It is not stated whether alfalfa is one such plant, nor is it stated whether alfalfa is typically mixed with other crops. This does not address the success of crops that are planted in the same field in successive years.

This simply describes the mechanism by which alfalfa increases nitrogen in the soil. It does not explain why alfalfa struggles when planted in successive years.

This explains why alfalfa struggles when planted in successive years. Though the soil accumulates nitrogen, it also accumulates toxic substances that are detrimental to alfalfa’s health.

This does not state whether the required bacteria exists in alfalfa fields. It establishes a condition necessary for alfalfa to be nitrogen-fixing but takes no position on whether that condition is satisfied.

This information is useless without more context. No given information suggests that black walnut leaves will more likely be present in fields where alfalfa is grown in successive years.