Smaller pore space reduces media drainage as water is held more tightly in the smaller pores. As the root system of the plant grows, large pore spaces become filled, decreasing media aeration porosity. Consider these factors when selecting media and monitor the media to detect any undesirable changes. Changes in media components Media particles undergo many changes that affect porosity. Uncomposted or improperly composted media particles such as sawdust or bark shrink as the particles deteriorate.
Peat moss expands and contracts when it is subject to wetting and drying cycles.
Although vermiculite compresses and breaks easily during mixing, bark and perlite withstand compression to a greater degree. The asymmetrical shape of perlite allows for large pore sizes in a mix. Water held inside pine bark can be absorbed by plants only if roots penetrate individual particles. However, if root penetration does not occur, the internal water is not available for absorption. Sphagnum peat moss and vermiculite also have high internal porosities.