Logical reasoning PrepTest 141 · Section 2 · Question 20

Incorrect. This answer is too specific in calling out alfalfa, which was just one example of the "green-manure" crops that can rejuvenate soil.

Incorrect. The argument relies on the chemical fertilizers somehow stopping the farmers from growing "green-manure" crops, not that these fertilizers don't in some way help those crops. Even if chemical fertilizers enhance the yield of these crops, farmers could still skip growing them (maybe other crops are more lucrative) to the detriment of the soil.

Incorrect. The conclusion is about improving the soil structure, not the soil quality, so this answer about the latter is out of scope. Additionally, even if the argument did make that jump, it's not necessary for soil structure to be the most important factor in soil quality for it to still be improved by a focus on soil structure—if you get better at the second-most-common question type on the LSAT, your score still improves.

Incorrect. The chemical fertilizers can have only indirect negative effects on soil and still be detrimental overall. They don't need to do direct damage to soil structure for this argument to work.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Chemical fertilizers led to decreased use of "green-manure" crops which led to poor soil structure. Therefore, abandoning fertilizers is necessary to make the soil better.

Answer Anticipation:
First, note that while the conclusion in this argument is causal, so are the premises—this isn't a correlation/causation flaw. However, the language in the conclusion does suggest another flaw—a degree shift. "[N]eed to abandon" is an exceptionally strong phrase, especially since the stimulus notes that the use of the "green-manure" crops was periodic. It's possible that the use of chemical fertilizers can continue as long as the farmers periodically use a "green-manure" crop to rejuvenate the soil—or even some other method! The correct answer, therefore, should highlight that these chemical fertilizers are somehow incompatible with the "green-manure" crops.

Answer Explanation:
This answer connects using chemical fertilizers to not growing "green-manure" crops. If many of the farmers will grow these crops even if they keep using chemical fertilizers, then it's not necessary "to abandon" their use to improve soil structure—the farmers can keep using the fertilizers while periodically growing the "green-manure" crops.

Key Takeaway:
First, causal premises can support causal conclusions, so not all arguments with causal conclusions feature correlation/causation flaws. Second, strong language in a conclusion suggests that there might be a degree shift flaw—when the conclusion has strong language, check the premises to see if they have equally strong language to back it up!