Keep the soil in place.

Learning goals and real-world question

A school wants to reduce sediment transported from a sloped soil patch. Students compare bare soil, straw mulch, and established planted cover using equal modeled rainfall and comparable soil trays.

They identify a treatment and controls, collect runoff, measure sediment with dry filters, use repeated trials, and support a provisional recommendation with data. The simplified interface supplies one task at a time, saves balance readings, and calculates group means and percentage reduction.

Background: soil cover can reduce erosion. This lab’s particular sediment quantities and treatment ranking are prescribed instructional outcomes, not results from a field trial. A real recommendation would consider rainfall, vegetation establishment, soil, costs, and site constraints.

Procedure

1. Predict which cover will transport the least sediment.
2. Place the rainfall simulator, prepared soil tray, runoff cup, filter funnel stand, balance, and clean filters.
3. Tare the empty balance. Weigh a clean, dry filter and place it in the funnel.
4. Apply the same nominal 100 mL of water over two modeled minutes from a fixed 20 cm source height.
5. Collect the tray runoff. Pour the collection cup through the weighed filter.
6. Drain and dry the loaded filter to modeled constant mass, then weigh it again.
7. Subtract the original dry filter mass. Repeat with three fresh prepared trays per treatment.
8. Compare means and observed ranges; explain drying, support a cover choice, and identify a limitation.

Controls: soil type, starting moisture, preparation, 500 g substrate mass, tray area, 10° slope, source height, applied water volume/duration, collection, filtering, and drying method. The app automatically supplies fresh standardized soil, a clean cup, and a new filter each trial. Planted trays represent established cover and roots, not plants grown during a two-minute test.

The changed factor is the ground-cover treatment. It includes the combined effects of mulch or vegetation/roots; the experiment does not isolate raindrop shielding from root effects or changes in infiltration. Fixed treatment order is used for this introduction; randomizing order is a worthwhile classroom extension.

Expected modeled pattern

Each trial varies, so students must use their own measured group means. The simulation’s ranking is not a guarantee that every planted cover will outperform every mulch at a real site.

Calculation examples

Dry filter 0.50 g; dry filter + sediment 3.00 g: collected sediment = 2.50 g. If a cover mean is 0.30 g and the bare mean is 2.50 g, reduction = (2.50 − 0.30) ÷ 2.50 × 100 = 88.0%.

The optional reduction check accepts ±0.2 percentage points. Report to one decimal place. Mean/range bars summarize three trials; the min–max interval is not a confidence interval.

Assessment · 12 points

3 = accurate and supported; 2 = mostly accurate but incomplete; 1 = limited or partly incorrect; 0 = absent. The app checks choices and writing presence; a teacher evaluates the scientific quality of the recommendation and limitation.

Discussion prompts

• Why could runoff color be misleading as a quantitative measure?
• How would incomplete drying affect the inferred sediment mass?
• Why must the empty filter mass be subtracted?
• Does collecting sediment in runoff account for all soil erosion?
• Which extra rainfall or slope conditions should be tested before making a site recommendation?
• What does a planted-versus-bare test tell us about roots alone?

Explicit measurement model and limitations

Captured dry sediment mass is prescribed as 2.5 g for bare soil, 1.0 g for mulch, and 0.3 g for planted cover, with independent trial variation up to ±10%. These values are synthetic instructional data. They are not derived from the rainfall volume, soil mechanics, or a calibrated erosion equation.

Dry filter true mass varies within 0.50 ±0.02 g. Each weighing adds noise within ±0.005 g and rounds to 0.01 g. The simulated filter captures all modeled transported soil; drying removes all water. Cover fragments, dissolved substances, splash outside the cup, infiltration, runoff volume, temperature effects, and real drying kinetics are omitted. Both time-lapses are visual aids, not real waiting periods.

The result is captured runoff sediment under a controlled teaching scenario, not a forecast of total field erosion. In a real classroom, filtration and drying methods must be selected and supervised by the teacher; blank-filter checks and capture-efficiency controls may be needed.

USDA NRCS: Cover crops for soil health
USDA AMS: Soil conservation and soil cover

The school scenario, simulated response, guided procedure, and assessment criteria are original instructional content.