Peters 20-20-20 + TE Professional

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Do not water before fertilizing. Proper concentrations of fertilizer will not harm roots. Watering first fills up velamen with plain water, and less fertilizer will be absorbed. Fertilizer applied after a plain watering is mostly wasted.

TDS meters don't measure total dissolved solids. They measure electrical conductivity, proportional to the number of charged particles in the solution. Unless you know exactly which chemicals are in the solution these meters don't yield useful data. Total dissolved solids are measured by evaporating a known quantity of water and weighing the minerals left behind. There aren't any meters that can do this.

Your water company publishes an annual water quality report. It will tell you what is in your water. It may report total dissolved solids, water hardness, or give amounts of each chemical species present. You can add the amount of fertilizer you plan to use and calculate TDS for yourself.