Before your water ever touches a nutrient reservoir, it's worth knowing what's already in it. Total Dissolved Solids (TDS) and pH are the two numbers that tell you whether your tap or well water is a clean starting point — or something you'll be fighting against all season.
"Test your source water before you test anything else. A lot of 'nutrient problems' are actually just what was already in the tap." — Andrew Woodward
TDS: what's dissolved in your water
TDS measures the total concentration of dissolved minerals, salts, and other solids in water, in parts per million (ppm). For drinking water:
| TDS (ppm) | Quality |
|---|---|
| Under 50 | Very low mineral content — can taste flat |
| 50–150 | Ideal — balanced mineral content, clean taste |
| 150–300 | Excellent — still well within EPA and WHO guidance |
| 300–600 | Good — acceptable taste, noticeable minerality |
| 600–900 | Fair — taste degrades, scaling becomes likely |
| 900–1,200 | Poor — unpleasant taste, potential health concerns |
| Over 1,200 | Unacceptable for regular drinking |
The EPA's secondary (non-enforceable, aesthetic) guideline sets 500 ppm as a reference ceiling. For growing, that same water is a different question — most tap water in the 100–300 ppm range is a fine base to build a nutrient solution on top of, since your fertilizer concentrate will add far more TDS than it starts with.
pH: acidity and alkalinity
The EPA's recommended range for drinking water is 6.5–8.5. The World Health Organization recommends staying under 8.0, since chlorine disinfection works less effectively as water gets more alkaline. Most municipal water in the US lands in the high-6s to mid-7s.
Why this matters before you grow with it
- High source TDS eats into your nutrient budget. If your tap already reads 300+ ppm, you have less room to add fertilizer before hitting your target EC — and you don't always know what that baseline TDS actually is (calcium and magnesium behave differently than sodium or chloride at the same TDS reading).
- High source pH can lock out micronutrients over time even if your reservoir tests fine right after mixing, since some source-water alkalinity buffers pH back upward between adjustments.
- Well water varies far more than municipal water — test it before you build a regular feeding schedule around it, and re-test seasonally since well TDS can shift with rainfall.
The bottom line
Test your source water once before you start dosing nutrients, and you'll know whether you're building your feeding schedule on a clean baseline or working around one. It's a five-minute check that saves a lot of guessing later.
The Aero-Gro Pro 5-in-1 Water Meter reads TDS, EC, and pH in a single dip, so you can baseline your source water and your reservoir with the same tool.
Tools for Serious Growers
Everything mentioned in this article — and more — available in the Aero-Gro store.