Critical Reasoning

Generally speaking, if the same crop is sown in a field for several successive years, growth in the later years is poorer than growth in the earlier years, since nitrogen in the soil becomes depleted. Even though alfalfa is a nitrogen-fixing plant and thus increases the amount of nitrogen in the soil, surprisingly, it too, if planted in the same field year after year, grows less well in the later years than it does in the earlier years.

Which one of the following, if true, most helps to explain the similarity described above between alfalfa and non-nitrogen-fixing plants?

(A) Some kinds of plants grow more rapidly and are more productive when they are grown among other kinds of plans rather than being grown only among plants of their own kind.

(B) Alfalfa increases the amount of nitrogen in the soil by taking nitrogen from the air and releasing it in a form that is useable by most kinds of plants.

(C) Certain types of plans, including alfalfa, produce substances that accumulate in the soil and that are toxic to the plants that produce those substances.

(D) Alfalfa increases nitrogen in the soil in which it grows only if a certain type of soil bacteria is present in the soil.

(E) Alfalfa is very sensitive to juglone, a compound that is exuded from the leaves of black walnut trees.


OA after discussion.

My ans - C.

IMO C

I'm also in the C bandwagon - it's the only answer that explains a similar situation to nitrogen depletion, except in the reverse: accumulation of toxins that inhibit growth of the alfalfa.

Let's look at the other answers:
(A) Some kinds of plants grow more rapidly and are more productive when they are grown among other kinds of plans rather than being grown only among plants of their own kind.
There was no mention of growing plants together in the question, so this appears to introduce random and unneeded information. This answer also specifics some kinds of plants, which means this would only account for some of the alfalfa growth problems, not all.

(B) Alfalfa increases the amount of nitrogen in the soil by taking nitrogen from the air and releasing it in a form that is useable by most kinds of plants.
This doesn't explain why alfalfa would suffer if planted in the same location year after year. This answer only explains the benefits that alfalfa gives to other plants

(C) Certain types of plans, including alfalfa, produce substances that accumulate in the soil and that are toxic to the plants that produce those substances.
Accumulation of toxins! This would explain why alfalfa thrives less and less each year: the alfalfa is creating toxins in the soil that are toxic to the alfalfa, and accumulate year over year. The situation applies to every instance of alfalfa growth, and explains why the alfalfa would grow less each year.

(D) Alfalfa increases nitrogen in the soil in which it grows only if a certain type of soil bacteria is present in the soil.
A certain type of soil bacteria is too specific: the question clearly states that 'Even though alfalfa is a nitrogen-fixing plant and thus increases the amount of nitrogen in the soil'. Again, this answer choice introduces new information that could only account for part of the problem (like in Answer choice A).

(E) Alfalfa is very sensitive to juglone, a compound that is exuded from the leaves of black walnut trees. This answer choice looks similar to A and D, in that it introduces information that could answer the alfalfa problem, but doesn't explicitly state that all alfalfa planted near black walnut trees will suffer from juglone, nor does it specify that all alfalfa is planted near black walnut trees. Both of those circumstances would need to occur to account for all alfalfa growth decline.

Thanks for your explanation, DCJen!

would go for C