Three readings that matter
Dissolved solids
Minerals in the water, mostly reported as parts per million, and estimated with an electrical conductivity meter
pH
The acid-alkaline balance of the water before and after you add fertilizer
Alkalinity
How strongly the water resists pH change when you add acid or a soluble feed
Soft water
Very pure water with almost no minerals, so a tiny acid dose swings pH hard
Hard water
High mineral load, usually calcium and magnesium, so pH barely moves until you add a lot of adjuster
Why source water differs
Regional spread: Rosenthal uses San Francisco tap near 65 ppm as weakly buffered and Los Angeles tap near 450 ppm as strongly buffered.
Public reports: Water districts test continuously and publish pH, dissolved solids, alkalinity and contaminants.
How to get them: Reports may arrive yearly, sit on the utility website, or come after a direct request.
Seasonal drift: Tap pH and mineral load can change after system flushes, so retest rather than assume last month's number.
A workable starting mineral load
Target band
About 125 to 150 ppm gives some buffering without forcing huge pH corrections
Raise minerals
Add a calcium-magnesium solution, or calcium nitrate if you still need vegetative nitrogen
Lower minerals
Run reverse osmosis or a charcoal de-ionizer, then blend purified water back with tap to near 125 ppm
Guesswork cost
Without a ppm or EC reading, fertilizer strength and pH adjuster doses are guesses
Practice
Test first
Measure source water before nutrients or pH products go in
Log the baseline
Keep the utility report and your meter reading so later lockouts are easier to diagnose
Treat water as feed
Dissolved calcium, magnesium and sodium already count toward the plant's salt load