Final answer:
Clay soil cannot be molded when moist and crumbles easily when dry because of its high proportion of fine particles that hold water tightly in small pores. Sandy soils, with larger pores, drain water more easily and are less cohesive when dry.
Step-by-step explanation:
A soil that cannot be molded when moist and crumbles easily when dry is clay soil. Clay soil is composed of fine particles of silt, which give it its plasticity. When water is added, clay soil becomes cohesive, but when it dries out, it crumbles easily. This is because clay holds water tightly in its small pores. Sandy soils, on the other hand, have larger pores and can drain water more easily, making them less cohesive and more prone to crumbling when dry.