Using Clay Pebbles For Hydroponics

Some growers use clay pebbles along with soil-less potting mix. But while it seems like an obvious choice to put porous materials like clay pebbles at the bottom of the pot in order for the water to drain better. But while it seems that it is a “no-brainer” option, it doesn’t work that way because of the nature of the potting mixes themselves.

Whether they are placed in pots  or in the ground, one thing soils and potting mixes have in common is that when the texture or character of the soil abruptly changes (such as from potting soil to expanded clay) it leads to a situation known as a perched water table. This normally occurs at the bottom of the pot or container, but if there is clay at the bottom you are basically creating a false bottom. The soil that rests on top of the clay pebbles will remain saturated to a specific height above the layer of clay pebbles after irrigation in the same way as it would if it were the bottom of the pot.

Once you irrigate a pot filled with clay pebbles and potting mix, you have to fully saturate the soil before the excess water can flow to the soil and into the pebbles themselves. And once you do that, the nutrient solution has filled in all the pores in the soil or potting mix that was once filled with air. And depending on the type of soil or mix you use, the saturated area can take up a significant chunk of the soil/mix. Thus, the actual effect of adding clay pebbles is to minimize the amount of the growing medium that you increase the proportion of saturated to unsaturated soil.

In any kind of potting media, the perched water table will have a depth that is similar for that medium. A saturated layer at the bottom that is one inch deep, for example, in a tall narrow pot will only occupy a small fraction of the total soil mass. But if you place the same amount of the same kind of medium in a wide, shallow dish that is only two inches deep, it will take up half the medium.

Of course, there are many technical details that influence the performance of water in pots, but overall the only reason to place a very shallow layer of pebbles on the bottom of a pot is to prevent the soil or potting mix from washing out from the drainage holes whenever you irrigate (and this is no longer even necessary in some cases).

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