These three factors and permeability are closely related. A compacted, hard potting mix will usually have poor aeration, present a physical barrier to root penetration and is subject to
waterlogging. Although water holding capacity can be quite high, permeability into the mix is usually impeded should the mix dry.
![](../../../../images/CSIRO/gypsum.jpg)
Gypsum (calcium sulphate), on the left, is
added to potting mix to reduce compaction.
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Gio. W. Fichera