Soil pH is determined by the concentration of which ions?

• A. Hydroxyl and hydrogen (H)
• B. Hydrogen and nitrogen
• C. Calcium and phosphate
• D. Phosphorous and gypsum

Answer: (A)

Soil pH measures how acidic or basic the soil solution is, and it is driven by the activity of hydrogen ions. The pH value reflects the concentration of hydrogen ions, but it’s also linked to hydroxide ions because of the water dissociation equilibrium where H+ and OH- balance each other. In soils, this balance is shaped by buffering and exchange processes, but the fundamental determinant remains the presence of hydrogen ions (and their complement, hydroxide ions) in the solution. That’s why the best answer involves both hydrogen and hydroxyl ions. Other substances listed are nutrients or contributors to buffering, but they don’t directly establish the pH the way H+ and OH- do.