Water holding capacity is greatly influenced by the size of the growing medium particles. Coarse particles create macropores which account for drainage and air porosity, while fine particles create micropores which retain water. A mixture of coarse and fine particles can be prejudicial for a substrate because the macropores will be filled by the fine particles, which reduces the air porosity of the growing medium.
A substrate with fine components is mainly used as a seed germination medium, and depending on the grower’s preferences, it can also be used to root cuttings. Therefore, this type of substrate is designed to easily fill small cells and to hold lots of water. Depending on the season of the year and the grower’s preference, a seed germination substrate may contain perlite or vermiculite. Usually, a germination substrate with perlite, such as PRO-MIX FPX, is more widely used during winter due to its lower water retention capacity, whereas a germination substrate with vermiculite, such as PRO-MIX PGX, is used during the summer since it has a higher water retention capacity. However, both PRO-MIX FPX and PRO-MIX PGX are designed to hold sufficient water and provide good aeration between watering cycles when used properly.