I was reading another blog today, and came across the advice to put gravel at the bottom of a planting hole to increase drainage. It makes sense–gravel has better drainage then clay, so put it at the bottom of a hole and its gotta help.

Well several soil classes, and a job at the extension service have taught me differently. It’s doesn’t help at all: it makes it worse. I’m now going to explain the science behind it. If you want to just trust me you can stop reading.

Soil texture refers to the size of soil particles. Clay particles are small, sand particles are large. Usually, across a soil profile there is little difference, or a very gradual difference in soil texture.

Now move into an urban soil. We disrupt it so much, many times there are drastic changes in the soil texture. That drastic change is made when we put gravel at the bottom of a planting hole. When there is a change in soil texture like that, and you procede to put water on it, an interesting thing happens. Say you have a clay soil, and then a layer of gravel. Instead of the water draining into the gravel, it collects in the layer of soil until all the pore space is filled with water. The water collects in the clay, not drains in the gravel. Once it does spill over into the gravel layer, the gravel layer also will collect water until all the pore space is filled with water before spilling out to the next layer of soil.

Confusing?

To get a better idea, imagine a sponge next to anything that isn’t a sponge. You put water on it, and it doesn’t start draining until it is absolutely full. Same concept with soil: the gravel acts as a sponge.

There are ways to get better drainage, and gravel is usually involved. The key is to allow drainage from the gravel layer, usually by a piper or other method. This site has some good illustrations.