Which nutrient is absorbed as both cations and anions by plants?

Nitrogen is unique among the options provided because it can be taken up by plants in both an ionic form as cations (specifically in the form of ammonium, NH4+) and as anions (primarily in the form of nitrate, NO3-). This duality allows plants to utilize nitrogen from various sources in the soil, enhancing their ability to meet their nutritional needs.

Apart from nitrogen, the other nutrients listed typically have more defined ionic forms for plant uptake. Sodium, for instance, is primarily taken up as a cation, while phosphorus is mostly available in forms such as phosphate that are considered anions. Manganese also exists largely as cations in soil, which plants absorb. Therefore, nitrogen’s capability to be absorbed in both forms makes it the correct answer.