space between soil particles that is relatively small and likely to be water filled (contrast with macropore).

Merriam-Webster Online Dictionary
micropore (noun)
a very fine pore
micropore (Wikipedia)

A microporous material is a material containing pores with diameters less than 2 nm. Examples of microporous materials include zeolites and metal-organic frameworks.

Porous materials are classified into several kinds by their size. The recommendations of a panel convened by the International Union of Pure and Applied Chemistry (IUPAC) are:

Micropores may be defined differently in other contexts. For example, in the context of porous aggregations such as soil, micropores are defined as cavities with sizes less than 30 μm.

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