Final answer:
Clay soil, characterized by its very smooth and sticky texture, contains fine particles and retains water well due to its small pore spaces.
Step-by-step explanation:
A soil that feels very smooth but sticky when touched is characteristic of clay soil. Clay particles are much smaller than sand and silt, often giving the soil a smooth texture. The presence of water adds to this soil's plasticity and cohesiveness, making it feel sticky. Due to the fine particles and small pore spaces in clay, it also has a significant ability to retain water, which contributes to its sticky nature when wet. Feldspar, silica, and alkalis like iron in the silt provide the distinct plasticity and reddish-brown color to the clay soil.