Irrigation water acid calculator
Bicarbonate (HCO₃⁻) in well water keeps the pH high, leaves lime in the emitters and makes nutrients harder for the plant to take up. Acid cancels the bicarbonate. Type in the bicarbonate from your water test, the target pH and your acid; the tool works out the mL of acid per tonne of water, the hourly injection, the acid a day, and the nitrogen, phosphorus or sulphur the acid adds.
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A worked example
A well in Aksu: the test shows 305 mg/L of bicarbonate (5 meq/L) and pH 7.4. The target is pH 6.0, with 65% nitric acid (density 1.39). One valve zone takes 36 m³/h, and the hottest day uses 150 tonnes of water.
- Bicarbonate to cancel 3.32 meq/L; about 103 mg/L of bicarbonate stays in the water
- The acid’s strength 0.65 × 1,390 ÷ 63 = 14.3 meq/mL
- Per tonne of water 3.32 × 1,000 ÷ 14.3 ≈ 231 mL of nitric acid
- Injection 231 mL × 36 = 8.3 L/h; about 35 litres a day
- Nitrogen it adds 3.32 × 14 = 46 ppm N, to take off the fertiliser plan
[The example’s numbers await Perge’s check.]
How it is worked out
Acid cancels bicarbonate one for one: one milliequivalent (meq) of acid turns one meq of bicarbonate into carbon dioxide and water. If the test gives bicarbonate in mg/L, divide by 61 for meq/L.
The carbonic acid balance tells how much bicarbonate is left at the target pH: pH = 6.35 + log(bicarbonate ÷ dissolved CO₂). A drip line is closed, so the CO₂ from the acid stays in the water; at pH 6 about a third of the bicarbonate is left. If you add the water’s pH, the CO₂ already in the water is counted too.
You can also set the target as the bicarbonate to leave instead of a pH. Leaving 30–60 mg/L (0.5–1 meq/L) keeps the pH from dropping suddenly.
The acid’s strength comes from its concentration and density: meq/mL = concentration × density × 1,000 ÷ equivalent weight. The equivalent weight is 63 for nitric acid and 49 for sulphuric acid. Phosphoric acid gives up only one hydrogen at pH 5.5–6, so it counts as 98. That makes 65% nitric acid about 14.3, 85% phosphoric acid 14.7 and 96% sulphuric acid 36 meq/mL.
Working safely with acid
- Always add acid to water. Never pour water onto acid: it heats up and spits.
- Wear gloves, goggles and an acid-proof apron, and keep plenty of clean water close by.
- Never mix acids with each other. Never put acid in a tank with concentrated fertiliser; feed it from its own tank and channel.
- Strong nitric acid gives off choking fumes. Keep the tank closed and in a place with good air flow.
Questions
How do I lower the pH of irrigation water?
By adding a measured amount of acid. The acid cancels the bicarbonate that holds the pH up. Drip systems mostly use nitric, phosphoric or sulphuric acid, fed through a channel apart from the fertilisers, with a pH sensor watching the result.
How much nitric acid goes in a tonne of water?
It depends on the water’s bicarbonate. To bring well water with about 300 mg/L of bicarbonate down to pH 6 takes about 200–250 mL of 65% nitric acid per tonne. Work it out from your own water test and check with a pH meter.
Which acid is best for drip irrigation?
Nitric acid lowers the pH and adds nitrogen; it is the most used. Phosphoric acid suits stages when the crop needs phosphorus, but in hard water it can form deposits and it may add too much phosphorus. Sulphuric acid is the strongest, so less is needed, but it is the most dangerous to handle.
Why is bicarbonate in irrigation water a problem?
Bicarbonate keeps the water’s pH high. At a high pH, calcium and magnesium settle out as lime and block emitters. Nutrients such as iron, manganese and phosphorus also become harder for the plant to take up.
What pH should drip irrigation water be?
Mostly between 5.5 and 6.5. Soilless growing works near the low end (5.5–6), soil near the high end. We can set the right value for your crop together, from your water test.
How do I turn bicarbonate mg/L into meq/L?
Divide by 61: 1 meq/L of bicarbonate is 61 mg/L. So 305 mg/L of bicarbonate is 5 meq/L.