Mulch, Runoff, and the Muddy Blacktop
Fifth graders at Ridgeway Elementary noticed a muddy stream crossing the blacktop after every heavy rain. Their teacher set up two identical 30 cm by 45 cm trays filled with the same soil mix and tilted both at the same angle using a 15 cm block. One tray was left bare; the other was covered with a 2 cm layer of wood-chip mulch. For each trial a student poured 500 mL of water through a watering can held 30 cm above the top of the tray for 30 seconds, and runoff was caught in a jar at the bottom. After three trials the class averaged the results: the bare tray produced 320 mL of runoff carrying 18 g of dried sediment, while the mulched tray produced 180 mL of runoff carrying 4 g of dried sediment. A classmate says, 'The mulch made the water disappear.' A parent asks whether mulch would still help on the steeper part of the hill.
1. Which claim do the class data support, and which numbers are the strongest evidence? Explain why the 'water disappeared' claim is not supported.
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The data support the claim that mulch reduces erosion; they do not support the claim that water disappeared. The strongest evidence is the sediment comparison: 18 g of soil washed off the bare tray versus 4 g off the mulched tray, about four and a half times as much, even though both trays received exactly 500 mL. Sediment is the soil itself being carried away, which is exactly the problem on the blacktop. The disappearing-water idea fails because the mulched tray still produced 180 mL of runoff. The water did not vanish; some soaked into the wood chips and the rest ran off more slowly. Everything poured in ends up as runoff or as water held in the tray, so the mulch changed how fast and how much water flowed off, not whether water existed.
2. Identify two things the class kept the same and describe one change that would make the comparison more trustworthy or more useful to the parent.
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Controlled variables include the 500 mL of water, the 30 cm pour height, the 30 second pour time, the 15 cm slope block, the tray size, and the soil mix. Each of these could change the runoff on its own, so holding them fixed is what makes the mulch the only difference between trays. The class already repeated three trials, which is good. To go further, add a third treatment, such as a tray planted with grass, so the class can see whether surface cover in general reduces erosion rather than mulch alone. To answer the parent, run the same comparison at a steeper slope, because a result that holds at two angles is much stronger evidence for the real hill.
3. Predict what would happen if the bare tray were planted with grass seed and watered for three weeks before the test. Explain using what you know about how plants affect soil.
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Expect far less runoff and much less sediment, probably near or below the mulched tray. Grass roots thread through the soil and bind particles together, so running water has a harder time lifting them. The blades also break falling drops into smaller splashes, so water hits with less force and soaks in instead of streaming downhill. The class would still pour 500 mL from 30 cm so the comparison stays fair. Because roots and mulch work in different ways, the strongest display compares bare soil, mulch, and grass side by side. One caution: three weeks of watering can pack or crust the soil surface, so the class should note whether the surface changed before testing.