In the age before plastic pots all flower pots were made of terracotta which is fragile and breaks easily. The pieces of the broken pots were placed in the bottom of flower pots to prevent soil washing through the drain hole and also to aid drainage. Today gravel is used instead of broken pieces of pot, but it seems that this is not such a good idea after all. This article which I found on the Balcony Garden Web site brands this as an old myth and provides evidence to show why we should not add gravel to our pots.

After reading this post, you?ll never put gravel or other coarse materials at the bottom of pots.?Must find out, WHY?

Every time you prepare a pot for planting you add a layer of gravels in the bottom for drainage! But, is it really necessary or not required at all?

Adding a layer of gravel, stones or pot shards in the bottom of the?container is a common practice that most of the gardeners (old or new or even experts) do. But do you really need to do this? We say NO!

We have grown plants in containers successfully without adding gravels to pots, and there seems no problem with it. Our plants have done really fine and never suffered from root rot or drainage problem. Let?s find out why it is NOT NECESSARY TO ADD GRAVELS or other coarse materials IN POTS!

WE CALL IT ABSOLUTELY A MYTH THAT IF YOU DON?T ADD GRAVELS IN THE BOTTOM OF THE POT THERE WILL BE A DRAINAGE PROBLEM AND YOUR PLANTS WILL DIE DUE TO WATERLOGGING; ALBEIT, THE OPPOSITE IS TRUE!

To support our claim, we refer to this educative article on Illinois University, according to them ?It is a myth that a layer of gravel (inside the bottom of an individual pot) beneath the soil improves container drainage. Instead of extra water draining immediately into the gravel, the water ?perches? or gathers in the soil just above the gravel. The water gathers until no air space is left. Once all the available soil air space fills up, then excess water drains into the gravel below. So gravel in the bottom does little to keep soil above it from being saturated by overwatering.?

Gravel Improves Drainage?

Adding a drainage layer looks like an important custom, and it seems a plausible chore. Many gardening experts, even some gardening show hosts, and experienced gardeners suggest this, BUT the soil SCIENTISTS have proved that adding a gravel or other coarse material instead of improving drainage impairs it. The soil scientist Kevin Handreck, the author of Gardening Down-Under and Good Gardens with Less Water, even believe that crocking increases the risk of damaging your plants by overwatering.
The Associate Professor Linda Chalker-Scott, an urban horticulturist at Washington State University, calls it a myth that refuses to die. In her report, she also questions gardening websites and books???Regardless of solid scientific evidence to the contrary! Nearly every book or website on container gardening recommends placing coarse material at the bottom of containers for drainage.?
Read her article on Sustainable Gardening here!

A Layer of Gravel Improves Air Circulation?

We know that plants need good drainage?so that their roots can receive adequate oxygen, and we also know that water passes through a?coarsely textured material faster than it does in fine material. But what we miss here is that water does not move easily from layers of finer textured materials to layers of more coarse-textured materials, which means instead of passing freely and easily the water sits between the soil and drainage layer and doesn?t start to drip until the soil is saturated completely. ABC SCIENCE also tried to debunk this myth in its article.

See more at Balcony Garden Web