Nutrient solution composition problem

[omerizm]

I'm having a hard time figuring out how to prepare my nutrient solution so I decided to ask for opinions. Any help is wellcome. Here it goes:

• My source water has 72 ppm ca, 30 ppm Mg and it is also high in bicarbonates.
• I am growing mainly lettuce and some other greens.
• Reagents I'm using for macros are: calcium nitrate, potassium nitrate, ammonium nitrate, phosphoric acid, nitric acid. I am not using magnesium sulphate since 30 ppm Mg is enough.
• I'm using 15% of total N as NH4.
• I'm trying to keep EC as low as possible, using a light formula for summer lettuce. Since my source water has an EC of 0,9 mS, the solution comes around 1.7 mS and it is already high for a summer crop.

Problems:

I'm adding little cal. nit. since I already have 72 ppm in source water, so I can barely reach around 100ppm N without adding too much of other elements. I can not use more ammonium nitrate bec. I don't want to raise NH4 ratio above 15%. I can not use more nitric acid bec. ph drops too much. I cannot use more potassium nitrate bec. I can not put any more K. (That is why I prefer to use phosproric acid rather than mono potassium phosphate as the P source.)

I have difficulty adding S to the solution, since I'm not using magnesium sulfate, potassium sulfate or any other suphate salts. (well I use some for micros but they are insignificant) I could use some sulphuric acid but I'm already using nitric and phosphoric acids and I can not add more bec. ph drops too much.

So far my only solution is to add some mag. sulfate to supply S, although I have enough Mg in my source water. up to 60-70 ppm Mg does not hurt the plants but It raises the EC which is something I really don't like.

Any ideas? How to supply S without raising EC even more?

Basically what I'm trying to do is to use my source water as it is. If I can not make do with it, I'll have to install a RO unit and dilute it, but it is costly. I can not use rain water or any other water source.

Thanks.