Understanding the perched water table really helped give me a better understanding of what it means to take care of plants.
Most containers will form a perched water table at the bottom, but they also occur when there is a sudden change in soil consistency, such as from clay to sand, or soil to gravel.
A layer of gravel in your container creates a higher perched water table that is more in the root zone of your plants, with adverse effects on plant growth and root formation. The gravel raises the perched water table in the bottoms of pots. Gravity can't quite pull all of the water from the bottom of the soil so it stays wetter than the rest of the pot and anaerobic.
The gravel takes up some space at the bottom of the pot so the transition zone is higher up in the pot where the soil and gravel meet, and potentially closer to the roots.
As you can see the perched water table occurs because of the bottom of the container, not because of the ground. As long as it's deep enough for your roots, you should be fine! And some species do prefer wetter soil too.
The reason that it is called the perched water table is due to soil retention.
I've literally never seen proper container soil sold anywhere, you need to make it yourself. Tapla Soil Retention is what forever changed how I go about preparing the soil for my pots. How? Now I make proper soil.
I idealy I try and mix my soil like this (based on Al's recip from Garden Web):
- 5 parts pine bark fines, dust – 3/8 (size is important)
- 1 part sphagnum peat (not reed or sedge peat please)
- 1-2 parts perlite (coarse, if you can get it)
- garden lime (or gypsum in some cases)
- controlled release fertilizer (if preferred)
- 3 gallons pine bark
- 1/2 gallon peat
- 1/2 gallon perlite
- 4 tbsp lime (or gypsum in some cases)
- 1/4 cup CRF (if preferred)
Any peat based potting media should be replaced yearly, 5-1-1 every two years or so, but the gritty mix will last for ages without losing pore space. All my potted perennials are grown in gritty mix now.