# Hygroscopicity and weights of fertilisers



## xim (5 Oct 2015)

I used to think that the caking of some ferts, especially micro nutrients, was trivial 
when it came to mixing a solution, as long as they were not deliquescent.
Just crush them, weight them, and mix.

But recently I've got a new bag of Fe-DTPA 7%. Its colour looks bright reddish brown 
and will turn sticky when exposed to the air. This makes me wonder about 
its weight.

So I tried leaving some portion of the new one exposed to the air overnight. 
And now it gets about 30% heavier. It seems it can gain even more weight 
because it has not turned into dry yellowish/greenish brown lumps 
like my old stock yet.

I'm also testing another commercial trace mix powder which is very hygroscopic 
that I've to keep it in a sealed container. It's about 100% heavier now and not stopping 

Yeah, some of you might say, "look at the plants and just add more if they need it".
But I think this issue highly affects our "communication" about dosing.
I also wonder if this affects the quality of commercial liquid ferts from batch to batch.


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## Edvet (5 Oct 2015)

Commercial ferts should be mixed based on dry matter percentages, and "good manufacturing practice" dictates samples are tested prior and post production,


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## rebel (7 Oct 2015)

fascinating. This could be a huge issue with 'correct' dosing when combined with fairly unreliable test kits! Maybe we are all in the dark?


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## Edvet (8 Oct 2015)

EI doesn't do "correct", it does "plenty"


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## dw1305 (8 Oct 2015)

Hi all, 
A lot of salts are <"hygroscopic">, and some are <"deliquescent">. 

You can take this into account to some degree, when you use a salt like magnesium sulphate or calcium nitrate it will always be hydrated with the <"water of crystallization"> , what-ever it says on the container. So it is MgSO4.7H2O and Ca(NO3).4H20 etc. when you work out the % content of the element 

As a general rule chlorides are nearly all hygroscopic, and calcium chloride (CaCl2.6H2O) is deliquescent. You can only keep it as a salt by oven drying and then storing it in an air tight container (freezing also works). 

If you are worried about measuring more accurate weights for micro elements and FeDTPA etc, you can use <"silica gel"> (or calcium chloride) stored with your salts, as a desiccant. You will need to oven heat the desiccant on a fairly regular basis for it to continue working.

cheers Darrel


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## rebel (8 Oct 2015)

Edvet said:


> EI doesn't do "correct", it does "plenty"


Hi Edvet, if you are measuring it wrong, then it might less than plenty, might even be low, which would be incorrect yes? 

Wonder what the error percentages could be?


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## xim (14 Nov 2015)

...So I had the brain dead moment. 

I thought the initial weight already included the weight of the cup.
But it's not. 

The correct result of leaving them in the cups for about 36 days is: 

The trace mix gets 21.72% heavier and remains damp and sticky.
I think it will never dry with this level of humidity in the air.

The DTPA 7% iron gets 1.03% heavier. It used to be a little heavier 
than this when it was still in the "damp and sticky" stage. 
Now it's completely dry and stable.


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## rebel (16 Nov 2015)

Wonder whether you can heat the mixtures in the oven to dry them off? Do you need to?


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## ceg4048 (17 Nov 2015)

Hello,
        As mentioned by Edvert, It is a complete waste of your time, energy and brain power to worry about the hygroscopic properties of dry salts. The values we are dosing are so high that the differences are negligible. In fact, if you remove the water from the salts, you end up dosing higher amounts which will have an increased effect on TDS, which everyone seems to be frightened of.

Since there is little to no gain in this endeavor, our advice is to simplify your life and allocate your energy to solving more urgent problems such as CO2 and flow/distribution.

Cheers,


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## xim (17 Nov 2015)

ceg4048 said:


> Hello,
> As mentioned by Edvert, It is a complete waste of your time, energy and brain power to worry about the hygroscopic properties of dry salts. The values we are dosing are so high that the differences are negligible. In fact, if you remove the water from the salts, you end up dosing higher amounts which will have an increased effect on TDS, which everyone seems to be frightened of.
> 
> Since there is little to no gain in this endeavor, our advice is to simplify your life and allocate your energy to solving more urgent problems such as CO2 and flow/distribution.
> ...



I don't give a -- about the 1.03% gain. Although the about 20% gain is "interesting" for me.
But it was the false 100% weight gain that made me worry and started the thread.

Funny that not long ago I just told someone not to worry about the time of dosing.



rebel said:


> Wonder whether you can heat the mixtures in the oven to dry them off? Do you need to?



I don't know if the high heat will affect the properties of chelating agents.
And heating it is just too complicated. So I would just compensate for the weight in that case.

But as I have already been keeping the trace mix in a sealed bottle. And as it turns out that
the Fe-DTPA's weight gain is insignificant. I think there is no need to change the way I store
the ferts.


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