I see recommendations talking about E.C., Cf, and ppm. What are they and what is the difference between them?
A. In hydroponics, they are all measurements of the amount of dissolved salts in your nutrient solution. These measurements include all salts, whether fertilizer salts or non-nutrient salts already in the water such as common salt. These salts are commonly referred to as the ‘background conductivity’ of your water and can be measured using a conductivity meter (see 11.).