Total water holding capacity (TWHC) and permeability

This is the total volume of water held in a saturated potting mixture after it has been allowed to freely drain for about 24 hours. This may be referred to as the potting mix's container capacity. Water held with little force in the larger pores is either readily available to plant roots or evaporates at the surface. This water is replaced by that held with stronger forces in smaller pores and on particles. As the potting mix dries, the water remaining is that held most strongly. A point is reached at which none of the remaining water is available to plants. When no water is available, plants wilt. The point at which plants have sustained irreversible root damage, and beyond which they fail to recover, is referred to as the permanent wilting point. The difference in water volume between TWHC and the permanent wilting point is known as available water. It follows, then, that a potting mix with good aeration will have a larger proportion of large pores, holding more water that is readily accessible for plants, compared with a potting mix with predominantly minute particle size, which may have a greater TWHC but not necessarily more available water.

Coarser material, such as composted
milled pine bark, should be incorporated
into the potting mix.

Permeability of water into open mixes is generally good. Reduced permeability, caused by water-repelling (hydrophobic) fine organic particles or very fine sand grains, can result if mixes are allowed to dry, i.e. have low available water. Incorporation of some coarser material into the medium counteracts this problem.

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