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"Homemade" Macro fertiliser precipitation issue!

Aleman

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28 Mar 2022
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88
Location
Blackpool, UK
So 8 weeks or so ago, I finally decided that I would make up my own ferts to resolve some algae issues I was having ... Turns out using RODI water on it's own with 1/3rd recommended dose of the commercial fert I was using (when I remembered) was actually a recipe to CREATE ALGAE! Well, whodathunkit??

As the fertiliser gods had provided the IFC calculator, and I have ready access to most chemicals (She is a lab tech ;) ) it's not really a difficult task, I already have a supply of beakers, and conical flasks plus a mag stirrer for use in my brewery, so lets go ...

Initially I made up 1500ml of a fertilser based on EI Full which gave me

52.41g of potassium nitrate
5.89g of dipotassium phosphate
108.65g of magnesium sulphate
117.9g of calcium chloride

of which I dose 50ml 7 days a week via an autodoser. Everything would be fine except for the precipitate that forms ... calcium phosphate as a guess, possibly apatite depending on the pH of the solution. So that wasn't particularly successful, so for Sh!ts n g!ggles I decided to make up 1500ml based loosely on EI Mid, but I'd dose 100ml of it daily, so I added

30.57g of potassium nitrate
3.93g of dipotassium phosphate
81.49g of magnesium sulphate

not adding any calcium salts this time as I saved that for the remineraliser, plus, I wanted to use calcium sulphate instead of chloride, which sort of forces you to dry dose.

So what's my issue? It threw a damned precipitate (but less) in the conical flask on the stir plate!!!!!!!! and that is even with the recommended amounts of ascorbate and sorbate added to the container first. Given that the concentration of ions in the flask was less than the first attempt and it was properly acidified, this time, surely no precipitate should have formed?

Why should I care ... well I use 22 gauge blunt syringe tip sat the dispense point, and NRV's to reduce any possibility of fluid flowing back should the pump head stop without the tubing compressed. (Jebao autodoser). The precipitate blocks the syringe needle and the check valves. At the moment I've removed the check valve and replaced the 22 gauge with an 18g, but I would be preferred to use 22g. I just can't work out whats happening chemically to cause that precipitate, anyone got any ideas or recommendations?

I've no such issues with the micro ferts
 

X3NiTH

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13 Apr 2014
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1,553
Your first solutions precipitate will be entirely Calcium Sulphate, any time you dose a Calcium Salt into solution it will precipitate out if it exceeds solubility rules for any other possible calcium compounds from the ingredients present, in this case you exceeded them mightily with sulphate so lots of precipitate!

In the second instance is the Potassium Nitrate you are using a pure compound free from Ammonia salts, if it isn’t then there is the possibility for Magnesium Ammonium Phosphate (MAP) precipitation but it will be a light dusting of precipitate sediment on the bottom.

Macro salt solutions generally don’t require acidification beforehand it’s mainly a recommendation only for Micro Fertiliser Solutions.

:)
 

ian_m

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Eastleigh
Try using mono potassium phosphate, as I have heard people having issues with the dipotassium phosphate when mixed with other salts. Not sure what the insoluble precipitate would be, but a quick Google and Wikipedia reveals may be solid precipitate of dimagnesium phosphate.

I suspect this is why mono potassium was used in original EI research & development.

Why are you using syringe tips ?
 

Aleman

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28 Mar 2022
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88
Location
Blackpool, UK
Try using mono potassium phosphate
May well give it a go, but I'm not hitting my potassium target as it is ... then again what is 1-2ppm between friends :D
Why are you using syringe tips ?
Not that it's critical, but slightly more accurate than just having an open tube.

I have the 4 channel Jebao autodoser, so I may well just split the macro into it's constituent parts and dose them at slightly different times/ locations in the tank.
 
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