bwaahahaa...I'm no "expert", but thanks...

Documenting what you do is always nice...for yourself and others...

Nope I've never used humic ore in anything, (wait actually I have, just nothing interesting) but had a chance to learn a bit about humic acids (and tannins) and how they may relate to helping to keep plant usable Fe2+ from reverting back to unusable Fe3+ (rust basically)... Turns out  it's pretty much how people made iron gall ink for for almost the last 1000 years. Good stuff...Like most processes it's pH and temp related (well within AP range really)... I can link the threads or give you the rundown on what I found (since it's scattered through different threads I think) if your interested...being iron related and all...but right now I've got to get to bed. It's half past midnight and I've got to be up in about 5 hours to do farming stuff...If it's not raining in the morning...

"world class expert"... good one Bob...Then again, flattery will get you everywhere

Bob Campbell said:

Thank you Vlad, you are a world class expert, and I feel confident saying we all appreciate your input. 

I will foliar feed with chelated iron in order to save the plants, and wait to see what happens.

Of course I will also document the water chemistry tests and results so that others can learn from this experiment. 

I think it will be very interesting when I am able to measure the iron.  I hope this test knows the difference between available iron and locked out iron.  We will see...

To present all of the details seemed like too much information, but it now appears recently lowering the pH was the most important of all information I could have shared.  Kudos to you for you hitting upon a very likely cause even though I left that out!  

I guess should also tell you I added about 100 grams of humic ore, 1/2 cup of dried seaweed and some fish flour last week.  I don't think that was enough in a 500 gallon system to make much different, but now I'm feeling like I should not hold any information back.

When I get this system stable I think I would like to do more experimenting with the humic ore.  Have you ever used it in aquaponics?


  

I have been reading about Cation Exchange Capacity, and now I wonder if it is a good idea to have both a gravel bed and a raft in the same system.  From what I have come to understand; the water in the raft will have a very low CEC, and in my mind will be in direct competition with the gravel for nutrients.

If the gravel is filled with anions which attract and hold on to the cations, won't this leave the water in the raft tank with very little nutrient availability forever?   How will the imbalance ever be resolved?

I don't know if I've just demonstrated my understanding of this topic or my complete confusion. 

Vlad Jovanovic said:

Hi Bob, 

Almost any type of media (with the possible exception of maybe perlite and some types of sand) has something called a CEC (Cation Exchange Capacity). People will often test their soil's CEC before planting or sowing a new cultivar in order to get a better idea of how much of what amendments to add. Hydroton, gravel, expanded shale all certainly have a certain CEC.

Cations are just ions with a positive charge (like plant usable potassium, calcium, iron, magnesium, zinc etc...)...and a medias cation exchange capacity is that medias ability to "hold onto" cations...This seems to make those nutrients more plant available physically (because they are there where the plants need them...around the roots..."clinging" to the media). This would appear to be why lots of deficiencies will first show up in NFT or DWC raft, and not in a media bed even though all those 3 components may share the same water...A water filled raft as a hydroponic sub-system in AP, so it seems, is going to show deficiencies first, before a media bed will... because its ability to "hold a store of" certain nutrients (it's CEC) is much less than any type of media.

Hope the above makes sense...I kinda suck at explaining stuff sometimes...

Personally, I'm pretty sure that you can relax.  I'd put that in the category of one of those many things that occur when your system is new (or maybe seriously out of whack...pH, under stocked etc)...There are a lot of folks running mixed/hybrid systems that do just fine...




Beginning Wednesday 13 June 2012 I began folar feeding chelated iron


On Monday 19, June I measured the iron and chelated iron 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
Vlad explained how dropping the pH from 8.2 to 6.6 could cause Cations rob the water of it available iron.


In the above photos you can clearly see a difference between the gravel and the raft.  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, Chelate=0
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, Chelate=0
On Monday I added another 60ml and finally got a reading of zero FE and 1 ppm Chelated Iron.
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 have 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. 


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

It seems that the cations are still robbing the water of all available iron.  
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.

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