Nitrogen Sources: Urea, Ammonium, Nitrate?

Hiya all, I'm new here and I'm helping a friend of mine grow some weed. I'm trying to design my own fertiliser from commodity chemicals to save money: If you buy ready made fertiliser mixes, the mark-ups can be pretty big.
I've run in to a few problems/got a few questions though:

I'm not sure what form of nitrogen to use. I'm thinking of using urea for most of it, but use a little bit of ammonium and nitrate where that's the easiest way to get hold of another element (eg ammonium phosphate and potassium nitrate are pretty easy to get hold of). Is there any problem with using urea? I've read that urea and nitrate both need to be conveted to ammonium by bacteria before they can be absorbed by the plant, whereas ammonium can be used straight away. I'm not sure if thats a good thing or a bad thing...

What pH does marijuana prefer?

And what specific nutrients will i need, compared to the needs of other plants? I've heard that I'll need more magnesium, and I guess I should add extra nitrogen at first to speed up growth, then cut down on the nitrogen to let the plants mature. Anything else I need to consider?

Also, how does everyone make their CO2? I'm thinking of either using respiration of yeast, or addition of citric acid to potassium carbonate (K2CO3). Another benefit of that would be that it'd leave me with potassium citrate, which I could use as a fertiliser for the plants, as a source of potassium.


Thanks for your time!

Nitrate provides the bulk of N for all plants, its also widely used for hydro.
Ammonia is converted thru nitrite to nitrate, it is available to plants in ammonia form but theres a lot of competition for it so it disappears rapidly.. unlike nitrate.
Urea can be converted into ammonia using an enzyme "urease", it also produces co2 in the conversion process.

Atomizer said:

Nitrate provides the bulk of N for all plants, its also widely used for hydro.
Ammonia is converted thru nitrite to nitrate, it is available to plants in ammonia form but theres a lot of competition for it so it disappears rapidly.. unlike nitrate.
Urea can be converted into ammonia using an enzyme "urease", it also produces co2 in the conversion process.

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Aha, I see. I thought it was the other way round, and nitrates were converted to nitrites then to ammonium before they were absorbed by the plant.
So what do you mean, that there's a lot of competition for ammonium? Surely the plants will be the only ones there, so there wont be competition?
Well, overall, does this sound like a plan?: Use urea as the main source of nitrogen cause its cheap, and its gets converted to nitrate eventually via ammonia and nitrite, but also use some nitrate (KNO3?) for quicker release.

If you let your res go anaerobic you can have the nitrite revert back to ammonia, its just a question of the environmental conditions to which bacteria thrive. There are plenty of bacteria and enzymes that can use ammonia..mother nature wastes no time in utilising everything and there is always competition even if you can`t see it
Overall it seems workable, as long as you provide the right conditions. what are you planning to do for the P,K,micro and macro nutes?

Atomizer said:

If you let your res go anaerobic you can have the nitrite revert back to ammonia, its just a question of the environmental conditions to which bacteria thrive. There are plenty of bacteria and enzymes that can use ammonia..mother nature wastes no time in utilising everything and there is always competition even if you can`t see it
Overall it seems workable, as long as you provide the right conditions. what are you planning to do for the P,K,micro and macro nutes?

Click to expand...

Ah, I see, thanks
Well, Im making a list of salts to mix together to get the other nutrients. Eg for K, I'm thinking K2SO4 or KNO3, and for P, (NH4)H2PO4 or KH2PO4, for Mg, MgSO4 etc etc. Then when it's all done, I'll adjust the pH using potassium hydroxide or citric acid.

I've been working out what mixes I'll use:

I think at first, I'll use a high nitrogen and phosphorus, low potassium mix, to make the plants grow big quickly (plants use N and P for growth):
450g Ammonium Phosphate ((NH4)3PO4)
250g Potassium Nitrate, Saltpetre (KNO3)
150g Urea ((NH2)2CO)
So that covers NPK, with a ratio of 34-23-14

Then for the trace elements I'd add:
100g Magnesium sulfate, epsom salts (MgSO4)
1/2 Teaspoon of trace element mix


Once the plants are nice and big, you want to slow down their growth and get them to mature. I've heard that cutting down on N and P, and adding more K is a good way of doing this:
250g Ammonium Phosphate
450g Potassium sulfate, sulfate of potash (K2SO4)
And no urea at all
That gets you an NPK value of 10-17-35
And I'd add the same mix of trace elements and MgSO4 as before.

Both of those mixes would be per 300l of water

Does that sound any good? Or have I missed anything?