What is the vadose zone?

The vadose zone, also known as the unsaturated zone, refers to the area in the ground above the water table where the soil is not fully saturated with water. This zone contains both air and water, which allows for water utilization by plants, as the roots can absorb moisture from the soil. In this context, the term "air and water mixture" aptly describes the vadose zone, as it represents the transitional area between the dry atmosphere and the saturation level of groundwater below.

In contrast, the other options refer to distinctly different geological or hydrological features, which do not accurately capture the characteristics of the vadose zone. The saturated zone, mentioned in the first option, describes the area below the water table where all pore spaces are filled with water. The bottom of a lake embodies an entirely submerged environment, while a rocky substratum refers to bedrock or solid geological formations that provide a foundation but do not pertain to the features of the vadose zone. Therefore, the definition of the vadose zone as a layer containing an air and water mixture is the most accurate representation among the choices provided.