The heavy dark soil associated with floodable valley-bottoms is a

The heavy dark soil associated with floodable valley-bottoms is a hydromorphic soil. Hydromorphic soil is soil that is saturated with water for a significant portion of the year, typically due to flooding or poor drainage. It is also known as waterlogged soil. Hydromorphic soil is characterized by a dark color, high organic matter content, and a heavy, compact texture. These soil characteristics are a result of the lack of oxygen in the soil due to saturation with water, which slows down the decomposition of organic matter and causes the soil particles to bind together more tightly. In the case of floodable valley-bottoms, the soil is periodically inundated with water during floods, leading to the development of hydromorphic soil. Therefore, the correct answer is: "Hydromorphic soil."