Why does clay retain nutrients better than sand?

• A. Clay has more negative charges and higher CEC
• B. Sand has more surface area than clay
• C. Organic matter primarily controls nutrient retention
• D. Gravel increases nutrient holding capacity

Answer: (A)

Nutrients in soil are held by electrostatic attraction to negatively charged surfaces, a property called cation exchange capacity (CEC). Clay minerals carry a lot of permanent negative charges from their layered crystal structure, and they also have a very high surface area due to their tiny particle size. This combination gives clay a high CEC, meaning it can hold and exchange more positively charged nutrients (like potassium, calcium, magnesium, and ammonium) in the root zone.

Sand, on the other hand, is coarse and has much less surface area and fewer negative charges, so its CEC is low. With fewer sites to hold nutrients, most of them are more prone to leaching with water movement.

So, clay retains nutrients better because its mineral structure provides more negative charges and a higher CEC, offering more sites to keep essential nutrients in the soil for plant uptake. Organic matter also helps with nutrient retention, but when directly comparing clay to sand, the high CEC of clay is the primary reason for the difference. Gravel has little effect because its large particles and low surface area yield very low nutrient-holding capacity.