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Nutrient solution observed through several days?

Posted by jamesvlad (My Page) on
Tue, Aug 10, 10 at 21:54

Hello everyone. Ive been preparing some nutrient solutions. With pH = 5.5. First and second day the solution is so clear, seems to be pure water . After three days, there is always a precipitate, just a little. I can see it in a glass jar using a white light. I suspect that is Calcium sulphate.

This is the nutrient solution concentration (in ppm) Im using:
N = 167
P = 27
K = 260
Ca = 114
S = 32
Mg = 41

I use calcium nitrate, Potassium nitrate, Magnesium sulphate, nitric acid and phosphoric acid, plus calcium and magnesium from tap water. Using the problematic anions (sulphate and phosphate at very low concentrations)

So, my question is...Has somebody observed your nutrient solution through several days? A lot of people claim that at pH = 5 - 6.5 all the nutrient are dissolved...but is that a fact?

If theres always a precipitate, regardless the nutrient solution you use, the "recommended" concentrations could be wrong because a hydroponic plant can only absorb nutrients dissolved, precipitated nutrients are unavailable. So we could be wasting salts. If that is true, then an article (that a found) could be right, that you need about half or less of sulphate and phosphorus (the problematic anions). Example:

Phosphorus = 15 ppm
Calcium = 80 ppm
S = 32 -16 ppm

If you want to read the article this is the link:
http://www.usu.edu/cpl/research_hydroponics3.htm

I suggest you to read other opinions in:
http://forum.sensiseeds.com/forum_posts.asp?TID=3270


Follow-Up Postings:

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RE: Nutrient solution observed through several days?

Hello Jamesvlad,

You indeed post an interesting query but you should bear in mind that salt solubility is dependent on many other factors, such as temperature and other ions that might be present in your water.

If you look at the solubility of calcium sulfate (2.4g/L 20C) you will notice that it can be a part of a solution without forming a precipitate up until a concentration of Ca of 557 ppm and S of 446 ppm is reached. The solubility of this salt has however some odd behavior as it increases and then decreases as water temperature increases.

In your case I would seriously doubt that your problem relates to calcium sulfate precipitation but probably due to some other insoluble salt that may be forming. Particularly transition metal salts (mainly iron salts) are known to easily come out of hydroponic solutions even as dihydrogen phosphates.

In my case I can tell you that I have observed nutrient solutions for even months without any metal salt precipitation with a similar formulation as yours, showing that this is indeed possible and that low concentrations should not be necessarily used.

I hope this helps :o)

Best Regards,

Daniel


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RE: Nutrient solution observed through several days?

Thank you danielfp. Ive prepared another solution without micronutrients, the solutions is clear after 4 days, and it has sulphate from epsom salt. You re right, the most probable salts that could precipitate are from iron. So, the tap water Im using already has iron or the micronutrients mix is too concentrated.
Well, Im going to prepare other nutrient solution with half the concentration of micronutrients to see what happens, then Im going to report the results.


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RE: Nutrient solution observed through several days?

Hello, lucas_formulas, thank you for the advice. Im going to test one by one.
About Mg and S, the tap water that Im using is hard water, about 45 ppm of Calcium and 20 ppm of magnesium. The 41 ppm of magnesium is an estimated.


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RE: Nutrient solution observed through several days?

Hello Everyone,

I am glad that my advice has been useful :o) Definitely I would say that your problem now relates with your micronutrient sources and that you are experiencing the precipitation of some of their salts. Potentially phosphates or carbonates depending on your actual conditions. Also you should be aware of the purity of your salts as most commercial fertilizers - as Lucas points out - contain some insoluble impurities from their fabrication process.

Also I would be extremely reluctant to believe that in this case precipitation comes from calcium sulfate, even if your water has indeed some sulfate the solubility is way higher than the concentration of micro nutrient additions. Perhaps some initial precipitation might occur when the nutrients are mixed but it should disappear with time as the salt reaches its equilibrium and dissolves. This is very unlikely to be the cause since the initial reservoir is clear according to James, so the precipitates formed should be a favorable product (much more insoluble salts than calcium sulfate like iron, copper carbonates, etc).

Regarding the S-Mg ratio I thought from the beginning that the higher S would be due to sulfuric acid additions (as he highlighted on the first post), although I don't know if it has been indeed taken into account for the calculation.

I hope this helps a little bit more :o)

Best Regards,

Daniel


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sorry, no H2SO4 addition

Sorry, I seem to have imagined the sulfuric acid addition, never mind that comment :o) Indeed I find this puzzling, how did you arrive at those S and Mg figures ?


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RE: Nutrient solution observed through several days?

Puzzling it is LOL... as the S content should anyway be higher - shouldn't it?

I've seen S and Mg figures with "reverse" S and Mg (higher Mg than S) content before, posted by some people. I guess that there is some false information source about S versus Mg content of Magnesium Sulphate somewhere at a web page and perhaps circulating in forums. The fun part is that rarely someone even notices the mistake at all. That's how fallacies and myths get propagated and kept alive ;-)

One more thing (Daniel) my suspicion doesn't come from sulphate content of the water but the high calcium content that can react with coper sulphate. If making a batch based on X gallons of nutrient solution you would use tap water to mix all salts, not distilled water. That's usually fine for dissolving most salts - BUT, if for some reason you have got hard tap water with high calcium content and mix your "micro nutrient concentrate" or even a pinch of coper sulphate with only a small quantity of this water (instead of using distilled water for the later purpose), you may indeed have the sulphate reacting with the calcium of the tap water and get typical precipitation. I have no idea with what exact specification this might happen, I only have got this reported from someone who experienced it. And indeed, when doing the cross check using distilled water there was no precipitation with the coper sulphate. It might also be due to the quality of the salt - which I can't say.

The other was Manganese Sulphate, but here it seems that the quality was simply bad and there was lots of insoluble residues (no precipitation) left. Testing with various available grades solved the problem. This is a report from some middle eastern country I will not cite in this context because of discretion for the concerned person.


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RE: Nutrient solution observed through several days?

Hi Lucas,

I did mean calcium, not sulfate, sorry for that. As I said on the first reply, the solubility of calcium sulfate allows for an S concentration of 446 ppm so it makes no sense that a small amount of micro nutrients, like copper sulfate, would generate a calcium sulfate precipitate. Of course, if you add it it could form temporarily but it should dissolve later on. Even if the amount of calcium in the water was high (like 200 ppm) the amount of sulfate needed to precipitate calcium sulfate would be MUCH higher than what micro nutrients can give.

As I say, you could have a precipitate of calcium sulfate when you first add the salt (because local concentration could be higher than the solubility limit) but as the salt dissolves completely calcium sulfate should disappear with time. In order to avoid this first precipitation to happen it is a good practice to first dissolve micronutrients in a small amount of distilled water and mix this rapidly with the nutrient solution instead of the solid salt or a much more concentrated solution. Either way, what I am trying to say here is that this problem most definitely does not come from calcium sulfate but probably from some other salt precipitating since the precipitate appear later and not when the salts are added.

I do agree that salt quality is an issue here, many salt providers, especially in third world countries do not have adequate recrystallization processes and quality can be very low (I have seen purities of around 60-70%).

Best Regards,

Daniel


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RE: Nutrient solution observed through several days?

After doing some experiments, I realized that the Iron Salt is the problem. The solution looks complete clear after 5 days without Iron salt.
Its amazing that only 2.4 ppm of iron can cause the solution gets cloudy, and to form a precipitate.
I also used ammonium sulphate and calcium chloride to rise calcium and sulfur concentrations.
So, now I can tell that calcium sulphate its not the problem as danielfp said.
Thanks to everyone who participated is this post.


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RE: Nutrient solution observed through several days?

Hello Jamesvlad,

I am glad you have found the source of the problem :o) Definitely this is inline with what I thought. Iron ions can form many insoluble salts in solution (phosphates, carbonates, etc) so you will want to stabilize your iron source, especially if your water is hard. I would suggest using FeEDDHA as you iron source which is the most stable iron chelate easily available for hydroponic growth. This chelate is extremely soluble and stable and it will keep your iron in solution.

I hope this helps you further ! :o)

Best Regards,

Daniel

PS : Precipitates can form as very small aggregates which can scatter light significantly. Even if you had a precipitate from an ion with a 0.5 ppm concentration it could still significantly cloud a solution.


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RE: Nutrient solution observed through several days?

Hi James,

I often go for simplicity.
Hence, do you actually need to add either ammonium sulphate or calcium chloride, to achieve your wanted- respectively a balanced and adequate formula for your purpose?

The amount of ammoniacal nitrate should anyway not be that high and is in most cases covered by the little percentage contained in calcium nitrate (or is supplemented with ammonium nitrate). Hence increasing (or only supplying both forms of) N with calcium nitrate should provide enough N and Ca in most cases. The amount of Sulphur can also be increased with Magnesium Sulphate. A few ppm more Mg in a formula as theoretically wanted, up to 50 ppm is no problem and should make no difference. On top of that, please note that for many crops, calcium needs (actual uptake from a nutrient solution) is often lower than expected or recommended. You could even say that relatively high amounts of calcium in some formulas are simply due to the relatively high content of Ca in calcium nitrate.

When looking at your target formula, without even calculating - I can almost tell that you are able to achieve it (or get close enough) without the use of your "additives". And here you should know that a difference of 5-10 ppm in major elements (except P) do not change much (if anything at all) with the effectiveness, grow rate or whatever. Have a look at some scientific research with parts of macro elements in a nutrient compositions. The variables in elemental composition are much rougher (sometimes steps of 50 ppm or more) to get any notable differences.

In fact, best accuracy in composing and making formulas is a good thing, because eventually you want to reproduce exactly what you had or modify it accurately. But also keep in mind, that plants do not really require such "human or scientific" accuracy and certainly do not react with some minor differences of a few ppm with macro elements in any final concentration of a formula. They can't even tell the difference ;-)


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RE: Nutrient solution observed through several days?

Hello lucas.

You are right about the balanced and adequate formula. The last experiment I made wasnt a adequate formula. I add Calcium chloride and ammonium sulphate to rise calcium and sulfur concentrations to observe only. With higher concetration is more likely to form precipitates. And, I dont have to wait more days to see what happens. Its like testing the maximun solubility. If at higher concentrations there are not precipitates, then at correct concentrations (balanced and adequate as you said) there are less probability to form precipitates. ;-)


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