Hey all, looking to mix my own fertilisers for a hydro system, but I need help.
So far, I've got two formulas, one for vegetative, lots of nitrogen:

One part ammonium phosphate, one part potassium nitrate (saltpetre/saltpeter) and one part urea
Which gives an NPK rating of 30-16-16

And another for during budding:
One part ammonium phosphate with one part potassium sulfate (sulphate of potash)
Which has an NPK of 14-24-27

I recon the NPKs are pretty good, but there are no trace elements or magnesium in the mixture. Can anyone suggest how much magnesium sulfate (epsom salts) and this trace element mix to add: http://www.gardendirect.co.uk/chelat...ment-mix-p-887


Cheers!

Theres a mixture called Azomite that would answer all your trace elemental needs. Google it I use it as a tea in hydro and as an amendment in soil. On epsom using 1/2-1 tsp per gallon is the norm. Mag should be at a 1 to 3 ratio to calcium.

Hmm, cant find it anywhere. But i found a typical analysis of it, so I can work out how much i'd need of the trace element mix.
I heard that Ca2+ is toxic to the cannabis family, to such an extent that you cant feed your plants tap water. Is this just pseudoscientific junk?

Too much Ca2+ will displace other micros, but its pretty damned important actually..
Amoniacal N isn't used to a great extent in most hydroponic systems due to lack of organic matter for NH4+ to bind with, thus allowing loss as NH3(g)..
And Urea isn't effective in hydro either due to lack of microbes to break it down..
All in all I think you should research a bit.. At this point micros are the least of your worries..

Hmm, I see.... So it all has to come as [NO3]-? I thought the plants themselves would be able to absorb [NH4]+ and (NH2)2CO... Do they only get metabolised in the soil then?
So the main ions I'm going to need will be [PO4]3-, K+ and [NO3]-.... Thats going to leave me with a lot of negative charges... How do people usually get around that?

Plants can uptake NH4+, but in hydro its really hard to keep it in the res long enough to do so.. And urea primarily ends up converting to NH4+ with the assistance of microbial life.. In our pH range though, NH4+/NH3 ratio likes to too far to the NH3 side which dissipates too readily as gas.. NH4+ likes to bind with soil matter thus holding it in place.. You're dealing with the same problem farmers face with N loss but to a much greater degree..
So yea hydroponics really leans toward the nitrate form of N..

Hmm I see.... But that means I'll need a lot of [NO3]- at first... What cation can I balance it with? Should I just use KNO3 and K3PO4? It'll leave me with a huge excess of K+ though....

Some Calcium nitrate perhaps, no real problem with ammonium nitrate if you can get it without DHS busting your grow, In hydro that would equate to pretty much pure nitrate, as would nitric acid.. (Nitric acid will kill you in a couple days very painfully if you breathe strong fumes..)
Really hydroponics fertilizers are dangerous to produce without proper knowhow, and dangerous to obtain reagents to create in this day and age.. Besides you'd need some decent chem background and apparatus to keep equilibriums in check and avoid unwanted byproducts..

I'm kinda ok at chemistry... I'm really interested in it and I have a little lab in my back garden
By "avoid unwanted byproducts" do you mean precipitation reactions and suchlike? Eg:
Ca(NO3)2(aq) + MgSO4(aq) ---> CaSO4(s) + Mg(NO3)2(aq)

Come to think of it, this does sound like more effort than its worth.... I can get hold of ammonium nitrate, but I think I'll probably be easier just to buy ready-made stuff...

I'm not entirely sure that would be an undesirable one..