Metre squared is a 2 dimensional thing, not a volume. You’re probably thinking of a metre cubed?

And there are 1000 litres in a cubic metre.

I suspect there's a misconception here: I think you need to converse square meters to square feet (or yards) to find out how much you need. So, one m2 being 10.8 ft2, you need 30 grams of fertilizer for every 10.8 ft2 of your garden.

Sorry you didn't get it.
For a liquid fertilizer for hydroponic you read 10 ml per Liter, so you if you have a reservoir of 2 liters, you will add 20 ml.
Got it until here?

NOW:
If you have instead a solid ferilzer which states 30g per m SQUARE not cubic.
How is converted in liters? how much of these should I put in my reservoir of 2 liters?
If it had something like30g per liter I would have add 60 grams but nothing is stated.
JUST 30grams per squared meters. like so:
https://www.compo.com/international/products/Controlled-Release-Fertilizer-for-Tomatoes-and-Vegetables.html

How would you use it in Hydroponics?

I got what you meant, I just think that is not what they meant. Because one cannot converse square meters into liters. Also, after I read your last comment, I checked that same fertiliser as it is sold here in Europe, and it says on the package: 800 grams for 30 plants...

Moreover, I don't think it is supposed to be mixed with water, as it is granular.

I am in Italy and it just says 30grams per m2.
I don't know how much to put into the reservoir =( =( !

gardengal, I wonder whether sonic_blu thinks you didn't get it either ;-)

It must be soluble somehow in order to get absorbed I understand is not fully soluble immidiatly but watering after watering it get dissolved so, the amount of minerals will be released I believe, so there must be somehow a conversions in liters if Crunch it and mix it...

Most CRF's are triggered to be released by temperature, not water. And they are NOT intended to be used in a hydroponic growing system so I would have serious doubts about their efficacy under those circumstances. And they are not a cheap means of fertilization. Why not just get something that is formulated for hydroponics?? Something like the Dyna Gro products.

I hope that sonic understands, now. The type of fertilizer in question is only meant to be applied to soil, where the particles can dissolve slowly over time. Period. Controlled release fertilizers are not manufactured to be dissolved in water.

That makes it a lot easier, right?