Which particles in soil are negatively charged?

B is the correct choice because clays and soil organic matter are both important constituents of soil that possess negative charges. This negative charge is primarily due to the ionization of functional groups and surface sites on these particles, which allows them to attract positively charged ions (cations) such as calcium, magnesium, and potassium from the soil solution.

Clays are made up of fine mineral particles that have a high surface area relative to their volume, allowing them to hold more nutrients and water. Soil organic matter is composed of decomposed plant and animal materials, and it also contributes to the overall nutrient-holding capacity of the soil. The negative charges on clays and organic matter are vital for nutrient retention, which is crucial for plant growth.

The other options include components that do not typically hold negative charges in the same way. Sand and silt are mainly neutral and do not contribute significantly to cation exchange capacity. Silica primarily contributes to the framework of sand particles and doesn’t carry a charge, while iron can form both positive and neutral charges depending on its oxidation state but does not maintain a negative charge in the context of soil particles. Limestone, primarily a calcium carbonate, tends to have a neutral pH effect and does not contribute negatively charged sites