Three Trays, One Rainstorm
Three identical trays were filled with the same soil, packed the same way, then tilted at 20°. Tray 1 was left bare. Tray 2 was planted with grass that had grown for four weeks. Tray 3 was covered with a 2 cm layer of straw mulch. Each tray received 2 L of water poured from a can held 30 cm above its upper edge over 2 minutes. Runoff was caught in buckets, left to settle, and the soil that settled out was dried and weighed.
| Tray | Surface cover | Soil lost | | 1 | none | 42 g | | 2 | grass | 6 g | | 3 | straw mulch | 9 g |
The runoff from tray 1 was by far the muddiest.
1. Which cover prevented the most erosion? Use the masses to compare, then explain the two ways a cover reduces erosion.
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Grass prevented the most erosion. Tray 2 lost 6 g of soil, compared with 42 g for bare soil and 9 g for straw mulch. That is seven times less soil than bare ground, and a little less than mulch.
Covers work in two ways. First, they shield the surface: falling drops hit blades or straw instead of bare soil, so individual soil particles are not blasted loose. Second, they slow the water down, so more of it soaks in and less runs off downhill carrying particles.
Grass has an extra advantage: its roots form a dense net through the topsoil that physically binds particles together, so soil resists being picked up even where water flows. Straw mulch protects and slows water too, but it has no roots, which is likely why it lost slightly more soil.
2. A fourth tray is prepared with a 3 cm layer of gravel on top. Predict how much soil it would lose compared with the other three, and explain your reasoning.
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Expect a mass between the bare tray and the mulched tray - roughly in the region of 15-25 g, clearly less than 42 g but more than 9 g.
Gravel helps in the same two ways as mulch: it covers bare soil so drops strike stones instead of loose particles, and it slows runoff so more water soaks in. That should cut erosion a lot compared with bare soil.
But gravel has two weaknesses here. It does not bind soil; fine particles underneath can still be washed out through the gaps between stones. And water runs quickly over a smooth gravel surface rather than spreading out. So it should protect well against splash but less well against the steady flow that carries soil away, leaving it behind both grass and straw mulch.
3. The trays were tilted at only 20°, but the school's hillside is much steeper. Predict how the results might change on the hillside, and state one thing these trays cannot tell you about the real hillside.
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On a steeper slope, water runs downhill faster and has less time to soak in, so every tray would lose more soil. The order would probably stay the same, because each cover works by slowing water and protecting the surface, and those benefits still apply on a slope. Bare soil would erode much more sharply than the rest.
One order could change, though: on a very steep slope, straw mulch can be washed or blown off in a real storm, after which the ground underneath behaves like bare soil. Grass, held by roots, would stay in place.
The trays cannot tell us about real rainfall. Natural storms have much higher drop energy, arrive after dry spells, and repeat over months, and the hillside also faces wind, freezing, and disturbance by people and animals.