Geography K–10 · Years 3–4
Erosion trays: bare soil, mulched soil and turf under the same rain
Geography (HASS F–6 sub-strand); NSW Geography K–10 (2015) Stage 2 focus area: The Earth's Environment (Protection of environments)
The idea
Plant cover and mulch slow water and hold soil, so the same rainfall carries far less soil off a covered slope than off a bare one.
Safety card
Hazards
- soil handling
- wet floor
Controls
- gloves, and hands washed after handling soil
- run outdoors or on a tarp
Note
No chemicals or heat are used. The activity is run under the school risk-assessment procedure; RiskAssess (riskassess.com.au) holds templates for school practicals and fieldwork. The NSW Department of Education Chemical Safety in Schools package is not triggered.
What you need
- 3 identical shallow trays (about 40 cm × 30 cm × 8 cm), each with a 15 mm wide V notch cut in the rim at the low end and the base of the notch level with the soil surface, so the jar catches surface wash; no hole is cut in the tray wall below the soil surface, which would drain water that had soaked through, would let soil pipe straight out, and would be blocked by the turf tray's root mat but not by the bare tray
- Garden soil to fill each tray to 5 cm, pressed level; tray 1 bare, tray 2 covered with 2 cm of straw mulch, tray 3 laid with a piece of turf
- 3 collecting jars (1 L) under the drainage holes
- 1 watering can with a rose (fine holes) and 1 L measuring jug
- A 10 cm wooden riser under the far end of each tray for the same slope
- An electronic scale reading to 0.1 g (a school science balance or a pocket scale) and 3 coffee filters to catch sediment
How to do it
- Set all three trays on the same riser so the slope is equal; place a jar under each outlet.
- Pour 1 L through the rose over 30 s onto the top half of tray 1. Time from the first drop to the last drip into the jar.
- Repeat with 1 L on trays 2 and 3 at the same rate.
- Measure the runoff volume in each jar with the jug.
- Filter each jar through a filter, rinsing the jar into it so nothing is left behind. Weigh the filters the same way before and after: dry each one in a warm dry place and weigh it once a day until two weighings in a row agree within 0.1 g (constant mass), because a paper filter's mass shifts with room humidity by more than the 0.1 g being measured. Sediment mass = loaded constant mass − empty constant mass; a difference of 0.1 g or less is recorded as too little to weigh on this scale, not as a number. A paper coffee filter passes some of the finest clay, so the mass is a lower bound on the soil that left the tray.
- Repeat the whole run twice more (3 trials) and record the mean runoff volume and sediment mass for each tray.
What you should see
The bare tray is expected to give the highest sediment mass and visibly muddy water, and the mulched and turfed trays much less sediment and clearer water. Runoff is less than 1 L in every case because some water stays in the soil; with the notch set at the soil surface, what reaches the jar is surface wash rather than water that has drained through the soil, so the runoff volumes and the sediment masses describe the same flow; a tray that takes more water into the soil gives less runoff. The class reports three means with units (mL and g), and records any tray whose sediment was too little to weigh as such. It worked if the bare tray gave the most sediment in all three trials; if not, check that the slopes and pouring rates were equal.
What changes
- What you change
- surface cover (bare, mulch, turf)
- What you measure
- runoff volume (mL) and sediment mass (g)
- What you keep the same
- slope
- water volume and pouring time
- soil type and depth
- tray size
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Rain only moves soil when there is a flood.
- Grass makes no difference to how much water runs off.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Geography K–10 Syllabus (2015), NESA; current and taught in 2026, replaced from 2027. The outcome codes are not printed on the NESA landing page for this syllabus, which only links the document, so the url given is the syllabus document itself (geography-k-10-syllabus-2015.docx); the code and its outcome text were read in that document on 23 September 2026.GE2-2
- Human Society and its Environment K–6 Syllabus (2024), NESA; implementation from 2027, not yet taught; code read on this page on 22 September 2026HS2-GEO-01
- Australian Curriculum v9AC9HS4K06AC9HS4S02AC9HS4S05
Sources
The pages the author read to write this activity.