Chemistry & Root Zone
Hard water contains higher concentrations of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺), commonly associated with bicarbonate (HCO₃⁻).
Key characteristics: - Higher alkalinity (buffering capacity) - Resists pH change - Can drive gradual pH rise in substrates
Practical risks if unmanaged: - Rising root-zone pH - Micronutrient lockout (Fe, Mn, Zn, Cu) - Carbonate precipitation and dripper blockage
Soft water has low Ca, Mg and bicarbonate.
Key characteristics: - Low alkalinity - Little buffering capacity - pH can swing quickly
Practical risks if unmanaged: - Ca/Mg deficiency unless supplied - Rapid acidification (especially with NH₄⁺ heavy feeds) - Greater sensitivity to dosing errors
Buffering capacity is the ability of water or substrate to resist changes in pH. In irrigation water, buffering is controlled primarily by bicarbonates (HCO₃⁻).
Hardness and buffering are related but not the same: - Hardness: Ca²⁺ and Mg²⁺ concentration - Buffering: HCO₃⁻ concentration
Two waters with the same pH can behave very differently depending on bicarbonate level.
Bicarbonates are neutralised by hydrogen ions:
HCO₃⁻ + H⁺ → CO₂ + H₂O
The goal in practice is usually to reduce alkalinity to a desired residual level rather than chasing a single pH number.
Once bicarbonates are neutralised, additional acid causes a rapid pH drop.
Risk: can push NO₃ levels higher than desired if used as the sole acid source.
Neutralising power is the ability to remove bicarbonates.
pH effect is the immediate measured change in pH.
A weak acid may lower pH temporarily without adequately neutralising alkalinity. Strong acids generally provide more reliable alkalinity correction.
Acid dosing should be based on alkalinity (bicarbonate), not pH alone.
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