Which phase of soil is not included in its principal phases?

In soil mechanics, the principal phases of soil are typically classified into three main categories: solid, liquid, and gas. The solid phase consists of the mineral particles and organic matter present in the soil. The liquid phase refers to the water content contained within the soil, which can influence various characteristics such as shear strength and permeability. The gas phase pertains to the air present in the voids of the soil, which can also affect the soil's behavior.

The vapor phase, while it can occur under certain environmental conditions, is not typically categorized as a principal phase of soil in standard discussions surrounding soil mechanics. Instead, vapor refers to water in a gaseous state, which is not a distinct phase in the context of soil composition as per the standard classifications. Therefore, the best answer reflects that the vapor phase is not included among the principal, primary phases of soil.