🌿 Soil pH determination
Potentiometric determination of soil pH in suspension with water (pH_H₂O) or KCl solution (pH_KCl). Basic agrochemical and environmental parameter.
Overview
Soil pH is one of the most important agrochemical parameters, affecting nutrient availability for plants, soil microorganism activity, and heavy metal mobility. Two values are determined: pH_H₂O (actual pH, in water suspension) and pH_KCl (exchangeable acidity, in 1 mol/L KCl suspension).
pH_KCl is lower than pH_H₂O by 0.5–1.5 units and better reflects soil buffering properties, as K⁺ ions displace H⁺ and Al³⁺ from the sorption complex. Agronomic soil pH classes: very acidic <4.5; acidic 4.5–5.5; slightly acidic 5.6–6.5; neutral 6.6–7.2; alkaline >7.2.
The method is simple, fast, and does not require expensive equipment, which is why it is performed en masse by Regional Chemical-Agricultural Stations (OSChR) in agricultural soil monitoring.
Method principle
The soil sample is mixed with distilled water or 1 mol/L KCl solution in a volume ratio of 1:5 (soil:liquid). The suspension is vigorously mixed for 5 minutes, left for 2 hours to equilibrate, and then pH is measured potentiometrically using a pH meter with combined glass electrode. Two-point calibration with pH 4.00 and 7.00 buffers.
Applications
- Agricultural soil monitoring (OSChR, IUNG)
- Fertilization recommendations (liming)
- Soil quality assessment according to Polish regulation
- Assessment of fertilization and cultivation impact on soil pH
- Assessment of heavy metal mobility in soil
- Environmental studies — acid rain impact
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 2–12 pH |
| Soil:liquid ratio | 1:5 (v/v) |
| KCl solution | 1 mol/L |
| Equilibration time | 2 h (±15 min) |
| Accuracy | ±0.1 pH |
Standard
- Standard number
- PN-ISO 10390:2022
- Title (PL)
- Jakość gleby — Oznaczanie pH
- Title (EN)
- Soil quality — Determination of pH
Step-by-step procedure
1. Sample preparation
Dry soil at room temperature. Sieve through 2 mm sieve. Measure 5 mL soil with graduated cylinder.
2. Suspension preparation
To 5 mL soil add 25 mL distilled water (pH_H₂O) or 25 mL 1 mol/L KCl (pH_KCl).
3. Mixing
Shake vigorously for 5 minutes on shaker (150–200 rpm) or manually.
4. Equilibration
Leave suspension for 2 h (±15 min) at room temperature. Do not mix during this time.
5. pH meter calibration
Calibrate pH meter with 4.00 and 7.00 buffers. Check slope (95–105% of theoretical).
6. pH measurement
Immerse electrode in suspension (2 cm below surface). Wait for stabilization (±0.02 pH for 30 s). Record result.
7. Rinsing and repetition
Rinse electrode with distilled water between samples. Perform duplicate every 10 samples.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Laboratory pH meter | Mettler Toledo SevenCompact S220, WTW inoLab pH 7310 | 3,000–15,000 PLN |
| Combined pH electrode | Mettler Toledo InLab Expert Pro, WTW SenTix 940 | 500–2,000 PLN |
| Laboratory shaker | IKA KS 130 basic, Heidolph Unimax 1010 | 3,000–8,000 PLN |
| PP beakers 100 mL | Disposable polypropylene beakers | 50–100 PLN/100 pcs. |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Potassium chloride KCl | 7447-40-7 | 1 mol/L solution (74.55 g/L), analytical grade |
| pH buffers 4.00 and 7.00 | — | Certified CRM for pH meter calibration |
| Distilled water | — | Conductivity <0.2 mS/m, pH 5.6–6.5 |
Health and safety (OHS)
- KCl — harmless, standard protective measures
- Glass electrode — fragile, risk of cuts
- Gloves and safety goggles — standard equipment
- Soil may contain contaminants — use nitrile gloves