Aging of Container Soils

by Brent Walston

Introduction

There is a changing dynamic relationship between an individual plant and its soil in a container. We realize that plants grow and change, but we don't consider that soils 'age' as well. In container growing, it is important to match the life expectancy of the soil to the frequency of repotting. This insures that your bonsai and other container plants won't be forced to struggle in a 'collapsed' soil mix. As soils age, they tend to break down, reducing the particle size and retaining more water. This process can actually match the growth rate and water needs of the plant, if carefully balanced.

In the Beginning...

Newly repotted plants don't take up as much water (in general) as leafy established plants. This is because the root system is compromised when repotting is accompanied by root pruning and combing out of old soil. We take this into account by reducing transpiration by reducing the foliage, or repotting while the plant is dormant.
In establishing a new root network, aeration and fertility appear to be the prime factors, not water holding capacity. Of course there must be enough capacity to allow for transpiration, but the rate of transpiration is much less at the time of transplant (and shortly afterward) than after root establishment and consequent new growth.

But Then...

As the roots grow and new shoots and foliage develop, transpiration increases. This can be dealt with by pruning to reduce transpiration, increasing the watering, reducing sunlight, etc. But one of the really nice things about having an organic component in the soil is that it begins to break down about the time the plant is demanding more water. This effectively increases the water holding capacity. A well designed soil and proper plant maintenance will help keep the moisture content in balance.
Once soil is thoroughly root colonized it is not subject to collapse for most species unless the plant is subjected to poor treatment such as massive overwatering, not enough direct light, etc. The root network will tend to keep the soil aerated by creating a woody framework. In fact, fine particles will often be washed out of a healthy root network.

Peat Moss and Nursery Mixes

Peat moss, when used in reasonable quantities of less than ten percent, does not add sufficient small particle volume to affect either drainage or aeration. That is what I really like about it. It is so efficient at retaining water without using up space, that it makes an ideal amendment for this purpose. Its lightweight fibrous nature also keeps it in position in the soil mix, rather than washing it quickly to the bottom. The old "UC" mix developed by the University of CA (Davis, I believe) was 50% peat moss and 50% fine sand. This of course was a nursery container mix, not for bonsai. It was used for years until the cost of peat moss forced them to change it.

Soil 'Shelf Life'

What you must understand is that the UC mix and all the other nursery container mixes are designed for quick growth and short 'shelf life'. Ideally, typical nursery plants don't stay in the same pot for more than one or two years (five gallon and under sizes). Any longer than that usually results in soil collapse or root bound conditions. Bonsai mixes must last longer, and they must be more flexible and stable to account for pruning and training. Except for training pots, we don't allow full flat out growth. This means that we have to pay a lot closer attention to soil characteristics than general nurseries do.
I try to make my soil mixes last as long as possible, even those in training pots. I use fresh bark and stable inorganic amendments (lava rock and perlite). This soil will last many years before the bark completely breaks down. Usually by that time the roots are in need of pruning and attention anyhow. That is how a soil should be designed, to last as long as the plant needs to stay potted.

All Inorganic?

Using only stable inorganic components such as lava rock or pumice will create a soil that will last longer than it really needs to last. Using only unstable inorganic amendments such as clay baked to much less than vitrification, akadama, etc. creates a soil that may not last as long as it needs to for some plants, although it is usually fine for two or three years. I have used pure fir bark, and it worked beautifully for about four years, but now I am repotting those plants because it is now quickly breaking down. A combination of stable inorganic and fresh organic amendment (fir bark), works fine for me. I get the right breakdown curve for repotting practices, higher CEC (cation exchange capacity), good aeration and drainage.

And finally

I think, and this is just my opinion, that you will simply get a different set of problems no matter what you use for soil. If you use purely inorganic components, you will probably have to use organic fertilizers and their attendant problems such as removing the surface residue, smell, insects, etc. If you use more than an optimum amount of organic components then you will get problems related to water and aeration, quick breakdown and collapse which you will have to overcome with closer attention to watering practices. There is no 'best' soil, there are only soils that work well in a set of environmental conditions that include the species of plant, how it is manipulated, who is doing the manipulations, watering, the climate, fertilizer type and practice, light/shade. All these things are interrelated.