I use the SSU as a foliar only, which is very economical when applied that way. The recommended dose is 5 ml/gallon but I typically just put 1 ml in a quart spray bottle. So, I apply just under the recommended dose, but it's close enough and my 1 ml syringe makes it easy to measure out in one shot!
That one quart bottle tends to last me several weeks, even with 24 plants, since it's only applied weekly.
You can add SSU to the nutes but they recommend you lower PPM's and while it shouldn't be a problem in soil (just more costly than foliar application), it's recommended NOT to use SSU in a res with an airstone.
One final comment that, for some reason, I just feel compelled to make!
Many growers are hesitant, to the point where they refuse, to apply anything foliar in the later stages of flowering for fear of budrot. Other growers will never appy foliars since they feel that ALL nutes should be taken in through the root system and a foliar only inhibits root development.
My experience is, assuming you have good/clean conditions in your growroom and spray earlier in the day, there's no issues applying foliars all the way up to harvest (although, I usually cut out the SSU a couple of weeks before). If you spray earlier and don't totally drench the buds, the lights will typically dry them out and, if you think about it, in an outdoor situation they'd be getting rained on at all hours of the day in an environment you couldn't control nearly as well, so I really don't understand all the fuss about foliars! Just my 420 cents on the issue!