Although it may seem like a good idea to utilize expanded clay pebbles to get better drainage by using them in the bottom of any potting soil mixture, the issue with it is that it simply doesn't work that way. Unfortunately the characteristics of potting soil is such that when you alter the texture suddenly, from a potting mix to expanded clay, after watering, you are creating the situation known as the perched water table.
When you irrigate, you introduce water into the top of the pot and it infiltrates down through the potting mix and out through the bottom of the pot. After you stop irrigating, the gravitational pull will continue to pull water down through the bottom of the pot until the point where the ability of the soil to retain the moisture actually overcomes the ability of the gravity to remove it. This leaves a saturated area at the bottom of the pot known as the perched water table.
By using the expanded clay pebbles, you are actually decreasing the depth of the soil within the pot. Essentially, this means that a perched water table will still happen above the clay pebbles after watering, the same as it would at the bottom of the pot.
By putting expanded clay pebbles in the bottom of the pot, you are making a false bottom for the soil; and this occurs in all types of pots and soil mixtures.
There are a few factors which govern the actual depth of the perched water table but the tightness or the openness of the medium is the most important factor. This soaked area can take up a large portion of the soil. The effect of introducing of the expanded clay pebbles is to simply reduce the amount of soil that is available for the plant to use. In any given medium, the perched water table is the same for that medium regardless of the plant size.
For instance, we take coco coir in a pot and the perched water table is one inch deep. If we would put that coco coir into a large, tall pot, then the proportion of the one inch that is saturated to the proportion of rest of the potting mix is very small. If you take the same volume of potting mix and put it into a very shallow pot which is, say 2 inches deep, and that 2 inches is the total volume of the soil, and of that 2 inches, 1 inch is still the perched water table, and that 1 inch is now 50% of the volume of the pot.
If the soil is saturated, it will prevent the roots from getting oxygen; and this will eventually kill the roots and the plant itself.
There is only one reason that you would use a thin layer of clay pebbles in the base of a pot and that is to prevent the potting mix from washing out of the drainage holes when you irrigate.
When you irrigate, you introduce water into the top of the pot and it infiltrates down through the potting mix and out through the bottom of the pot. After you stop irrigating, the gravitational pull will continue to pull water down through the bottom of the pot until the point where the ability of the soil to retain the moisture actually overcomes the ability of the gravity to remove it. This leaves a saturated area at the bottom of the pot known as the perched water table.
By using the expanded clay pebbles, you are actually decreasing the depth of the soil within the pot. Essentially, this means that a perched water table will still happen above the clay pebbles after watering, the same as it would at the bottom of the pot.
By putting expanded clay pebbles in the bottom of the pot, you are making a false bottom for the soil; and this occurs in all types of pots and soil mixtures.
There are a few factors which govern the actual depth of the perched water table but the tightness or the openness of the medium is the most important factor. This soaked area can take up a large portion of the soil. The effect of introducing of the expanded clay pebbles is to simply reduce the amount of soil that is available for the plant to use. In any given medium, the perched water table is the same for that medium regardless of the plant size.
For instance, we take coco coir in a pot and the perched water table is one inch deep. If we would put that coco coir into a large, tall pot, then the proportion of the one inch that is saturated to the proportion of rest of the potting mix is very small. If you take the same volume of potting mix and put it into a very shallow pot which is, say 2 inches deep, and that 2 inches is the total volume of the soil, and of that 2 inches, 1 inch is still the perched water table, and that 1 inch is now 50% of the volume of the pot.
If the soil is saturated, it will prevent the roots from getting oxygen; and this will eventually kill the roots and the plant itself.
There is only one reason that you would use a thin layer of clay pebbles in the base of a pot and that is to prevent the potting mix from washing out of the drainage holes when you irrigate.
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