As to respond to Fluid1... you are more than welcome to thread jack me anytime
I don't know what post I found this on but here it was I cut and paste to my cheat sheets.
ppm goes up, PH goes down=plants require less nutes.
ppm goes down, PH goes up=Plants require more nutes
ppm stable, PH goes up=Equilibrium=Good thang.
I found this also:
Most hydroponic solutions should be in the range of 150-600 parts per million in disolved solids. 300-400 ppm is optimum. It is possible to test your solution or soil with a electrical conductivity meter if your unsure of what your giving your plants.
Keep in mind most disolved solids readings are usually on the low side, and actual nutrient levels are usually higher. It is possible with passive hydroponics, to get nutrient build-up over several feedings, to the point the medium is over saturated in nutrients. Just feed straight water now and again, until you notice the plants are not as green (slightly), then resume normal feeding.
If this is true, I'm running ppm's too high, but it doesn't seem to adversely affect my plants. I had thought I'd heard that when they are in their 3+ week of veg you can go as high as 1000ppm and in flowering as high as 1200... but I've not been able to find where I got that information from.
Hope this helps. ~ Rav