If most of your experience is in growing terrestrial plants – whether that is houseplants, gardens, or even lawns, trees and shrubs – the concept of “flushing potting media” is foreign. Those plants take their nutrition from the minerals in the soil, with or without the occasional supplement of a granular fertilizer. The grower merely adds water to the process, allowing the minerals to dissolve and become ions in solution – the only way the plants can absorb them. As orchids generally don’t take their nutrition from the medium in which they’re growing, but get it directly from the nutrient solutions we provide, the “game” is changed, so we have to approach things a bit differently.
When we feed our plants, we start by taking fertilizer minerals and dissolving them in water, again so they are ions the plants can absorb. Those minerals are often in a powder or granular form, but if you use a liquid fertilizer, it just means that the manufacturer already did that for you, and you’re just diluting it further, to “usable” strength.
When we pour the dilute solution over and through the potting medium, some of it gets absorbed by the plant immediately. But far more of it is soaked up by the medium, to be available for the plant in the near future – as long as it remains a solution.
Unfortunately, the water evaporates, which results in the concentration of those dissolved minerals increasing over time. Eventually, when their solubility limits are reached, they precipitate out of the solution as solids on the surfaces and in the pores of the potting medium. When you water with fertilizer solution the next time, some, but not all of the minerals will re-dissolve. This means the solution is now stronger than what you intended to apply, and as we don’t really know which minerals re-dissolved the most, we don’t know the formula of that solution either. What’s worse, however, is that as the stronger solution dries, the buildup in the pot gets worse and worse. Repeat this process often enough and the potting medium becomes so mineral laden that it throws off the osmotic balance in the roots, and is actually toxic to the plants.
Flushing periodically with clear water goes a long way to removing the buildup, and most growers find that flushing the pot thoroughly every fourth watering does the trick. It never eliminates all of the buildup, but it does keep it to a minimum. This means it normally isn’t an issue during the normal life of the medium; even better, when you repot, you’re basically starting over with a “clean slate”
One thing to keep in mind is your water quality: if your water supply is high in dissolved solids – calcium and magnesium carbonates being the most common – they will build up in the potting medium as well and you may need to shorten the time period between necessary repotting.
Ray Barkalow has been growing orchids for over 45 years, and owns First Rays, which offers horticultural products to the hobby grower. He may be contacted at firstname.lastname@example.org and you can visit his website at FirstRays.com.