Which feature of soils contributes most to nutrient retention in soils like clay?

• A. Lower water-holding capacity
• B. Higher pH stability
• C. Lower mineral content
• D. Higher charge density on surfaces

Answer: (D)

The key idea is that nutrient retention in clay-rich soils is controlled by the charged surfaces of clay minerals. Clay minerals have a large surface area with a high density of negative charges on their surfaces due to substitutions in their crystal structure. Those charges create a strong attraction for nutrient ions in the soil solution and give clay soils a high cation exchange capacity, meaning ions can be held on the surface and exchanged with others rather than being washed away. The higher the charge density on the surfaces, the more sites there are to bind nutrients, so nutrients stay in the soil longer and are more available to plants. That makes the feature described—higher charge density on surfaces—the best explanation for why clay soils retain nutrients so effectively. (Lower water-holding capacity would actually reduce retention, clays typically have high water-holding capacity; pH stability is about buffering, not direct nutrient binding; lower mineral content would reduce, not explain, the retention.)