PrepTest 106, Section 1, Question 8
Generally speaking, if the same crop is sown(70 more words not shown)
Which one of the following,(15 more words not shown)
- ASome kinds of(28 more words not shown)
- BAlfalfa increases the(26 more words not shown)
- CCertain types of(21 more words not shown)
- DAlfalfa increases nitrogen(20 more words not shown)
- EAlfalfa is very(15 more words not shown)
Reading the argument
Alfalfa puts nitrogen back. So why does it still grow worse?
The ordinary explanation is that repeated planting uses up nitrogen in the soil. But the passage specifically gives us a crop that increases the soil's nitrogen and still shows the decline. We need to explain that result while keeping the nitrogen increase.
Maybe nitrogen isn't the only thing the plant needs. What if another nutrient, such as potassium, is being depleted? I might also ask whether the nitrogen being added is in a form the plant can use. Those are possibilities to investigate, not facts we know about alfalfa. The point is that replenishing one resource doesn't settle every condition needed for growth.
What we’re looking for
The question asks us to explain why alfalfa resembles the other crops in declining over successive years. I'd look for some other change that makes later years worse. A fact about growing plants is useful only if it helps explain that change over time.
Working the answers
- A
Growing with other kinds of plants might help some crops, but this doesn't explain the comparison between earlier and later years in the same field. We aren't told that alfalfa was initially mixed with other plants and later grown alone. Nor are we told that alfalfa is one of the kinds that benefits. Without those connections, this doesn't account for the decline.
- B
This explains how alfalfa adds nitrogen. It doesn't explain why alfalfa grows less well despite doing so. Knowing where the nitrogen comes from and that most plants can use it gives us the mechanism behind the fact we already accepted, not a reason for the worsening growth.
- CCorrect
Here's a different change in the soil: harmful substances build up. Alfalfa adds nitrogen, but it also produces substances toxic to itself. Planting it repeatedly allows those substances to accumulate, making the soil worse for alfalfa even while nitrogen increases.
This takes a different route from the missing-nutrient guesses. We don't need another resource to run out; an unwanted substance can build up instead. The accumulation also explains the timing. Later crops face a buildup left by earlier crops, which gives us a reason growth could worsen year after year.
- D
We aren't told that the required bacteria are absent, disappear over time, or differ between earlier and later years. A requirement for nitrogen-fixing doesn't by itself establish that alfalfa fails to meet it. More importantly, the puzzle grants that alfalfa increases the nitrogen. We still need to explain the decline despite that benefit.
- E
Sensitivity to a compound could matter if the alfalfa were exposed to it. But are there black walnut trees near this field? Does exposure increase in later years? The answer supplies neither connection. A harmful substance from those trees is only a possible story we would have to add; C puts the harmful buildup into the repeated alfalfa planting itself.
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