We often hear the recommendation to “flush the pot” periodically in order to remove mineral buildup in potting media that occurs as your fertilizer solution evaporates. We are told to irrigate with fresh water only to accomplish that, so we merely suspend our feeding regimen once a month, every other watering, or however we feel comfortable.
Unfortunately, that may not be particularly effective. In the September 2006 Orchids magazine, Dr. Yin-Tung Wang of Texas A&M University published an article about the retention of mineral salts, and the difficulty in flushing them from diatomite. Let’s look at the mechanics of why that might be:
Start with a “virgin” particle that is saturated with a fertilizer solution – the mineral ion distribution is uniform throughout. As the water evaporates from the surface, in order to stay in solution – the natural drive to stay less concentrated, as becoming a solid is as concentrated as it gets – those ions migrate toward the center of the particle, increasing their concentration in the remaining liquid. Eventually the driving force to stay in solution loses out, and those ions will precipitate as the mineral salts in a highly concentrated “core”.
Now you flush the dry pot, watering with clear water only. The particle absorbs from the outside, in, and once the water reaches that “core”, it starts to dissolve that outermost part of the concentrated minerals. Some of those minerals will be moved to the surface for extraction, but unless you continue to flush heavily, evaporation will “push” it right back to the middle. To truly “flush” the medium, long-term soaking is needed, which allows the minerals to migrate into a large volume of liquid.
That scenario is true for both organic- and inorganic media particles, but with organics, if you repot appropriately, you throw the problem away! With inorganic media, because of their long (essentially unlimited) life spans, the tendency is to go longer periods of time without repotting, which exacerbates the problem.
So how do we avoid, or at least minimize the problem, as in traditional culture, it is inevitable?
We know that we must feed our plants adequately in order to get them to grow and bloom well. Infrequent watering and the use of concentrated fertilizer solutions favor buildup, and as we can see, clear-water flushing is not particularly effective in reversing it. If, on the other hand, we tend to water more frequently and use lower levels of fertilizer additions when we do, we reduce the evaporative “driving force” and keep the mineral concentration low to begin with.
None of that applies to semi-hydro culture, by the way, as you’re essentially keeping the medium mass at a constant level of moisture and mineral content, slowing the rate of buildup. Note that I did not say “preventing” buildup, as all media components will accumulate minerals and plant wastes over time. There is a sister article on this subject as it relates to inorganic media components.
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