The vast majority of gardeners use and recommend the use of gravel or any other similar material to create a drainage layer in the bottom of the pot. I have to recognize that out of laziness I’ve never used and I have always gone well. But it was what they recommended all over the world, gardeners, experts and professionals. And continues to be recommended since there is quite a controversy about it and there are supporters of a option and its opposite.
Now apparently at the root of a publication of the University of Illinois this myth comes crashing down. As regards the water on it impregnates the substrate and occupies the space that previously did the air and only then trickles over the gravel, but if not, there will come out the same for the drain holes of the pot, these are essential. The thing goes beyond and there are even experts in soil that discourage the use of gravel by being counter-productive. Some think that the layer of gravel can harm plants by overwatering.
The plants need good air circulation in their roots but this is not the brings the gravel but the very composition of the soil. And it is shown that the water does not leave the soil until it is well saturated. That is to say that when the water reaches the layer of gravel is because it has already filled all the empty spaces located between the soil itself.
It is easy to check by yourself this point. We can do it with a sponge and a pitcher of water. One must only place the sponge on the gravel and pour water on the sponge. The water does not drain out on the gravel until the sponge is saturated full of water. And gives like, place the sponge on gravel or any other surface. According to these theories and studies do not fill the pot more than with sutrato current. Add a layer of gravel does no more than steal space to the substrate, which is essential for the plant, and make that the pots are heavy and difficult to move.
For my personal experience, as I said at the beginning, I can assure you that the opinions of the experts are certain. I believe that I have never used gravel or broken pottery to create a drainage layer, at least that I remember, and my plants are still there. I guess it all came from trying to recreate the typical structure of the soil but it is clear that the proportions are not the same not by a long shot. In addition, the substrates that we currently use in landscaping drain very well. It is not the same of course if we used normal earth, with clay, that is compacted and drains poorly. In short, we can forget about the gravel and spare us unnecessary effort.