Which of the following is a source of nitrogen for plants?

Nitrogen is an essential nutrient for plants, playing a crucial role in their growth and development. Ammonium nitrogen is a form of nitrogen that plants can readily absorb and utilize. It is converted into amino acids and proteins, which are vital for plant metabolism and overall health. Ammonium ions (NH4+) are absorbed by plant roots from the soil and can be a primary source of nitrogen, especially in soils that are rich in organic matter or subjected to fertilization practices.

The other options refer to elements that are not sources of nitrogen for plants. Sulfur is important for synthesizing certain amino acids but does not provide nitrogen. Similarly, calcium and potassium are also essential nutrients for plant health but do not supply nitrogen. This makes ammonium nitrogen the only correct choice, as it directly contributes to the nitrogen nutrition of plants.