The pores that are filled with water at permanent wilting point. These pores are too small for a plant to use without great difficulty.
The water associated is usually absorbed onto the surfaces of clay molecules. The water held in micropores is important to the activity of microbes creating moist anaerobic conditions. The water can also cause either the oxidation or reduction of molecules in the crystalline structure of the soil minerals. Size <30 μm. μm = micron or micrometre. `