🌿 Soil pH determination
In short
Potentiometric determination of soil pH in suspension with water (pH_H₂O) or KCl solution (pH_KCl). The test is performed according to PN-ISO 10390:2022; Measurement range: 2–12 pH. The procedure comprises 7 steps (about 2 h 25 min in total); it is used for: Agricultural soil monitoring (OSChR, IUNG), Fertilization recommendations (liming), Soil quality assessment according to Polish regulation.
At a glance
- Standard: PN-ISO 10390:2022
- Category: Environmental
- Procedure steps: 7
- Total time of stages: 2 h 25 min
- Measurement range: 2–12 pH
- Soil:liquid ratio: 1:5 (v/v)
- KCl solution: 1 mol/L
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
-
Sample preparation
Dry soil at room temperature. Sieve through 2 mm sieve. Measure 5 mL soil with graduated cylinder.
-
Suspension preparation
To 5 mL soil add 25 mL distilled water (pH_H₂O) or 25 mL 1 mol/L KCl (pH_KCl).
-
Mixing
Shake vigorously for 5 minutes on shaker (150–200 rpm) or manually.
-
Equilibration
Leave suspension for 2 h (±15 min) at room temperature. Do not mix during this time.
-
pH meter calibration
Calibrate pH meter with 4.00 and 7.00 buffers. Check slope (95–105% of theoretical).
-
pH measurement
Immerse electrode in suspension (2 cm below surface). Wait for stabilization (±0.02 pH for 30 s). Record result.
-
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
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-ISO 10390:2022 — “Soil quality — Determination of pH”.
How does this method work?
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.
What is the measuring range and accuracy?
Measurement range: 2–12 pH; Soil:liquid ratio: 1:5 (v/v); KCl solution: 1 mol/L; Equilibration time: 2 h (±15 min).
How long does the test take?
The times given for the individual stages add up to approximately 2 h 25 min. The procedure comprises 7 steps.
What equipment is required?
Laboratory pH meter, Combined pH electrode, Laboratory shaker, PP beakers 100 mL. Indicative cost of the core instrument (Laboratory pH meter): 3,000–15,000 PLN.
Where is this test used?
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.
What safety precautions apply?
KCl — harmless, standard protective measures; Glass electrode — fragile, risk of cuts; Gloves and safety goggles — standard equipment; Soil may contain contaminants — use nitrile gloves.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-ISO 10390.