Carbonic Acid and Bicarbonates in Irrigation Water

What Bicarbonates Do in Soil and Water

Alkalinity measures a water source's ability to neutralize acid — its buffering capacity. In most agricultural and turf irrigation water, bicarbonate (HCO₃⁻) is the dominant species driving that alkalinity reading. Alkalinity above roughly 150 mg/L as CaCO₃ is a concern for most crops and turf; bicarbonate above about 120 mg/L starts to cause problems.

The effects are both physical and chemical. Inside pipes and drip emitters, bicarbonate combines with calcium and magnesium at higher pH to precipitate calcium carbonate (CaCO₃) — the white scale that narrows emitters, reduces flow, and lowers distribution uniformity. In the soil, high-bicarbonate water arriving at elevated pH pushes the soil solution toward alkaline conditions that lock up phosphorus, iron, manganese, and zinc. Plants respond with yellowing leaves, reduced root growth, and lower nutrient use efficiency.

Key numbers to read from your water report: pH, HCO₃⁻ in mg/L, and alkalinity as CaCO₃.

How Carbonic Acid Changes Bicarbonate Levels

Carbonic acid forms when CO₂ dissolves in water:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

The form that carbon takes in the water depends on pH — this is called speciation. Saruhashi (1955) published the defining tables: at pH 8.0, roughly 98% of the carbon in solution exists as bicarbonate (HCO₃⁻). At pH 6.3 — the crossover point — H₂CO₃ and HCO₃⁻ are at equal concentrations. Targeting pH 6.5 converts roughly half of all bicarbonate to carbonic acid.

When carbonic acid lowers irrigation water from pH 8.0 to pH 6.5, it is converting existing bicarbonate into carbonic acid — not adding new bicarbonate. The pH goes down because of the H⁺, but the alkalinity stays the same.

Carbon speciation at different pH values. At pH 6.5, roughly half of the bicarbonates have shifted to carbonic acid. Source: Saruhashi (1955).

How CO₂ Makes Calcium Available

Carbonic acid does not stop at adjusting the pH of free water. CO₂ present in treated irrigation water and in the soil atmosphere reacts with calcium carbonate (lime) in the soil:

CaCO₃ + CO₂ + H₂O ⇌ Ca²⁺ + 2HCO₃⁻

This reaction converts solid calcium carbonate — a form unavailable to plants — into dissolved calcium ions (Ca²⁺) that the plant can actually take up. The liberated bicarbonate is more mobile and can leach through the soil profile rather than accumulating near the root zone.

What Water pH Control with Carbonic Acid Actually Changes

Carbonic acid water pH control targets the soil solution at the wetting front — the pH the plant encounters during active feeding. Water pH control is not primarily about reducing the alkalinity reading on a water report. It is about delivering water at the right pH so nutrients are soluble when the plant needs them. What remains in the soil after carbonic acid treatment is CO₂ and H₂O: no sulfate, no chloride, no contribution to rising electrical conductivity over time.

Reading Your Water Report for Bicarbonate Risk

The key numbers to check:

• pH — above 7.5 is a concern; above 8.0 is significant for most crops and turf
• Bicarbonate (HCO₃⁻) — above 120 mg/L is a concern; above 180 mg/L is high
• Alkalinity as CaCO₃ — above 150 mg/L is a concern for turf; above 200 mg/L for most crops warrants treatment
• SAR (Sodium Adsorption Ratio) — elevated SAR combined with high bicarbonate accelerates sodium hazard
• RSC (Residual Sodium Carbonate) — positive RSC indicates the water will deposit carbonates in the soil

Research Evidence

Lampreave et al. (2022) studied carbonated irrigation water in calcareous vineyards with iron chlorosis — conditions similar to high-bicarbonate agricultural water. At approximately pH 6.5, carbonated irrigation increased phosphorus, iron, manganese, zinc, and calcium availability in the leaf tissue. Yield increased approximately 30–42% per vine by year three. Chlorophyll content increased from 2.13 to 3.41 mg/g dry weight. Leaf iron nearly doubled.

ECO2MIX has documented declining soil EC in internal field sampling from farms treated with carbonic acid. Two mechanisms drive this: no salt ions are added by carbonic acid, and lower-pH irrigation water at the wetting front improves infiltration and helps existing bicarbonates and salts leach through the soil profile.

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