What does porosity measure?

• A. The pH level
• B. The amount of nutrients
• C. The volume of open spaces in rock or soil
• D. The water holding capacity

Answer: (C)

Porosity is the fraction of a rock or soil’s volume that consists of empty spaces, or pores. Those pores are the open spaces that can be filled with air or water, so porosity tells you how much fluid storage capacity the material has, usually expressed as a percentage. It’s about how much space exists, not how easily fluids move through it—that latter idea is about permeability.

This differs from pH, which measures how acidic or basic a solution is, and from the amount of nutrients, which concerns the chemical content available for organisms. It also differs from water holding capacity, which describes how much water a soil can retain after drainage due to capillary forces and pore-size distribution; porosity provides the potential space for that water, but doesn’t by itself determine how much water will be held. In short, porosity answers: how much empty space is there for fluids in the material?