TDS

Total Dissolved Solids

Since dissolved ionized solids, such as salts and minerals, increase the conductivity of a solution, a TDS meter measures the conductivity of the solution and estimates the TDS from that reading.

Evaporation leaves fertilizer behind and you can easily kill plants, had done it. Once I have TDS, I won't have to worry anymore, just measure and add and I know it makes no difference if I'm a little high or low, but I have to have the EXACT TDS to get started or I set myself up for disaster.