Alkalinity is the acid-neutralising capacity of water. In irrigation water it comes almost entirely from bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻), with small contributions from hydroxide and other weak bases. It is reported as mg/L (ppm) of calcium carbonate equivalent. Alkalinity is not the same as high pH: water can be at pH 7.5 with low alkalinity and move easily, or at pH 7.5 with high alkalinity and resist acid stubbornly.
In practice
Rough guide for container growingbeing reviewed:
| Alkalinity (mg/L as CaCO3) | Effect |
|---|---|
| Below 40 | Little buffering; pH moves easily (RO, rain) |
| 40–100 | Comfortable for most container growing |
| 100–200 | Needs acid; media pH tends to creep up |
| Above 200 | Needs significant acidification or RO blending |
Water utilities publish alkalinity in annual water-quality reports, and inexpensive aquarium KH test kits give a usable estimate. Acid dosing to neutralise most of the bicarbonate, rather than simply chasing pH, gives a stable result.
Units and formula
1 dKH = 17.86 mg/L as CaCO3. Bicarbonate (mg/L) ≈ alkalinity as CaCO3 × 1.22.
Don't confuse with
- pH: the acidity level, not the reserve.
- Hardness: calcium and magnesium content; often related, but not the same thing.
