Which of the following factors does NOT affect radon's movement through soil?

Radon's movement through soil is primarily influenced by factors that control how easily the gas can travel through the soil medium. Soil moisture, size of mineral grains, and permeability all play significant roles in determining the gas's mobility.

Soil moisture can impact the movement of radon because water in the soil can fill pore spaces, thus affecting the routes through which radon can migrate. The size of mineral grains affects the porosity and the spaces available for radon to travel; smaller grains can create a denser structure that might restrict movement compared to coarser materials. Permeability refers to how easily fluids and gases can move through the soil; higher permeability generally allows for more effective radon movement.

On the other hand, temperature does have an indirect effect on radon movement yet is not a primary factor affecting its migration through the soil. While temperature can influence gas diffusion rates, it does not fundamentally alter soil structure or its capacity to facilitate radon transport in the same way that moisture, grain size, and soil permeability do. Therefore, temperature is the least direct factor among the choices provided with respect to radon’s movement through soil.