Sulfur: real need and uptake with hydroponics.

14 years ago

I've always wondered about the high Sulfur content in classical (and my) formulas. Well, as/when we use Magnesium sulfate to add the necessary Mg to our formula, we can't avoid having relatively as much sulfur as the content of MgSO4 * 7H2O has anyway (50 PPM of Mg equals automatically 70 PPM of sulfur). So far so good...

Actually, when calculating a nutrient formula we consider P and K as elemental and actual P and K, but what about Mg and Sulfur from MgSO4 * 7H2O? Because Sometimes also Mg is given as MgO. I recently have found this formula, to transform MgO convention into actual or elemental Mg. It goes like this: Mg = MgO x 0.60. But what about Sulfur, is there only elemental S or is there any "industrial standard convention" as well?!

Also I got this data which shows minima- and maxima content of sulfur and Mg in nutrient solutions.

Mg = 25 -75 PPM

S = 60 -330 PPM

The extreme wide range for Sulfur is probably due to the huge differences of sulfur needs among different plant types, isn't it?. Or do plants have a good tolerance of S, or is it simply pushed that high because of the mentioned high S-content of our MgSO4 * 7H2O? In the later case, it must rather be a convention as an indicator for real S-uptake, right?

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Comments (5)

greystoke
14 years ago
Plants require as much sulfur as phosphorus, one of the elements usually considered a major plant nutrient. Sulfur is found in cystine, cysteine, and methionine, amino acids that make up plant proteins. It activates certain enzyme systems and is a component of some vitamins (vitamin A).

So . . . ALL my hydroponic nutrients contain Â±70ppm P, 70ppm S and Â±50ppm Mg (derived from MgSO4.7H2O, Epsom salt).

I don't vary these quantities at all.
gringojay
14 years ago
Hi jean-luc,
Reviewed some tomato (only) data giving minimum (borderline deficient) - maximum (borderline excessive); please read these as % of total fertilization formula (not ppm) & also tagging along P (for comparison) min.-max.:
(A) with newly mature leaves still on & onset of initial flower:
S = 0.3 - 0.8 %
Mg=0.3 - 0.5 %
P = 0.2 - 0.4 %
(B) with newly mature leaves still on & initial ripe fruit:
S = 0.3 - 0.6 %
Mg=0.25-0.5%
P = 0.2 - 0.4 %
O.K.
So for tomatoes, if take the min. of Mg & max. of S that is their extreme ratio of use (at those 2 stages, A or B).
The drop in the S % maximum, with fruit ripe (stage B), probably indicates less S amino acid (protein) complexes being needed. It can be inferred to reflect a shift in type of enzyme (protein) needing to be formed for ripening activity & want less S competition at binding sites.
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Is the yellowing prevalent in older foliage or newly emerging leaves? Did you check for mites/scale ...? What are you using for fertilizer? If you're using a soluble fertilizer at regular intervals, it's very unlikely there is a physical deficiency of Fe; more likely something pH induced or some other issue. S deficiencies in containerized plants are rare, BTW. If the plant has been in the same soil for more than a year, and your fertilizer doesn't contain Mg, it's a very real possibility that you're seeing a Mg deficiency. If you suspect a Mg deficiency, try mixing 1/2 tsp of MgSO4 (Epsom salts) in a quart of water and spritzing some of the leaves. If they green up, it was probably a Mg deficiency, but the nice thing about the MgSO4 is that it will also correct any S deficiencies. There ARE other cultural conditions that can cause deficiencies, but usually, when you start adding a little of this & that to try to correct a singular nutrient issue w/o strong indication of what's wrong, you end up creating problems rather than solving them. It's better to start from the bottom up, with a close look at your soil, then your choice of fertilizer, because in the end, your soil choice is the most probable source of trouble, if you can eliminate light as the issue. Al Here is a link that might be useful: Also, clicking me might be helpful
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jean-luc
Original Author
14 years ago
Thanks to both of you for the conclusive info, which actually confirms the way I looked at it. I also looked up my formula for Mg and S substraction from MgSO4.7H2O, which simply takes 9.9 Mg vs. 13 % of S. And these are actual PPM. This problem was solved.

But because MgSO4*7H2O amazingly delivers exactly the amount of Mg and S needed... it simply made me suspicious and it came to my mind, that we deal with a conventional compromise here.

I have a tomato nutrient recipe here from the University of Florida, where the S-content vs. P is even higher from the state of so called "last cluster". And I see this as a confirmation too:

N-153 PPM
P - 50 PPM
K-206 PPM
Mg-48 PPM
S-66 PPM
Ca-174 PPM

PS:Part of the problem was a little discussion with a fresh chemistrist, who probably just graduated. When I told him about the S and Mg content in a nutrient, he wouldn't agree. Among other kinda hilarious things, he pretended that it was proven by ash analyses, that Mg uptake was much higher than S anyway. He also claimed that most of the sulfate remained in the solution then. Anyway, people who are smart with chemistry, but have no practical experience with nutrients and actual components, can truly be a real p.i.t.a. He also pretended that there couldn't be any Mg in a nutrient solution, because Mg0 wasn't water soluble. As I told him that MgSO4.7H2O is generally used for the purpose, he wouldn't believe taht either. Later he pretended that there could be release of sulfuric acid then. Here he didn't realise that such acid couldn't do any harm to roots or plants, as it is diluted as soon as it forms.

Well, it took me some time time too to understand the principles of nutrient manufacturing ;-)

PS2: in the meantime I found other info about uptake of Sulfur and it says that S-uptake varies extremely with different plant varieties.
tranzformation
12 years ago
Hi Jean-Luc
Please could you tell me if you are also lucas_formulas http://forums.gardenweb.com/forums/load/hydro/msg022050446641.html?27503
I have used the lucas formula, up to now the reults are excellent. The plants have grown very large and are now flowering like mad, however they seem to be sticking at the flowering stage and not going to fruit.
If you are lucas formula please could you tell me if you change the nutrient formula at the flower stage.
joe.jr317
12 years ago
Yep. He is. He is also Lucas. He changes his name every once in awhile. I'll let you figure out why for yourself. . .

Sulfur: real need and uptake with hydroponics.

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greystoke

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Sulfur: real need and uptake with hydroponics.

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greystoke

gringojay

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