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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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Bob, Iron is a cation. Lowering the pH (like you did) will make any iron (Fe2+) in the system available to the plants. The generally recognized minimum is 2ppm to stave of deficiencies...

Though honestly, looking at your photos, it doesn't look like the worst of the chlorosis is on the new forming leaves (as the case would be with iron). 

Magnesium deficiency looks the same as an iron deficiency (inter-venal chlorosis), except that it usually occurs on the older leaves (Mg is somewhat mobile in plants, so it gets "pulled" out of older leaves to support the new growth, iron is not so mobile)...in a brand new system Mg can be just as problematic as Fe depending. People mitigate this with either Epsom salt, or buffering with dolomitic lime...

Some of those new leaves look green while others don't...were they that way befor you added the iron as well?

And don't look at cation exchange capacity as "robbing" you of anything...LOL...

i find this very interesting.  i've got a fully mature low-density hybrid system and i see a similar pattern.  plants in gravel are fine.  plants in NFT show yellowing deficiencies on new leaves.

i'm attempting to increase nutrient levels as summers temps rise, feeding the fish more.  we'll see if anything changes, but so far it seems like some critical nutrients are locked up in the gravel bed.

The leaves have gained green color since adding iron, even the new leaves were yellow.   I poured the iron right on the hydroton and let the excess leak into the water (I read your post about two root zones).  I have also been foliar feeding with a strong FE2+ mix .  

Before I ever added any FE2+ even the plants in the gravel were yellow.  They have completely returned to a very healthy green.

I too have been thinking about Magnesium.  How much Epson Salt should I add?  I only have a few mosquito fish in the sump tank and they seem to tolerate anything so I'm not concerned right now about fish..


A quick search for the amount of Epsom Salt to use revealed 1 tsp per 100 gallons.  Does that sound about right? 

Or should I use my salt meter and go for a specific dilution?

The thing I find interesting is that the gravel still seems to be stealing the FE2+ citions.   Thank you for correcting my use of the term cation.  As you can tell I've never taken chemistry, but I'm learning

I hope that by continually adjusting the FE2+ and Mg chelate I will eventually satisfy those cation hungry rocks and find some balance. Did I use 'cation' correctly this time?



Vlad Jovanovic said:

Bob, Iron is a cation. Lowering the pH (like you did) will make any iron (Fe2+) in the system available to the plants. The generally recognized minimum is 2ppm to stave of deficiencies...

Though honestly, looking at your photos, it doesn't look like the worst of the chlorosis is on the new forming leaves (as the case would be with iron). 

Magnesium deficiency looks the same as an iron deficiency (inter-venal chlorosis), except that it usually occurs on the older leaves (Mg is somewhat mobile in plants, so it gets "pulled" out of older leaves to support the new growth, iron is not so mobile)...in a brand new system Mg can be just as problematic as Fe depending. People mitigate this with either Epsom salt, or buffering with dolomitic lime...

Some of those new leaves look green while others don't...were they that way befor you added the iron as well?

And don't look at cation exchange capacity as "robbing" you of anything...LOL...

I'm not sure what meter you have...but the way I'd do it is...

First figure out a target ppm. Most hydroponic formulations (Benton, Papadopolous, Arnold et al) will call for a 50ppm concentration of Mg in solution. Though keep in mind this will not be enough for things like cucumbers and tomatoes. So you need to choose a target threshold (ppm). Once you've chosen a threshold amount...I'd convert my system water volume from gallons to litres (in this case ppm is equal to mg/L and metric becomes much easier to work with than Imperial...so we'll use ppm and mg/L interchangeably)...

Magnesium sulfate (MgSO4) in the form of Epsom salt (MgSO4-7H20) is roughly 10% Magnesium.

Knowing this, and knowing your target threshold and knowing your total system water volume...it should be easy to calculate how much Epsom salt to use. I'd stay away from 'teaspoons' for now and stick to grams...My particular teaspoon holds about 3 grams according to my gram scale. But you should measure yours...So only 10% of those 3 grams is actually Magnesium...So 300 miligrams of Magnesium.

Though it is my opinion that it would be really keen to get your hands on some dolomitic lime (CaMgCO3)2 and buffer your pH with that, no need to figure out anything, just get to your target pH... and choose a target threshold for the Epsom salt way lower than the 50 mg/L (20-30 mg/L perhaps...)

Magnesium deficiencies can be (have been at times) pretty hard to correct sometimes...but the next batch of plants should fare better...and hopefully by then you'll have more fish...eating quality feed and less of these kind of things to worry about. (Though it might still be a good idea to alternate buffers...dolomitic lime one time, potassium bi-carbonate the next, then calcium based lime the next...repeat cycle...blablabla...)

I've slowly come to find that a 'balance' and ratios between the major cations (NH4, which you don't have to worry about in AP...K, Ca, Mg, and to an extent Zn) may be more important than their specific actual amounts...

I want to give you an update.  I tested the MicrobeLift Chelated Iron I was using and discovered the product has only a trace amount of chelated iron.  I've posted the test and included pictures of my plants since I began using a different product.

It seems my lack of iron was due to putting my trust in a very poor product.  But I'm not looking at this as a negative experience because you have sparked my interest in chemistry. I've learn so much in the past few weeks, and now I want to learn more.

Thank you for all your help and the many discussions you have contributed to!

Click Here for a link to my test results



Vlad Jovanovic said:

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...

Wow! What a difference in the amount of Iron in those two products! (Usually they'll list the percent...6%, 10%, or 12% seem to be popular numbers). I bet your right, that MicrobeLift is probably meant for aquariums with clear water...

The plants look way better now. Nice going!

Bob just know that that product also has an N-P-K value of 0-5-8 (in addition to the iron) so long-term and regular use might not be such a good idea because of the phosphates (-5-). It is entirely possible (and eventually probable) to induce an algae bloom in your AP system with the excessive phosphates (especially once you have fish and all). For long term iron dosing, it might be wise to purchase a product that is just iron with no other nutrient content.

There is a 6% EDHHA iron chelate from 'Growmore' that should be available to you...(they also make a 10% EDTA iron chelate that is a bit cheaper to buy...I know it seems weird that the one with almost twice as much iron is cheaper, but you really might want to sport the extra couple of bucks for the EDHHA 6% because of the plant toxicity issues related to EDTA).

Looks like that Fe test kit was well worth it already. 

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