What is a Perched Water Table?

The term perched water table is being used more and more in gardening circles. What is it and how do you control it?

It’s a phenomena found in pots and containers and can end up rotting plant roots. It should be understood by every gardener.

What Happens When a Pot of Soil is Watered?

There are two forces acting on the water. Gravity tries to pull it down through the soil and out the bottom. At the same time a second force called capillary action pulls water back up the pot.

If you have ever watered a pot by setting it in a tray of water you have witnessed capillary action as it draws water up the pot right to the top. Is top watering or bottom watering better?

At the top of the pot, gravity is stronger than capillary action and in this region of the pot the soil particles get wet but the spaces between the particles, the pore spaces, are drained of water. However near the bottom of the pot both forces are of equal strength resulting in pore spaces that are full of water. This region is called the perched water table or saturation zone. It is fully saturated with water.

The Amount of Water is Important

If you only add a small amount of water there will not be enough to create a perched water table.

As you add more water you reach a point where nothing has drained out the bottom of the pot and the perched water table is fully formed.

As you add more water there is no space left in the pot and the excess runs out the bottom. Even though water is dripping out the bottom, the perched water table remains in the pot.

Still Not Convinced?

If you are not convinced that a perched water table exists, have a look at this video where I show you a way to demonstrate it with your own pots.

The Size of The Perched Water Table

How high is the perched water table and what determines its height?

The height is determined by soil properties including particle size and charge characteristics. Small particles have small pore spaces making it easier for capillary action to move water higher, resulting in a higher saturation zone at the bottom of the pot.

Soil particles also have an electrical charge which attracts water. You might know this as the EC properties of soil. Organic matter, compost, clay, peat moss and coir all have higher levels of charge than sand and silt. A higher charge increases capillary action resulting in a higher perched water table.

The size and shape of the pot does not affect the height of the perched water table.

The above diagram shows the perched water table (in blue) for 4 different pots. The left one is a standard pot. The second one is tall and skinny and has the exact same saturation zone height as the first one. A wide pot also has the same height. The pot on the right is small but even it has the same height.

A gardener can reduce the relative size of the perched water table by using taller pots. Small pots makes the problem worse.

There are a couple of key points to understand.

• The height of the saturation zone depends on the characteristics of the soil.
• The saturation zone exists in the bottom of all pots, provided enough water has been added.
• The size of pot (width or height) does not affect the height of the saturation zone.

What Happens When You Add Gravel to the Bottom of a Pot?

Many sources claim that adding gravel to the bottom of a pot increases drainage. To do that, the gravel would have to change the soil characteristics, but it doesn’t do that when layered below the soil.

Once you understand how water moves through the soil, and the effect of these two forces, you quickly realize that rocks at the bottom of the pot can’t reduce the saturation zone. Therefore they can’t increase drainage.

A bottom layer of stones moves the perched water table higher in the pot, but its height remains the same. This results in less unsaturated soil for plant roots. Adding gravel actually makes the situation worse for plants.

Is a Perched Water Table a Problem?

It occurs in all pots and containers and plants grow well in most of them. Why is that?

If you only add small amounts of water with each watering you never create a wet zone, even above stones, so it is not a problem.

If roots fill the pot and use the excess water quickly, it is also not a problem. Nor are plants that like a wet root zone. Plants with a small root system that stays above the perched water table, as in large containers, also don’t have a problem. Plants in small pots that dry out quickly grow just fine.

Remember that all pots, even ones without stones, have a perched water table and that not all plants in pots have rotten roots! If you have ever bought a root bound plant you can easily see that it was not harmed by the perched water table.

A claim such as “adding stones to the bottom of a pot will rot your roots” is a myth or at least an exaggeration. I am not suggesting it is a good practice, but it does not always lead to rotten roots.

How Do You Change the Size of the Perched Water Table?

The size is a function of particle size and charge characteristics. If you increase the particle size or decrease charge, drainage increases and the height of the perched water table decreases.

Some potting media has almost no perched water table. For example, orchids are potted in large chunks of bark or coconut husk. The pore size of these is so large that the capillary action is almost zero, so gravity pulls all the water out the bottom of the pot.

Cactus soil contains lots of sand and stones, which are relatively large and have a low charge resulting in a very small perched water table.

You can decrease the perched water table in any soil by increasing the average particle size. Adding sand, perlite or vermiculite to most potting soil will accomplish this. The key is to mix it in with the soil, not to lay it at the bottom of the pot.