Converted result
| Hydrogen ion [H+], mol/L | 3.16 × 10−7 |
|---|---|
| Hydrogen ion [H+], µmol/L | 0.32 |
| Hydrogen ion [H+], mg/L as H+ | 0.0003 |
| pOH | 7.50 |
| Hydroxyl ion [OH−], mol/L | 3.16 × 10−8 |
| Acidity vs neutral water (pH 7) | 3.2× more H+ |
Converted from your pH input at 25 °C (Kw = 1.0 × 10−14).
Understanding the pH scale: hydrogen ion activity in soil and water
The pH scale is a negative logarithm, not a linear one. It compresses a concentration range that spans twenty orders of magnitude into fourteen readable numbers, which is exactly why two readings that look close together can represent very different chemistry. This converter undoes that compression: give it any single member of the set (pH, pOH, [H+] or [OH−]) and it reports the whole equilibrium.
1. The four conversions the calculator runs
[H+] = 10−pH
pH + pOH = pKw = 14.00 (at 25 °C)
[OH−] = 10−pOH = 10(pH − 14)
mg/L H+ = mol/L × 1008 (molar mass of H+ = 1.008 g/mol)
Worked example. A soil paste reads pH 6.50. [H+] = 10−6.50 = 3.16 × 10−7 mol/L, which is 0.32 µmol/L or about 0.00032 mg/L as H+. The companion pOH is 14.00 − 6.50 = 7.50, so [OH−] = 3.16 × 10−8 mol/L. Because pH 6.50 sits 0.50 units below neutrality, this solution carries 100.5 ≈ 3.2 times more hydrogen than pure water at the same temperature.
2. Why a "small" pH difference is a big chemistry difference
Moving from pH 7.0 to pH 6.0 does not make the soil "a little more acidic": it multiplies hydrogen ion activity by ten. Moving from 7.0 to 5.5 multiplies it by about 32. That is the practical reason lime and sulphur rates are specified in tonnes or kilograms per hectare rather than in pH points: the amendment has to neutralise real equivalents of acidity, not a number on a dial. When you need the actual rate rather than the conversion, the lime requirement calculator turns a target pH and a soil CEC into tons per acre.
3. Temperature matters more than most charts admit
Self-ionisation of water is endothermic, so pKw falls as temperature rises: 14.17 at 10 °C, 14.00 at 25 °C, 13.79 at 40 °C. A hydroponic reservoir at 30 °C therefore has a neutral point near 6.80, not 7.00, and the same [H+] reading maps to a slightly different pOH. This tool holds the standard 25 °C basis so lab and textbook numbers agree; for reservoir work that must be temperature-corrected, read the EC and ionic balance alongside it in the PPM to EC calculator.
4. Reading soil pH: water, salt solution and buffer pH are three different numbers
- pH in water (1:1 or 2.5:1 soil:water) is the number most growers see on a report and the number this converter handles.
- pH in CaCl2 or KCl typically reads 0.5 to 0.8 units lower than the water value because the salt displaces hydrogen off the exchange sites. Quote the method with the number.
- Buffer pH (SMP, Sikora or Adams-Evans) measures reserve acidity held on the exchange complex, not active acidity, and it is what a lime rate is actually calculated from.
For the full method comparison and when each number changes a decision, see the soil reaction (pH) guide, and for pH targets by crop look up your plant in the soil pH plant database.
5. Target soil pH for common systems
| System | Target pH (water) | What drifts first |
|---|---|---|
| Blueberries, azaleas, rhododendrons | 4.5 – 5.5 | Fe and Mn lock out above 6.0 |
| Potatoes, potatoes scab-prone ground | 5.0 – 6.0 | Common scab increases above 5.5 |
| Most vegetables, strawberries | 6.0 – 7.0 | P ties with Ca below 5.5 |
| Lawn turf, pasture grasses | 6.0 – 7.0 | N efficiency drops as pH climbs past 7.5 |
| Alfalfa, legume seed | 6.5 – 7.5 | Rhizobium fails below 6.0 |
| Asparagus, spinach, brassicas on calcareous ground | 7.0 – 8.0 | Fe, Zn and Mn availability falls |
6. Low pH versus high pH: which tool takes over
- pH below target (acidic). The fix is a liming material, and the rate depends on buffering capacity: use the lime requirement calculator, or the field walkthrough in how to calculate lawn lime.
- pH above target (alkaline). Elemental sulphur, iron sulphate or an acidifying fertiliser; see how to lower soil pH fast. Above pH 8.3 also check whether sodium, not calcium carbonate, is driving the number: calculate exchangeable sodium percentage (ESP).
- pH drifting between seasons. Track the trend, not single readings: how to test lawn soil pH and the best soil for grass and lawns cover repeat sampling and correction.
References
- IUPAC (Gold Book): the operational definition of pH and the activity basis of the scale.
- Richards, L.A. (1954). Diagnosis and Improvement of Saline and Alkali Soils, USDA Agriculture Handbook No. 60 — soil reaction methods and interpretation.
- US Salinity Laboratory Staff, and McLean, E.O. (1982), "Soil pH and lime requirement", in Methods of Soil Analysis, Part 2 (ASA Monograph 9) — water, salt and buffer pH methods.
- Soil and Water Testing for Analytical Laboratories, FAO Soils Bulletin — field and laboratory pH protocols.
Sources and method: the publications this tool cites are listed in the calculator sources register, one table per calculator, with a link to each source that still answers and the formula this page prints kept as the primary record.
More soil chemistry tools
Turn buffering capacity (CEC, buffer pH or OM%) into tons of ag lime per acre, adjusted for ECCE and ENV.
Elemental sulfur, iron or aluminum rates to hit a target pH — with acidifier comparison and rebound warnings.
Convert lab ppm to meq/100g and read total CEC, base saturation and cation balance from one soil test.
Balance Ca, Mg, K and Na against Albrecht targets with a saturation donut, what-if simulator and plan.
Sodic soil reclamation: exchangeable sodium percentage into tons of gypsum per acre by source purity.
Irrigation water quality — sodium adsorption ratio, CROSS and adjusted SAR with dispersion diagnostics.
Exchangeable sodium percentage from Na and CEC in meq/100g or ppm, with the 6% and 15% sodicity thresholds.