calcium nitrogen oxygen protein
I gave my students this multiple choice question: Fertilizers can enable farmers to grow the same crop in a field for several years in a row. Farmers who use less fertilizer often rotate their crops by planting the crop one year and legumes, such as beans and clover, the following year. Fertilizer use and crop rotation with legumes both increase the availability of which of the following nutrients in soil?

Only one answer is correct among these 4 choices:
- calcium
- nitrogen
- oxygen
- protein

Could you tell me which one is correct?

nitrogen