I believe a grower should have a quality nute from a reputable company (unless you mix supersoil), but it's not absolutely necessary.. I feel that they are PH buffered and have better ingredients than MG. That could just be good marketing though. I've seen decent plants grown with store-bought stuff. Personally, I like peace of mind.
When using bottled nutes you need to follow the feeding schedule listed on the company's website. The bottles always recommend way too much nutes because they are assuming that you are not using the whole line of nutes.. This info will conflict with what's on the feeding chart online. The feeding chart is the way to go.
http://generalhydroponics.com/site/gh/docs/feeding_sched/GH_FloraNova-Simple.pdf
Also consider getting a TDS Meter. These things are awesome and cost around $15 - $20 on amazon. HM Digital is a good one,, I use ALL of the time. I use it to check the quality of my water that I drink and I use it to measure my nutes.
In soil you want to start out with low PPM and then work up to around 800 ppm which wold be full strength nutes. It's nice because you can be assured that you are delivering the correct amount of nutes. Also, if you are using tap water you want to know what your starting PPM is. Mine is 500 PPM. That is ridiculously high and it's not ideal for growing.
One nice and very affordable method of Nuting is to use the Lucas formula. I think it's tried and true. just get the Floranova bloom and add 8ml, use this throughout your entire grow. No TDS required.
http://lucasformula.com/
Yes, Feed/water/water. Allow for some runoff. This clears the roots and soil of built up salts (fertilizer). I find that I end up watering or feeding every 3 days or so in bloom. So I give her nutes every 9-10 days.
What's funny is nutes have been marketed as plant food. They're really not. They're more like vitamins and minerals that allow our bodies to function. Light is the food source.
