Rain, tap or well water always comes pre-loaded with some native substances before fertilizer gets added to it. When water has more substances than average it’s called hard water. When water has less substances than average it’s called soft water.
The predominant minerals found natively in tap water are calcium and magnesium.
A measurement of your water taken before nutrients are added will determine its hardness:
Very Soft Water – rain water and R/O water.
0 – 0.14 EC
Soft Water – average tap water.
0.14 – 0.28 EC
Slightly Hard Water – average tap water.
0.28 – 0.42 EC
Moderately Hard Water
0.42 – 0.64 EC
Hard Water – Very difficult to work with.
0.64 – 1.06 EC
R/O or rain water is Very Soft, close to 0 EC (0 TDS). Tap water is often around 0.1 EC (150 TDS), but this depends on municipality. Use Cal-Mag to raise the mineral content of your water. An EC metre can tell you when you’ve added enough Cal-Mag to your irrigation water to reach a target dosage of 0.3 EC (300 TDS).
Excessively hard water is very difficult to work with because adding fertilizers to water that is already crowded with minerals creates a toxic solution. To fix hard water use a reverse osmosis process to take minerals out, then put fertilizer in.
After treating water with a reverse osmosis machine, add calcium and magnesium back to it at a 2:1 ratio, raising its mineral content to 0.3 EC (300 TDS).