Agriculture and Food 7–12 · Years 9–10

Soil pH in water and in calcium chloride

Plant Production 1

PracticalLow risk

The idea

Soil pH measured in 0.01 mol/L calcium chloride reads lower than in water because the salt releases acidity held on soil particles, and the calcium chloride figure is the standard used to decide on liming.

What you need

air-dry soil sieved to less than 2 mm from three sites, 30 g each
0.01 mol/L calcium chloride solution (1.47 g of calcium chloride dihydrate per litre of deionised water), 250 mL
deionised water, 250 mL
50 mL screw-top tubes, 18; balance reading to 0.01 g; 25 mL measuring cylinder
pH meter with pH 4 and pH 7 buffer solutions for calibration
field soil pH test kit (indicator dye and white powder)

How to do it

Calibrate the pH meter with the pH 7 and pH 4 buffers.
Weigh 5.00 g of each soil into each of six tubes; add 25 mL of deionised water to three and 25 mL of calcium chloride solution to the other three (a 1:5 soil to solution ratio), so each soil and solution has three separate extractions.
Cap and shake the tubes for 1 hour (by hand at 10 minute intervals if there is no shaker), then let them settle for 30 minutes.
Rinse the electrode, measure the pH of each suspension and record the difference, pH in water minus pH in calcium chloride.
Test the same soils with the field kit and record its colour reading.
Report the mean and range of the three tubes for each soil and method.

What you should see

For every soil the calcium chloride pH is lower than the water pH; the soilquality.org.au NSW fact sheet gives the difference as 0.6 to 1.2 pH units. The field kit reads close to the water value. Read against the NSW guidance, a topsoil below pH 5.2 (calcium chloride) needs recovery liming and a subsoil below 4.8 needs liming whatever the topsoil; one pH unit lower means ten times more acid. The learner knows it worked when the triplicates agree within 0.1 pH unit and the calcium chloride reading is the lower one for each soil.

What changes

What you change: extracting solution (water or 0.01 mol/L calcium chloride) and soil sample
What you measure: pH
What you keep the same: 1:5 soil to solution ratio
shaking and settling times
meter calibration
temperature of the suspensions

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

A soil at pH 5 is only slightly more acidic than one at pH 6.
A soil pH number is the same whatever liquid the soil is mixed with.
Once a paddock has been limed it will not need lime again.

Safety card

Low riskLearners carry it out

Hazards

calcium chloride dust and solution irritate eyes
glass pH electrode can break
soil contaminants

Controls

eye protection when weighing and pouring
gloves for soil
wash hands afterwards

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1 (general information for all staff) and Volume 2 Appendix D (generic risk assessment advice for each chemical). Complete a CSIS or RiskAssess site-specific risk assessment before the lesson.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Agricultural Technology Years 7–10 Syllabus (2019)AG5-6 AG5-8 AG5-11
Agriculture Technology 7–10 Syllabus (2024), new syllabus, implementation from 2027AGT5-APG-01 AGT5-ENV-01
Australian Curriculum v9AC9TDE10K04AC9S10I03

Sources

The pages the author read to write this activity.

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