Design an investigation

Cover crops and soil erosion

Units 4 and 5
10 points
About 23 minutes

You can use a calculator on this question, just like on exam day.

You read about a real-world environmental situation with a described figure or model, then pick out the parts of a scientific investigation (hypothesis, variables, controls), explain the science behind it, and propose solutions to the problems it raises. On the exam: Question 1 of 3 in the 70-minute free-response section (Section II is 40% of the exam score).

The question and its sources

Agricultural scientists set up twelve identical test plots on a hillside farm to compare ways of protecting soil after the fall harvest.

Figure 1: Diagram of one test plot

The diagram shows a side view of a rectangular plot, 20 m long, running down a hillside with a 6% slope. Raindrops fall on the soil surface. Water that does not soak in flows downhill as runoff, carrying soil particles with it.

At the lower edge of the plot, a metal gutter collects all of the runoff and directs it into a covered tank. After each storm, the scientists dry and weigh the sediment that settles in the tank.

All twelve plots have the same soil: 40% sand, 40% silt and 20% clay, which is classified as loam. The plots are divided into three treatments with four plots each.

Source: Hypothetical study design

Table 1: Soil lost from October to April

Treatment	Description	Mean sediment collected per plot (kg)
Conventional tillage	Soil plowed after harvest and left bare all winter	410
No-till	Soil not plowed; crop stalks and leaves left on the surface	95
Cover crop	Soil plowed, then planted with winter rye that grows until spring	70

Source: Hypothetical data

Suggested time: 23 minutes

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Part (a)

1 point

Describe one property of a loam soil that makes it well suited for growing crops.

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Part (b)

1 point

Identify the independent variable in this investigation.

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Part (c)

1 point

Identify the dependent variable in this investigation.

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Part (d)

1 point

Identify the control group in this investigation.

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Part (e)

1 point

State a null hypothesis for this investigation.

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Part (f)

1 point

Explain why the scientists used four plots for each treatment instead of one.

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Part (g)

1 point

Based on Table 1, explain how planting a winter cover crop reduces soil erosion compared with conventional tillage.

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Part (h)

1 point

Describe one way that sediment eroded from farm fields can harm organisms in a nearby stream.

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Part (i)

1 point

Describe one disadvantage to a farmer of using the no-till method.

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Part (j)

1 point

Propose one sustainable farming practice, other than no-till or cover crops, that would reduce soil erosion on sloped fields.

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