Tomato Magnesium Deficiency

bbrushJuly 1, 2010

Is this magnesium deficiency?

The problem is that I make my own nutrients and I am sure I added magnesium sulphate to my B solution, although I was very tired when I made it up :/. Is there anyway of testing that there is or not magnesium in the solution? Doesn't calcium nitrate react with magnesium sulphate to form gypsum?

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Hi bbrush,
Magnesium deficiency shows indeed up in intervenal chlorosis but it differs from iron deficiency (which shows similar symptoms) that it shows up in older and bigger leafs first. Compared to iron, Mg is one of the more mobile elements and thus moves from older leafs in young shots when missing or deficient. Iron deficiency will, due to its immobility, affect shots and younger leafs first. That's why and how you distinguish both pretty easily.

If I get you right, you are sure but not 100% to have added Mg and want to take a small amount of A and B and mix them together in concentrated form - to test if you haven't forgot to add it. Because the calcium nitrate would then react with the sulphate part of the magnesium sulphate and precipitate calcium, and otherwise not?

Well I wouldn't consider this as a good idea (or a reliable test), as there are other sulphates (a few trace elements, perhaps potassium sulphate) in B solution that would be potent enough to provoke calcium precipitation as well. I guess the presence of magnesium sulphate would show a stronger reaction with much more precipitation, - but that fact doesn't qualifies it as a reliable test, if you ask me.

The actual (pragmatical) confirmation would be the final amount of B concentrate when finished. If ever you forgot to add your magnesium sulphate (which represents a considerable amount in mass and volume), you would end up with a much smaller (as unusual) quantity. If you don't remember having noticed such irregularity, it would be good enough (for me) as a confirmation that you have actually added it. ;-)

PS: how many ppm of Fe and Mg did you add to your formula? Some "classic" vegetative formulas may come short in Mg and Fe.

    Bookmark   July 1, 2010 at 11:40PM
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Thanks Lucus, I am using this formular The Best of Growing Edge p58 (veg) and have my EC at 1.5 so according to Daniels calculator Mg is 26ppm and Fe is 3.85.

Yes my new leaves and shots were effected, so it must have been iron deficiency? The problem seems to be corrected as 2 days before I posted that I bumped my EC up by adding some of my general solution as well as bringing my pH down from about 6.7 to 6. Could it be that the pH was to high for iron? As it is obvious that I did add iron to my A solution.

    Bookmark   July 2, 2010 at 5:57AM
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I have looked up the recipe from the link and typed the formula in my personal calculator and ended up with a bit over 50 ppm of Mg with this formula and also with 6 ppm of iron. But for the actual formula, not "downsampled" at EC -1.5 mS/cm. The actual EC of the formula turns around 2.0.

I guess there is an disambiguation on two levels to do, before concluding anything:

1. Have you done a sure disambiguation between:

A. Magnesium sulphate heptahydrate (MgSO4.7H2O)
B. Magnesium sulphate (MgSO4)
Normally (mostly) formulas refer to A. Magnesium sulphate heptahydrate.

2. What was your input data? I've calculated all weights as shown for 2000 Liter total (20 Liter diluted to 1:100 equals 2000) and all other ppm values as in NPK look in fact plausible to me. Although Phosphorus is abundant while potassium comes quite short from my own understanding of a balanced tomato formula.

PS: yes Fe availability is affected by pH, but (as it is rather immobile anyway) such short time swing in PH would normally not end up in Fe deficiency in an earlier growing stage with tomatoes. I've grown tomato seedlings in random pH (probably around 6.5-7 which is a frequent non corrected pH) and with only 1.2 ppm of Fe, low EC (lettuce formula) and a fast growing rate, without any signs of iron deficiency. Well, this part is just a selective observation from my end, - no evidence for anything and not a recommendation! 3.5 ppm of Fe is what I'd recommend with toms for all growing stages;-)

    Bookmark   July 2, 2010 at 8:21AM
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Hello BBrush,

It would be great if you could post the exact weights you put into your formulation so that we can adequately calculate your concentrations. Using the formula given on that page and my calculator I get an Mg level of around 40 and an Fe level of about 5.9 for the amounts given when taking into account that you are working at an EC of 1.5 mS/cm and assuming that the original formula gives 4.0 mS/cm after a 100 factor dilution.

Iron deficiency has many causes besides lack of iron in solution. Antagonism from P and high pH are some of the most common causes.

I agree with Lucas in that if you added both Mg and Fe it is probable that you didn't have a lack of those nutrients in solution. Also if you used FeEDTA the shift of pH towards 6.7-7 in a short time is not have bound to cause any other problems. As always addressing the true cause of this problem will prove to be a monumental task due to the incredible number of variables involved but the important thing - of course - is that you have solved it !


Here is a link that might be useful: Everything Hydroponics

    Bookmark   July 2, 2010 at 3:08PM
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Thanks, maybe I am using your calculator wrong, but I added the amount of salts from that page and added it to 7200 litres to get the ec at 1.5, that site says ec of 4 but the calculator says a ec of 5.39 if add it to 2000 litres?

Yes you are correct, impossible now to figure our what was wrong as I changed a few things to correct the problem. I am going to do a reservoir change this weekend and will add the same nutrients and the same EC and will see if the problem comes back.

    Bookmark   July 3, 2010 at 3:12AM
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thanks, I have already made up the stock solutions, scaled it down to 5 litres of A and 5 of B, thats why I was a bit worried that I forgot to add magnesium.

    Bookmark   July 3, 2010 at 8:21AM
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Seems that for some reason we are not exactly at the same page here. I have rechecked, as I did the input in a hurry and in fact did use 4000 Liter. Notably because with this amount the formula makes much more sense (as a final concentration) as with 2000 Liter.

With the previous amount of water I got indeed 53 ppm from 2136,00 of Magnesium sulphate heptahydrate (MgSO4.7H2O). But actually that would mean 108 ppm for 2000 liter. I counterchecked with another piece of code and got 105 ppm of Magnesium.

I also don't exactly see how bbrush's problem is "solved"...?!

My calculator estimates the EC for 4000 liter to 4.18 and to 1,11 for 7500, which is in fact week for tomatoes and could very likely be part of the problem.

I have always said that I am no friend of EC calculations based on LEC equivalency. While perfectly valid for lab grade chemicals and laboratory studies, the formulas do not exactly translate properly to nutrient making because of purity issues and contaminants of components that may vary and are difficult to evaluate.

I remember very well having a rather though debate on this topic a while back here, when revealing that I actually measure all components one by one with my instruments and note their conductivity. That was when I was finally "caught with using non-scientific methods" LOL. Now I am actually glad that with another case, my own method is much closer to the facts. ;-)

We can try to get on the same page here, but we are not obliged. I am happy with my tool set, methods and results, which have grown and proven to be reliable over the years. Thus I have no need for any debate nor discussing matters all over again that are apparently not meant to find consensus.

So, never mind ;)

    Bookmark   July 3, 2010 at 8:24AM
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Little correction: I like rounded numbers (7500) - with 7200 Liter I would get 1,16 mS/m, which is not much more after all and would likely be equally insufficient, if so.

    Bookmark   July 3, 2010 at 8:45AM
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Hello Lucas,

You cannot draw me away from a provocative discussion with a "never mind", lol :o).

I do agree with you on the Mg ppm with a 2000 liter volume. My calculator indeed gives 105 ppm and the difference with your 108 could be due to an addition of factors like rounding, exact molar masses used, etc. Our differences in EC might be due to the different values we may have for limiting conductivities but they are close enough for the whole rough approximation this is anyway.

You are right in that limiting ion conductivities do not give very accurate representations but the reason you give is actually not entirely correct. The problem is not purity but in fact the problem is that limiting ionic conductivities are only accurate and additive when ions are in what we call "infinite dilution" that is, when concentrations are below 0.0001 M and ion with ion interactions are minimal. Literature on the field shows you that purity may only account for about -10% of your error while ionic interactions when using concentrated solutions account for +90% of the problem.

In hydroponic solutions, concentrations are higher and ionic interactions are present and in fact important. What you do is not so "non scientific" but it is actually an empirical method that take into account ion interactions due to your direct measurement (of course you also take into account any purity factors). Obviously, your results are bound to be much better, so I would in fact expect your method to be closer to the real life measurement :o).

What I am trying to do right now is to build a set of empirical rules based experiments similar to yours that allow a better estimation of the ionic conductivities. I think that it should be fairly simple to do and it will probably allow the estimations to be at an accuracy of around +/- 2%. We'll see if I can achieve that :o)

So I think we are pretty much on the same page anyway :o)

Best Regards,


Here is a link that might be useful: Everything Hydroponics

    Bookmark   July 3, 2010 at 8:53AM
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Provocative? Maybe figuratively speaking but no... mot meant to be actually provocative. But right, I knew that what I am doing is in fact not that non-scientific and here I was getting ironical because of the history and the unfair criticism I earned with it a while back. It's in fact nothing else than a well known empirical method that was already used formerly with hydroponics.

OK, then let's see if we are entirely on the same page for the rest of it.

1. Would you agree with me that with such experiments (actually only a series of measurements) to determine empirically confirmed conductivity data, one should use a dilution that is close to "reallife" or typical measurements, as in 1-1.5 mS/cm and 1.5-2.0 mS/cm? This would imply very distinct and chosen solutions that are close to these conductivities, and for each component. The crosscheck could in fact be done with measuring conductivity of several combinations (sets) of components when mixed in "typical" proportions plus typical total concentrations. With this method one would come extremely close to real life, right?

2. What is your opinion about the "Growingedge" tomato formula that bbrush was/is using? Do you think it is suited and balanced for tomatoes. In my opinion it could be improved. In fact it could use a different N to K ratio, even more Potassium for growing stage. Not more than 150-160 ppm N, which is already reached at 1.5 mS/cm. And notably less Phosphorus to avoid any P antagonism in the first place. With P, less is indeed more.

I have "downgraded" the "Growingedge" formula to 1.5 mS/cm (which I can do with my empirically determined data set more accurately) with my calculator, as this is what bbrush was using, if I understood that part right. It happens that you end up with 38 ppm of Mg and the "dilution factor" would equal 5590 Liter then. That's more than enough. The P content on the other end is as high as 50 ppm with only 1.5 mS/cm, which is clearly too high with this concentration. Probably part of old school of more generous P content.

As suggested, bbrush'es problem is only solved as soon as the reasons for Iron (or other) deficiencies are found and corrected - that's what I meant and that is not provocative, is it? Hence from my perspective, getting on the same page implies to actually trying to solving the real problem. ;-)

Regards and cheers,

    Bookmark   July 4, 2010 at 12:16AM
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One more correction about measuring conductivity: in fact the optimal concentration to measure with each component, would be lower than 1-1.5 or 1.5-2.0 mS/cm because their actual proportion in a nutrient solution is in fact lower ;-) These would rather be used with crosschecks of combinations and formulas.

    Bookmark   July 4, 2010 at 12:24AM
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Hello Lucas,

Well I meant "provocative" in a positive way regarding the spark of interesting discussion :o) I didn't mean you wanted to start a heated discussion or anything like that.

I think we are pretty much on the same page.

1. Yes I agree with you in that this approach should probably give a conductivity estimation closer to that found in the real problem. Since you are developing an empirical method which is a closer match to the real life problem. Of course, such an approach is bound to give better results but its actual accuracy will depend on the actual overall approach you use. The accuracy of your measuring equipment for volume, weights, EC, etc. Using adequate analytical chemistry techniques with adequate error propagation should give you an idea of your expected error from measurements and your expected errors from the method itself. How "extremely close" you can come depends on the above errors and the overall design of the technique.

2. I do agree with you in that it is not what I would call a "perfect" tomato formula and indeed I consider that the N:K ratios and P values could be improved. It is a general "old school" thing - as you cleverly put it - to use too much P and cutting back on this should give better results. However the actual "ideal" nutrient levels for Bbrush should also depend on other factors including weather conditions, tomato varieties, if other plants are present, etc. However I do agree that lowering N and P and increasing K would be a good first approach.

The "real problem" is a little bit hard to address here because of the lack of information. There is a myriad of causes why the deficiency could have shown which could range from pH changes to temperature to nutrient solution concentrations, etc.

To address the true cause of this problem I would need much more information. I would need at least a daily pH/EC, nutrient temperature, air temperature log, pictures of younger leaves, old leaves, nutrient makeup information, scale information, volume information, etc. At least for me, I cannot do anything but guess if I cannot have the above mentioned information and therefore the only thing I could advice is to gather the information so that we can know what is the true cause.

However I am glad to help whenever I can and if anyone wants my opinion about something they can simply ask :o) I can't however give adequate advice without adequate information. Bbrush is currently repeating the addition to see if the problem is indeed caused by the nutrient makeup. We will see if this is actually the cause or if it was merely a temporary effect due to something else.

As you see, we pretty much are on the same page Lucas. I believe you have become a little bit defensive from your previous forum discussions :o)

Best Regards,


Here is a link that might be useful: Everything Hydroponics

    Bookmark   July 4, 2010 at 8:17AM
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Haha! Wow, and I thought I had some knowledge of Chemistry. Bravo you two, bravo. This is such an interesting discussion between two very good chemists/hydro-gardeners.

Hopefully reading your posts will teach me something about chemistry in relation to mixing my own nutrients. I would love to save the money and make my own nutrients but so far it seems all too complicated. I get what the ion interactions mean (sort of.) Is the "limiting ion conductivity" something like the limiting reagent??

    Bookmark   July 4, 2010 at 4:22PM
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Hello Ethnobotany,

Thank you for your comment :o) I am glad you like the discussion and find it interesting.

I will write a tutorial in a few days about making your own nutrients so that you may have an easy "start-up guide" to make your own nutrients using my calculator with some basic general formulas. :o)

Regarding your question, limiting ion conductivity is a very different concept and it doesn't have anything to do with limiting reagents. The limiting conductivity of an ion is merely the value of its conductivity per mole at infinite dilution. It is called limiting conductivity because it is the limit reached by conductivity for an ion when approaching a concentration of 0 M. It allows us to calculate the conductivity of complex mixtures with extremely good results when concentrations fall below 0.00001 M and ion interactions are minimal (limiting ion conductivities are additive in this case). Higher concentrations introduce errors which must be corrected through empirical models or through much more complex calculations.

I hope this answers your questions :o)

Best Regards,


Here is a link that might be useful: Everything Hydroponics

    Bookmark   July 5, 2010 at 7:44AM
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Thank you very much! That did answer my question. I will be looking forward to seeing your easy-to-do guide for the nutrients! I know there has to be a better way than spending $40 - $50 on a set of 3 tomatoes!

    Bookmark   July 6, 2010 at 1:10AM
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Hi again Daniel (and ethnobothany)!
Actually relieved that I can lower my guards for a second LOL.
It is with great pleasure I realise that we are indeed at the same page and that consensus is more likely to be produced among "objective subjects" than with opponents. ;-)

Let me just add one more thing about the precision of the described empirical method to determine conductivity coefficients for nutrients. Of course that the precision is still limited and that sources of error aren't excluded (btw: you didn't mention the interaction of pH and EC reading in the context). But then again the conditions and the toolsets-, actually the precision that will be used in real life when putting the results to practice, aren't any better or more precise. The error quote in practice will most likely be much higher, due to individual variables and imprecision of less precise equipment in use. This doesn't mean that you wouldn't aim for best precision with any method, but that the precision that is used eventually when using it in real life, will determine the actual precision and outcome anyway.

That was actually an argument I used to defend the relative "imperfection" of my method formerly - and yet it was generously ignored with all other then. I'm not defensive here, but merely expressing my disappointment about how "not-invented-here" alternatives typically end up in rejection and denial. Not so with you Daniel, - you are smarter, as you manage to reshape, enhance- actually give it a scientific touch and use the idea for your own purposes and ends. How smart indeed LOL ;-)

About solving or diagnosing a plant deficiency problem; of course that without complete information it is rather difficult to achieve. But a good pirate isn't afraid to even sail unknown waters (including the Red Sea as they say). And a good physician also needs to treat comatose patients, even-though they are hardly able to tell their medical history or describe any acute symptoms. I know, analogies suck, as they never actually describe the real thing - but they're fun to elaborate and are pretty illustrative then and now... ;-)

In other words, I am used to working something out from the guts with what I've got. And yet my quote is actually pretty good over time.

This part I'd like to quote:
>>I believe you have become a little bit defensive from your previous forum discussions :o)That's what happens if you make concessions, - they do not result in mutual concessions, but get used against you eventually LOL. Aren't you stating the obvious here, while using what I confessed earlier!? I know you're kidding, as I am - and yet! Hahahaha...

In fact, the term "defensive" in such or similar context has become quite a clichand is often used in a rather inflationary way. Having a closer look at the thing-in-itself, you discover that in fact it's the hell of a paradox as the term is often used in a defensive way! Hence telling someone they are defensive- means actually acting defensively ;-)

One more analogy about colloquial abuse of terminology, for the fun of it?
Imagine a top ten rated "public enemy" chased around the world for nearly a decade. Finally tracked by joined forces of C.I.A, N.S.A and Interpol, plus Mossad to top it of. Eventually trapped somewhere out in the sticks by a sniper team where no escape is possible. The shooter that has got the guy in his cross-hairs for some time already, awaiting the "go" for the take down coming in from some dubious chain of command any second,- thinking out loud: "can't you hold still for a nanosecond you paranoid brat"?! LOL

Btw: You may prefer using a distinct personal charm mixed up with a pinch of America Del Sur soft soap at your end Daniel, to avoid any conflict in the first place - which is actually smarter. That's just fine with me and I promise I will not call it silky or hypocritical nor blame you for that, ever ;-)

My most sincere apologies for getting that "offensively defensive", and bit rude again- far off topic also! ;-)
Really nothing personal involved or intended here, apparently I just can't prevent myself from thinking out loud!

Cheers and regards,

    Bookmark   July 6, 2010 at 1:29AM
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