Your starting ppm is only accurate for the time you mix and first use them.

The plant will uptake some nutrients and then in turn deposit back and discard other by-products back into your rez. Thus this changes the PPM reading. You don't know what the mineral mixture is due to this fact.

That is why I and many others say to change your rez every 7-10 day's and that should never be a problem. You can even use less water because of the 7-10 day count. I have grown in systems over the years that used 10 gallons for 40 plants and another 5 gallons added at day 5 to fresh up the mixture. All 15 gallons are mixed at the same time so everything is all the same when the fresh mix is added. That is a total of 15 gallons for 40 plants growing in a sea of green system.

I hope that helps you understand ppm and the plants roll in your numbers you read.

You can use the Ph to increase or decrease the available minerals to the plant to match it's cycle of life. There is a chart in our FAQ section here to show you the different minerals available to the plant at any given Ph in both hydroponics and dirt.