A pair of common causes of root issues in container culture are rapid changes in soil particulate size and two or more dissimilar soils in the same container. The use of drainage material at the container bottom raises the level of the perched water table in the container. This is never an advantage, but usually container plantings with well-established root systems tolerate it well as long as the remaining soil doesn't hold enough water to keep roots soggy for time measured in days rather than hours. The most important roots, the fine feeder roots are fragile & when deprived of O2 begin dying within hours. When the plant uses enough water so that O2 again returns to the soil, root regeneration begins. This death & regeneration of roots is taxing to the plant and uses resources the plant might better have spent in branch extension, blooms, foliage, fruit, or storage for future use. A soil that drains well & always holds sufficient air does not see the seesaw death & regeneration that poorly aerated soils do & offers the potential for plants to grow very near their potential genetic vigor. A poorly drained soil or one that remains saturated for long periods is a cultural restriction on genetic growth potential.

Per your hypothesis, a gradual change in particulate size does greatly diminish or eliminate the perched water issue. However, the most effective method of structuring your container soils is to use only one soil from top of container to bottom. If you are concerned the planting might be over-potted, add a wick to drain excess water temporarily. When it's obvious the planting will need water more than once per day, remove the wick. This method allows a maximum volume of soil in the container and gives you the ability to adjust the volume of water the soil will hold at container capacity. (Container capacity is a horticultural term that describes a container soil after a thorough soaking and just as the container stops draining.)

Additionally, those soils that remain saturated for long periods break down much more quickly than those that drain well & are well aerated. As soil ages, particulate structure weakens and causes break-down into increasingly smaller sizes. At this point, the speed of soil breakdown/collapse increases and the lower regions of the container become a bacterial soup. This is why I choose to turn old soil into the garden & start containers with fresh soil at each potting/repotting instead of trying to "revitalize" it. If you add perlite and fresh soil to soup, you have soup with fresh soil and perlite in it. ;o)

You'll find that it's not only the larger size of particulates alone that facilitates water movement through soils but the uniformity in size as well. In talks to groups about soils, I often use this illustration: I fill a jar with marbles, essentially uniform soil particles, and point out the excellent aeration and drainage potential. I then add BB's and shake the container until they settle, illustrating the negative effect that dissimilar size particles has on aeration/drainage. Then, I add play sand, which obviously, totally destroys aeration. This is the same effect that sand or other small particulates, compost, and collapsing soils have on drainage and aeration.

Good luck in your endeavors.

Al

Al, thanks for your detailed reply and great information. Somehow I don't quite get what the problem with dissimilar soils is though. I think I am missing something. I had thought that because the perched water issue was diminished, the plant would be happy. Are you saying its OK but not worth the hassle, its easier to just wick it if things are not drying out once per day?

Scott

Dissimilar soils can take two forms. One is the drainage layer transition, the other is actually mixing two soils. E.g., if you have a plant that is several years old & you decide to pot-up and use a good, fast draining soil, there could be problems. The soil in the root ball could be in an advanced state of collapse & could hold water for long periods while the new, fast draining soil seems to dry quickly. Your natural inclination would be to water when the new soil is dry. This could easily cause root rot because the old soil remains saturated for extended periods. Even if rot isn't an issue, lack of aeration can have a negative effect on root metabolism. At the other end of the spectrum you could see hardening of the old soil in the root mass or it could become hydrophobic to the extent that water penetration doesn't occur. The effect of this is the death of fine rootage due to lack of adequate moisture and or gas exchange.

Drainage layers are ineffective and don't do what they're supposed to do. At best, plants tolerate their use. What they do, is use up valuable space that could be holding properly aerated and moist soil that is effective at holding nutrients. I often hear "I've used packing peanuts for years and the roots grow right into them." Great - why bother with soil then. If peanuts are a good growing medium, forget the soil, just use them - simplifies the whole container growing thing. How many nursery operations or greenhouses use drainage layers? Ever see one that uses packing peanuts?

The actual reason that your idea would reduce the perched water table impact is not because it does anything to improve drainage, it simply reduces the amount of water in the perched water table because you would be using gravel or some other media that takes up some of the space that would normally be occupied in part by water. The perched water table would remain as high as if you did not use the material, but the volume of water in it would be reduced as would the volume of soil available for root colonization.

Siting your container so that it is buried an inch or so into the ground, using a pot-in-pot method of nesting containers (soil in the lower container), or using a wick are much more effective at removing unwanted water from containers. Drainage layers alone will not do it. These methods allow you some adjustability in how much water you allow to remain in the container for plant uptake. Depending on a drainage layer forces you to wait until the plant uses water in the perched water table or it evaporates.

Excluding those adapted to aquatic conditions, plants are never happy with their roots in a saturated soil. They only tolerate that condition to varying degrees. Choose a soil that remains moist, but aerated from container top to bottom & I guarantee your plants will thank you by exhibiting superior vitality. If you can manage the schedule, shoot for soils and/or drainage set-ups that require/allow you water daily. The price you'll pay is more frequent watering and fertilizing requirements, but the benefits are many.

Al