Agriculture and Food 7–12 · Years 11–12
Soil pH profile and the liming decision
Plant/Animal production: Soil, nutrients and water
The idea
Soil acidity changes with depth, so measuring pH in calcium chloride in the 0–10 cm and 10–20 cm layers shows whether a paddock needs maintenance or recovery liming before aluminium toxicity restricts roots.
What you need
- soil auger or push tube marked at 10, 20 and 30 cm; sample bags; phone with GPS for site records
- 0.01 mol/L calcium chloride solution, 500 mL
- pH meter with pH 4 and 7 buffers; 50 mL screw-top tubes; balance reading to 0.01 g
- 2 mm sieve and trays for air-drying
How to do it
- Sample, if possible, in autumn two to ten days after good rain, as the NSW fact sheet advises. Record ten GPS-marked points across each of two paddocks with different histories (for example lucerne hay cut for years, and native pasture).
- At each point take a core and split it into 0–10, 10–20 and 20–30 cm sections; combine each depth across the ten points into one bulked sample per paddock.
- Air-dry, sieve to 2 mm and measure pH in 0.01 mol/L calcium chloride at 1:5 (5.00 g soil in 25 mL), shaking for 1 hour and settling for 30 minutes, in triplicate.
- Plot pH against depth for each paddock.
- Apply the NSW guidance to decide between maintenance liming, recovery liming and liming for subsurface acidity, and list the practices in each paddock's history that would acidify it.
What you should see
The profile places each paddock in one of the three NSW cases: topsoil above pH 5.2 with subsurface above 4.8 needs only maintenance liming; topsoil below 5.2 needs recovery liming; subsurface below 4.8 needs lime whatever the topsoil reads. A paddock with a long history of hay or legume removal or ammonium fertiliser is expected to read lower, because the fact sheet names inefficient use of nitrogen (including ammonium-based fertilisers) and removal of produce, especially lucerne hay and legume crops, as the main causes of acidification. The learner knows it worked when triplicates agree and both paddocks were sampled the same way.
What changes
- What you change
- paddock history and sampling depth
- What you measure
- pH in calcium chloride
- What you keep the same
- ten points per paddock
- depth increments
- 1:5 ratio, shaking and settling times
- meter calibration
- time of year
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Spreading lime on the surface fixes acidity at every depth straight away.
- Soil acidity is natural, so farming practices cannot change it.
- One sample from the middle of a paddock represents the whole paddock.
Safety card
Hazards
- strain from augering
- calcium chloride irritates eyes
- soil contaminants
Controls
- share augering and lift correctly
- eye protection when handling the solution
- gloves for soil; wash hands
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.
Sources
The pages the author read to write this activity.
- www.soilquality.org.au/factsheets/soil-acidity-new-south-wales
- www.soilquality.org.au/factsheets/making-sense-of-chemical-indicators
- www.nsw.gov.au/education-and-training/nesa/curriculum/tas/agriculture-stage-6-2013
- www.nsw.gov.au/sites/default/files/noindex/2025-03/agriculture-amended-st6-syl-2013.doc
- curriculum.nsw.edu.au/learning-areas/tas/agriculture-11-12-2026/outcomes
- education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/5._Volume_2_Appendices.pdf