Tom learned about S/H from me, during a presentation I gave to an orchid society in Virginia, and he has apparently changed what I taught him. Here's why I disagree with that approach, but why some might get away with it, at least for a while:
In a semi-hydro pot, the vast majority of the evaporation occurs in the top portion of the column of medium, in a gradient, of course.
As the water evaporates, dissolved solids in the liquid - fertilizer nutrients, whatever was in the water, and plant wastes - reach their maximum solubility levels and precipitate.
If you water properly, which means frequently flooding the pot to the top with nutrient solution and letting it drain, you flush the pot, mostly redissolve those residues, saturate the medium, and refresh the chemistry of the solution in the reservoir.
If you only top up the reservoir, that evaporation/precipitation process continues unabated, allowing the residues to accumulate to the point of being toxic to the plant.
Now some caveats to that:
If your growing environment has very high humidity, the wicking ability of the LECA outstrips the evaporation rate, so the top stays wet, and precipitation is kept to a minimum. While the "Flush At Every Watering" (FAEW) technique is still preferred, it is far less crucial to the long-term health of the plants.
Likewise, if you sell that plant before the buildup becomes an issue, "it's never a problem" (to you), and if you dump the old LECA when you repot the plant - something I don't bother with because I water correctly - you're dumping the precipitates, too.
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