Which nutrients are examples of cations?

• A. Calcium, potassium, magnesium, ammonium
• B. Nutrients that are anions like sulfate
• C. Only nitrogen and phosphorus
• D. Trace elements only

Answer: (A)

Cations are positively charged nutrients that soil particles hold and release through cation exchange, making them available to plants. Calcium, potassium, magnesium, and ammonium are classic cations. Calcium and magnesium are divalent, potassium is monovalent, and ammonium is the positively charged form of nitrogen that plants can absorb. These contrast with sulfate, which is an anion (negatively charged), and with nitrogen and phosphorus, which in soils are commonly taken up as nitrate and phosphate anions (though nitrogen can also appear as ammonium). This is why the listed nutrients are cations.