Water Movement in Potting Mixes

By Sharon Rosenzweig

This discussion is geared mainly for growers who do not wick water their AVs or who use self-watering planters.

Water’s Natural Forces

Water is essential to growing plants. There are two forces of nature that cause water movement through the potting mix – gravity and capillary action.

Capillary action is a function of adhesion and cohesion. Adhesion is water’s tendency to stick to the soil particles. Cohesion is the tendency for water to stick to itself.

Every plant pot has a “perched water table” (PWT) which is water that occupies a layer of soil that is always saturated and will not drain through the hole in the bottom of the pot. The capillary pull of the soil equals the force of gravity and the water will not drain, it is “perched”. This is the area of the pot where roots seldom penetrate and where root problems begin due to a lack of aeration.

Various potting mixes have different size soil particles. Large soil particles have less surface area and less overall adhesive action. Gravity is stronger and the mix has better drainage. The PWT is lower in coarse soils than fine soils. Large particles mixed with small particles will not improve drainage because smaller particles fit between the large, increasing surface area which increases capillary action and water holding potential. The key to good drainage is size and uniformity of soil particles.

Myths Dispelled

The following information is contrary to what we have always been taught about potting African violets.

Using a coarse drainage layer under the potting mix does not improve drainage. It moves the level of the PWT higher in the pot. This reduces available soil for roots, reduces total usable pot space and limits potential for beneficial gas exchange. It is better to use a mix with uniform particle size throughout the pot.

Taller containers provide better drainage than squat ones. The PWT will be the same in both size containers, but the taller one provides more usable air holding soil above the PWT.

 

Using Wicks

The use of wicks in pots will help to eliminate the PWT. The wick must extend into the PWT, so have the wick running from the top of the potting mix to an inch or so out of the drainage hole. The wick makes the water think the pot is deeper, so water begins to move downward seeking the new bottom of the pot, pulling the rest of the PWT with it.

Experiment

Fill a pot with potting mix to the top. Add enough water to fill to the top so soil is saturated. Allow water to drain out the bottom hole(s). When drainage stops, insert a coffee stirrer or narrow straw several inches up into the drain hole. Observe how much additional water drains out. This is the water that occupied the PWT.