Some thoughts on the ''drainage'' layer.

[MikeInOz]

It has often been recommended - especially in the older books - to add a layer of coarse material in the bottom of pots in the mistaken belief that this will improve drainage of water from the pot. Drainage is basically the movement of water due to gravity. In reality, a coarse layer does nothing to remove excess water from a pot although in some cases it may facilitate the movement of water out of the drainage holes by stopping finer material clogging them.

But let's look a little deeper...
Roots need oxygen to function. By far, the most important way roots get oxygen is by diffusion - not as is commonly thought, by water dragging in air as it moves though the soil. This is why cacti roots function for months or even years without rain even though they may be growing nothing but dust. As roots take up oxygen, it is replaced by more diffusing through the soil via the millions of pores within it.

After drainage, water is help in the medium by surface tension but gravity will tend to concentrate the water in the lower part of the pot. This is known as the saturation layer and will remain there unless it is in contact with a surface which has finer pore spaces than the mix itself. Usually, the pot sits on the bench and surface tension is not interrupted so this saturation layer remains until the water is slowly removed by the plant and evaporation. Naturally, this saturated layer contains less oxygen than the mix directly above it because it's pores are filled with water (more or less)

The height of the saturated medium remains the same regardless of the height of the pot.
This does not happen in the field. In natural soils, the surface tension is not interrupted and water continues to drain away to lower levels. There is no saturated layer unless the soil is sitting on some impervious material.
This is what modern commenters on this subject use as evidence of the course layer being undesirable. (That is that the saturated layer remains above the drainage layer and reduces removal of excess water. This is of course wrong. The amount of water held is higher in a mix which does not contain a coarse layer because there is lower amount of larger pore (air) spaces. The actual saturated layer is simply moved from above the coarse layer to the bottom of the pot. It is not removed and the drainage of water does not change.
Does all this actually matter? Yes and no. If water is managed well, the amount available to the plant can be well controlled as long as it is remembered just how much water can be held at the bottom of the pot even when the surface be seem quite dry. Some Cacti may even benefit from the saturation layer. Those from higher rainfall areas particularly.
An observation....
Last year I sowed a large number of cactus seeds of a wide variety of species. With some pots, I included a layer of coarse perlite (about 4-5mm) about 10mm deep at the bottom of the pot followed by a very thin layer of fine perlite (2mm) above it followed by the seed raising mix (more on this later).
With some pots just straight mix was added before sowing the seed. Over a period a 12 months some most of the seedling grew well regardless of the course layer. Some however stopped growing strongly after a while, and while they remained healthy I noticed their roots had stopped growing and their development was much reduced.
These seedling were invariably in the pots without the coarse layer of perlite.
Obviously, some of the species did not appreciate the lower amount of oxygen available to them in the unaltered mix.
So what does the coarse layer REALLY do? It allows more efficient diffusion of oxygen to the lower part of the roots. Instead of the roots finding less air as they go down, they find more. This is why I will from now on name this layer the AERATION layer and dispense with the misleading name ''drainage'' layer.
How does it work? If you consider using this layer in your pots of plants which you believe may enjoy faster drying or at least more available oxygen in the depths of the pot, I recommend the following.
Use a material which is porous and able to absorb water. This will allow water to be drawn down as it is used by the roots and evaporation through the drainage holes. Solid materials like stones will not do this.
Because these particles of coarse porous material continue to draw water from the mix as it is removed from them, they will create a highly aerated yet humid environment for the roots.
I use perlite because it is readily available here but no doubt any similar inert and porous material will do just as well.
It is important that the particle size is correct. Too large and the environment will be too airy and defeat the purpose. To fine and you defeat the purpose again. I have found 4 to 6 mm particles just right.
It is also important that the growing mix does not infiltrate the coarse layer thereby rendering it useless. We can do this by using a finer grade of the same material to cover the course layer and so stop finer particles from the growing mix finding their way into it.
I hope this at least creates some interest among those of us who like to tinker.

[One Windowsill]

Seems reasonable to me.

[Steve Johnson]

IMHO, there are only two reasons for tinkering:

• Cacti have a lot of trouble establishing, which may or may not be determined by the first full growing season under the grower's care.

• The cacti establish, grow well and flower, then eventually stall out.

Based on personal experience and conversations with expert growers over the years, it takes 5 years to know if one's growing practices are sound or not. If those practices hold up after 5 years, then as the old saying goes -- "if it ain't broke, don't fix it". No need for tinkering on my end, but for people having trouble with their cacti, Mike's concept is worth serious consideration.

Edit: Guess I did do a bit of tinkering when I started supplementing my Dyna Gro 7-7-7 with Potassium sulfate. Mike, the preliminary results are better than I could've expected. The growing season here in L.A. is just about over, and I can't wait for the next one!