What type of soils are composed of silt and clay and generally drain poorly when wet?

Fine grained soils are characterized by their composition primarily consisting of silt and clay particles. These soils possess small particle sizes, which result in a high specific surface area. Because of this property, fine grained soils have a tendency to hold water and do not drain well when saturated. The small void spaces between the particles make it difficult for water to flow through, leading to poor drainage characteristics.

In contrast, coarse grained soils, composed of larger particles such as sand and gravel, typically allow for better drainage due to their larger pore spaces. Organic soils contain decayed plant material and have different physical properties than silt and clay, often affecting their behavior in terms of drainage as well. Plastic soils refer to soils that can undergo deformation without fracturing, and while they may be fine grained, this term is more specific to their mechanical behavior rather than their drainage characteristics.

Overall, the defining features of fine grained soils lead to their classification as having poor drainage when wet, which is why this option is the most accurate.