Soil pH is a crucial chemical property that influences plant growth and nutrient availability, being a measure of the soil solution's acidity and alkalinity. Various testing methods exist, such as different soil-to-water ratios and standardized procedures, highlighting the importance of consistency in testing for accurate data. Factors affecting pH readings, including suspension measurement protocols and electrode position, must be standardized to ensure valid comparisons over time.

1. Item 13: Soil pH

2. Basic theoretical principles
• Soil pH is considered the single most important
chemical property of soil because it affects plant
growth and nutrient availability in many different
and complex ways.
• Soil pH is a measure of the soil solution’s (soil
water) acidity and alkalinity.
• pH is the negative logarithm of the hydrogen ion
concentration [H+]
pH = -log [H+]

3. Testing for Soil pH
• soil to water ratios of 1:1 (most common in
most of the U.S.) or 1:2, 1:2.5, 1:5, 1:10
• saturated soil paste.
• Dilute salt solution (KCl, CaCl2)
• it is important to be aware of the soil pH test
being used and to be consistent between
methods to ensure comparable data over time

4. • ISO 10390:2005, has unified the pH analysis at
a soil:solution ratio of 1:5
• ASTM D4972-13, provide the standard
method for the soil:solution ratio of 1:1
• Kalra, Y.P., & Maynard, D.G. (1991) suggest to
measure at soil:solution ratio of 1:4
• Diagnosis and Improvement of Saline and
Alkali Soils ( 1954) suggest saturated paste
method for saline-sodic soils
Testing for Soil pH

5. • several European and worldwide initiatives
including the GlobalSoilMap and the
international soil classification have unified
the pH measurement (soil:water at the ratio
of 1:5)

6. • Soil pH in water at various soil:water ratios
o 1:1
o 1:2
o 1:2.5
o 1:5
Methods used by SEALNET’s Labs for measuring soil pH
• Suspension measurement
o Stirred
o Settled
• Electrode bulb position
o In supernatant
o In sediment

7. Will these differences affect results?
• Highly likely that suspension measurement protocol
and electrode position will affect readings in an
unpredictable way.
• If suspension method protocol and electrode position
are standardised in an SOP, it is likely that pH at one
soil:solution ratio can be converted to another ratio.

8. Clay loam
Sandy clay loam
Loamy clay
source: Afzel and Yasin, 2002
Difference variation from 0.1 – 0.4 pH unit

9. pH
Soil:water ratio
1:1 1:5 1:10
7.7
8.3
8.6
Brazito Sandy Loam pH
CaCl2 7.4
KCl 7.1
Difference variation 0.6 pH unit

10. 200 soil samples from SW Poland
Source: Kabała et al 2016.

11. Fig. 2. Linear regression model of pH
1:5 water (pH 1:5W) versus pH 1:2
CaCl2water (pH 1:2 CaCl2) (black
circles) and pH 1:5W versus pH 1:1W
(open circles).
Source: Z. Libohova et al. 2014
RMSE 0.44

12. What critical factors will the SOP for soil pH need to address?
• Soil and water ratio
• Suspension measurement- stirred versus settled
• Electrode bulb position

13. raw
pH data
YES NO

14. To check the electrode

15. Quality of water use
• distilled water (DI) for rising electrodes;
however, deionized , RO, pure are also
recommend

16. Interpretation of soil pH
Source: Australian Centre for International Agricultural Research. 2008

17. Thank you for your attention…