After heavy rain, why does soil wash from one patch while another stays mostly in place? A pair of shallow trays lets children compare bare soil with soil protected by ground cover. The most important lesson is not simply that one tray “wins.” It is learning to change one condition, measure consistently and describe what the evidence actually shows.
Materials
- Two identical shallow trays with a small lip or drainage opening
- The same amount and type of soil in each tray
- Dry leaves, grass clippings or mulch for one tray
- Two clear collection cups
- A measuring jug or bottle with small holes in the lid
- Books or blocks to create equal slopes
- Towels and an outdoor or washable workspace
Use clean garden soil, not soil from a site that may contain animal waste, glass or chemicals. Avoid breathing dry dust; dampen it lightly if needed and wash hands after the activity. An adult should lift wet trays and manage slippery spills.
Set up a fair comparison
- Put equal amounts of soil in both trays and press them with the same light pressure.
- Cover one surface with a measured layer of leaves or mulch. Leave the second bare.
- Raise both trays to the same gentle angle and place a cup below each lower edge.
- Predict what will happen to runoff colour, soil movement and water volume.
- Pour the same amount of water from the same height for the same length of time over each tray.
Watch where small channels form. Compare the colour and sediment in the cups after the water settles. A darker cup may contain more suspended soil, but colour is only a rough observation. Older children can filter each sample through matching paper and photograph the results.
Keep a useful record
Create a table with columns for tray surface, water added, slope, runoff volume, runoff appearance and soil moved. Record unexpected events too: perhaps one pour was stronger, a leaf shifted or a drainage hole clogged. Those details help explain uncertainty. If you repeat the test, change only one factor, such as slope angle or ground-cover thickness.
A homemade rain gauge and weather log can connect the model with actual rainfall. The seed germination investigation offers another chance to practise fair comparisons.
Questions for different ages
- Ages 6–9: Which tray changed more? Where did the soil collect?
- Ages 10–12: Which variables did we control, and which were difficult to control?
- Teenagers: How could we measure sediment more reliably or repeat enough trials to compare variation?
Children who prefer not to touch soil can pour measured water, time the trial, photograph the trays or dictate observations. Use a stable table height and lightweight jugs for accessible participation.
What this model does not prove
Two trays cannot represent every landscape. Root depth, soil type, slope length, rainfall intensity and land management all matter. Loose leaves are not identical to living vegetation with roots. Phrase the conclusion narrowly: “In our setup, the covered tray lost less visible soil,” rather than “Plants always stop erosion.”
Improve the next trial
Ask children to identify the largest source of error and make one repair. They might mark a pour line on both bottles, use a timer, weigh equal dry soil or support trays on the same block. Run at least two trials before comparing. If results differ, report the variation instead of choosing the preferred result. Real investigations often show that a procedure needs refinement.
To trace where the runoff could travel next, make a water-cycle model or sketch a watershed. Dispose of muddy water on soil where permitted, not down a storm drain, and return clean natural materials responsibly. Wash containers before reusing them for another non-food activity.
