LETS CREATE POTTERY EMAIL 27 FREEStatements such as, “Placing gravel at the bottom of the container can prevent the free flow of water through potting medium and out the drainage holes.” are also incorrect. I am not suggesting it is a good practice, but it does not always lead to rotten roots. You can clearly see that a claim such as “adding stones to the bottom of a pot will rot your roots” is a myth or at least an exaggeration. Remember that all pots, even ones without stones, have a perched water table, and that not all plants in pots have rotten roots! If you have ever bought a root bound plant you can easily see that it was not harmed by the perched water table. Plants in small pots that dry out quickly grow just fine even with a layer of stones. Plants with small root systems that stay above the perched water table, as in large containers, also don’t have a problem. Nor are plants that like a wet root zone. If roots fill the pot and use the excess water quickly, it is also not a problem. If you water by adding small amounts of water, you never create a wet zone, even above stones, so it is not a problem. Is a Perched Water Table a Problem?Īfter telling readers not to use pebbles because it causes a perched water table, writers go on to warn them about the dangers of the wet zone, but is this really a problem? For these reasons, I think its use is acceptable. However, the term is now well entrenched in gardening and horticulture writings and one could argue that the bottom of the pot is a “solid formation” trapping the water. This is not exactly what happens in a pot, and so the term is being used incorrectly when gardeners use it to talk about the wet layer in pots. In geology, a perched water table is an area in soil were water collects above a solid formation, such has hard clay, or rock. As already mentioned, the saturation zone exists with and without the stones, and in fact the height of this zone is the same in both cases. The understanding of this has led to a new myth adding stones to a pot “decreases drainage” which results in a saturation zone. Once you understand how water moves through the soil, and the effect of these two forces, you quickly see that rocks at the bottom of the pot can’t reduce the saturation zone, so they don’t increase drainage. Stones will not change the characteristics of the pores in soil and so they won’t change the capillary action of soil. To do that, the stones would have to overcome gravity and the capillary forces. Many sources claim that adding stones to the bottom of a pot increases drainage. What Happens When You Add Stones to the Bottom of a Pot? Particle size is one of the parameters that are at play here. Water does not easily move from soil into the rock layer (picture b). The silt loam is the soil, and rocks are the sand. The movement of water in this experiment is no different than water moving in a pot of soil. Water moving between silt and sand, showing the saturation zone At some point the two forces are again in equilibrium no more water runs out and the saturation zone remains. There is no air, which is bad for plant roots.Īs even more water is added, the capillary forces can’t overcome the gravity, and excess water runs out the bottom of the pot. In this zone, all of the pores are filled with water – that is why it is called saturated. The result is a saturated zone at the bottom of the pot. At the same time capillary action pulls it up, preventing it from running out the bottom of the pot. As the amount of water increases, gravity has more of an effect and starts pulling water to the bottom of the pot. Initially water just coats the outside of particles with a film of water. Charged particles, like clay and compost, have a stronger capillary action than non-charged ones (sand and silt). A smaller pore size increases capillary action. Capillary action is not very strong and so water only moves up a fixed amount depending on the type of soil.
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