Earth and Environmental Science 11–12 · Year 11

Soil pH and organic content by loss on ignition

Module 1: Earth’s Resources

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Soil forms from rock, air, water and living things together; its pH and organic fraction are measurable records of the biotic and chemical part of that formation.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

Bunsen flame and crucibles above 400 °C
barium sulfate powder inhalation
smoke from burning organic matter

Controls

tongs and heat mat
fume cupboard or open window for ignition
safety glasses
let crucibles cool in the desiccator

Note

Heat or hot water is used: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package, Section 1, and record a RiskAssess risk assessment before the lesson.

What you need

the same three soils as the texture test, sieved to 2 mm
soil pH test kit (barium sulfate powder and indicator) or a calibrated pH meter and 1:5 soil to water suspension
crucibles, tongs, muffle furnace set to 400 °C (or, where no furnace is available, a Bunsen burner and pipe-clay triangle), desiccator, balance to 0.01 g
drying oven at 105 °C

How to do it

pH: mix 10 g soil with 50 mL deionised water, stir for one minute, stand five minutes and read the meter in the suspension (or use the kit on a moist paste).
Dry 20 g of soil at 105 °C overnight and weigh the dry mass to 0.01 g.
Heat the dry soil in a crucible in the furnace at 400 °C for 4 hours, the same temperature and time for every sample, cool in the desiccator and reweigh. A crucible over a full Bunsen flame reaches red heat, where carbonates break down (calcium carbonate loses 44 % of its mass as carbon dioxide) and clays lose bound water, so Bunsen losses overstate organic matter, most for limy or clayey soils, and cannot be compared with furnace losses; if only a Bunsen is available, heat every sample the same way and report the loss as an upper limit.
Loss on ignition (%) = (dry mass − ignited mass) / dry mass × 100; repeat for each soil.
Tabulate texture class, pH and loss on ignition against site (garden bed, sports field, bush, roadside).

What you should see

Garden and bush soils with visible litter give the larger losses on ignition; for example a 20.00 g dry sample that ignites to 19.10 g has lost 4.5 %. The soilquality.org.au NSW acidity fact sheet states that plant growth is favoured between pH 5.5 and 8, that topsoil pH measured in calcium chloride should be kept above 5.2, that a reading in water can be 0.6 to 1.2 units higher than in calcium chloride, and that field kits give results similar to water measurements; the learner reports the kit and meter readings side by side and states the method behind each.

What changes

What you change: soil site
What you measure: pH and loss on ignition (%)
What you keep the same: 1:5 soil to water ratio
same drying temperature and time
same ignition temperature and time (400 °C for 4 hours)
cooling in a desiccator before weighing

Common misconceptions

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

pH 7 is the natural pH of all soil.
Burning the soil removes the minerals (it removes the organic carbon; the mineral ash remains).

Curriculum references

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

Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES11-8 EES11-11 EES11/12-3 EES11/12-4 EES11/12-5
Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-11-03
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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