OK that is a clear goal:
Measuring the concentration of minerals is very difficult. There is probably no simple (e.g. linear) relation between the concentration of the minerals and the conductivity of the liquid. If there is only one mineral used you can make different concentrations, measure conductivity and create a lookup table. If you have two or more minerals it can be quite complicated. Conductivity will not discriminate between Na+ and K+ ions if they are both available.
To have a robust system you should have multiple sensors that are specific for some type of ion.
One thing I would at least consider is using a pH sensor as plants are sensitive for both too high and low pH.
NASA has a lot of knowledge, you can contact them at - http://natureandscienceagriculture.co.uk/Index/Pages/MAIN.html -