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Difficulty: MediumMethod of Reasoning

Soil scientist: Conventional agricultural wisdom holds that planting legume cover crops between growing seasons enriches soil fertility solely by fixing atmospheric nitrogen. However, this view is incomplete. When non-nitrogen-fixing brassica cover crops are planted instead, soil nitrogen retention across subsequent crop rotations matches that achieved with legumes, even though brassicas fix no atmospheric nitrogen at all. Therefore, the primary mechanism by which cover crops preserve soil nitrogen is not biological fixation, but rather the physical prevention of nitrate leaching by dense root networks.

Which one of the following most accurately describes the method of reasoning employed in the soil scientist's argument?

  1. A
    concluding that atmospheric nitrogen fixation plays no role in plant growth because an alternative factor can also prevent nutrient loss
  2. B
    drawing an analogy between two plant species to establish that they share identical cellular mechanisms for nutrient absorption
  3. undermining a proposed explanation of a phenomenon by showing that the phenomenon occurs in the absence of the alleged causeAnswer
  4. D
    inferring that because root network stabilization is sufficient for nitrogen retention, it is a necessary condition for all soil enrichment
  5. E
    discrediting a widely held belief by demonstrating that the empirical data cited in support of that belief was gathered under unrepresentative conditions

Answer

The argument proceeds by undermining a proposed explanation of a phenomenon by showing that the phenomenon occurs in the absence of the alleged cause.
The author casts doubt on the claim that atmospheric nitrogen fixation is the sole explanation for nitrogen retention by showing that another plant variety (brassicas) produces the exact same retention effect despite lacking the ability to fix nitrogen. Thus, the argument undermines an explanation by pointing out that the phenomenon occurs in the absence of the supposed cause.

Step-by-Step Solution

1
Identify the target claim and conclusion of the argument
The target claim is the conventional view that legume cover crops enrich soil fertility solely through atmospheric nitrogen fixation. The main conclusion is that the primary mechanism is physical leaching prevention by root networks, not biological fixation.
Accurately identifying the conclusion is essential for mapping the overall argumentative method.
2
Analyze the empirical evidence introduced by the author
The author introduces brassicas—a cover crop that fixes zero atmospheric nitrogen—and shows that soil nitrogen retention is identical to that achieved with legumes.
Understanding the function of the comparative evidence reveals how the author refutes the conventional causal model.
3
Abstract the logical relationship into structural terms
The author demonstrates that effect E (nitrogen retention) persists even when purported cause C (nitrogen fixation) is absent, thereby proving that C is not the sole or primary cause.
Matching this abstract template to the answer choices identifies the correct method of reasoning.

Key Concept

Causal Refutation via Method of Difference / Absent Cause
Estimated Time:1m 15s
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Topics

Method of Reasoning

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