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Difficulty: MediumStrengthening or Weakening Author Assertions

For decades, agrarian historians attributed the dramatic increase in grain yields during the eighteenth-century Norfolk Agricultural Revolution primarily to the adoption of the four-course crop rotation system, which incorporated turnips and clover alongside wheat and barley. According to the traditional consensus, turnips served predominantly as winter fodder, enabling farmers to sustain larger livestock herds through the winter months. This increase in herd size supposedly produced unprecedented volumes of animal manure, which, when applied to fields, restored depleted nitrogen levels and propelled cereal crop yields.

Recently, however, historical agronomists analyzed organic nitrogen isotope ratios in soil cores extracted from eighteenth-century Norfolk agricultural estates. Finding that soil nitrogen concentration increased substantially during this period even in sectors where livestock density remained strictly constant, these researchers argued that manure application was secondary. Instead, they hypothesized that clover—a legume that hosts nitrogen-fixing Rhizobium bacteria—directly synthesized atmospheric nitrogen into the soil. Based on these isotopic findings, the author asserts that clover's biological nitrogen fixation, rather than manure generation from increased livestock herds, was the primary mechanism behind the regional yield surge.

Which of the following, if true, would most weaken the author's assertion regarding the primary mechanism behind the Norfolk yield surge?

  1. Widespread burning of timber across eighteenth-century Norfolk released large quantities of airborne nitrogen compounds that deposited uniformly across both cultivated fields and uncultivated lands.Answer
  2. B
    Agronomic trials replicating eighteenth-century soil conditions demonstrate that clover crops fix atmospheric nitrogen at twice the rate of any other European legume from that era.
  3. C
    Eighteenth-century British estate owners who implemented the Norfolk four-course rotation earned greater long-term livestock profits than farmers who retained traditional fallow rotations.
  4. D
    Nineteenth-century agricultural theorists widely maintained that manure application was superior to natural crop rotation for maintaining soil fertility.
  5. E
    Turnip cultivation required significantly more manual labor for weeding and seedbed preparation than the cultivation of either wheat or barley.

Answer

The statement that widespread timber burning deposited airborne nitrogen compounds across all lands in Norfolk most weakens the author's assertion.
The correct answer weakens the argument by offering an alternative explanation for the observed soil nitrogen isotope increase. The author relies on soil core isotope spikes in livestock-constant sectors to conclude that clover was the primary driver. If timber burning introduced atmospheric nitrogen across all lands, the soil isotope increase was caused by external environmental deposition rather than clover, neutralizing the author's core evidence.

Step-by-Step Solution

1
Identify the author's main assertion and supporting evidence
Author asserts clover's nitrogen fixation was the primary driver of yield increases, based on evidence of rising soil nitrogen isotope ratios in areas where livestock numbers remained constant.
Understanding the precise link between evidence (soil isotope ratios) and conclusion (clover caused the yield surge) is required to evaluate potential weakenings.
2
Evaluate how new information disrupts the argument's logic
If airborne nitrogen from timber burning deposited nitrogen compounds uniformly across all soils, the isotope spike occurred independent of clover cultivation.
An alternative cause for the key piece of evidence undermines the claim that clover fixation was the principal cause of the nitrogen surge.
3
Select the choice that provides this alternative cause or flaws the inference
The option describing regional atmospheric nitrogen deposition from timber burning provides a compelling alternative explanation.
It directly attacks the validity of using soil isotope spikes as proof of clover's dominant role.

Key Concept

Weakening an argument by introducing an alternative explanation for the evidence
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Topics

Strengthening or Weakening Author Assertions

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