Is Small Gravel Good To Mix With Soil For Gardening

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Garden Hits & Myths: Can rocks really aid soil drainage?

When water is poured onto a pile of rocks, it drains through almost instantaneously. It's a science lesson that most of us learned before we could walk. Dropping a bunch of rocks into a plant pot would, therefore, seem to be the perfect solution for keeping our potting soils from becoming too wet and boggy. Unfortunately, the only thing that rocks will increase is the strain on your back.

Nov 04, 2016 • November 4, 2016 • 3 minute read • Join the conversation

Some people believe that adding rocks to the bottom of your plant pots can improve soil drainage, but science suggests otherwise. Photo by John Mahoney /Canwest News Service

'Always put rocks in the bottom of a container to improve soil drainage'

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A bit of science

A professor of soil science first clued me in to the fallacy of using rocks and gravel to improve drainage. I admit that I was skeptical at first, and I had never seen soggy gravel so I asked him why it wouldn't work.

"Soil physics," he said.

Now, I doubt that anyone reading this column really wants to delve into the world of soil physics, but suffice it to say that in the 1800s a very smart fellow named Henry Darcy explained how water moves in soil. He proved that before water will move from a fine-textured soil to a coarse-textured soil, the fine-textured stuff must first become saturated.

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What this means for gardeners is that if you slowly pour water onto a fine-textured potting soil that sits atop a layer of gravel, the potting soil must be saturated before the water begins to drip. Granted, once the water hits the rock, it will quickly drain to the bottom of the pot, but the potting mixture above will still be saturated.

But that's not the end of the drainage story. To make matters worse, the thicker the layer of rocks in a pot and the thinner the layer of fine-textured potting soil sitting above, the worse the drainage. The powerful wicking action of fine potting soils will dominate the downward pull of gravity. As a result, gardeners are often left with some pretty soggy soil.

In our greenhouses, I remember a particularly fine-textured seedling mixture that drove me crazy. It held water wonderfully, but the soil surface would always develop a thick, green layer of algae that proved to be a nearly impenetrable barrier for some of my precious seedlings.

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What should you do?

If you are concerned about soil drainage, forget about adding rocks to your container. The best strategy to improve drainage is twofold. First, choose potting soils that consist of coarse-fibred peat moss combined with a high percentage of perlite. Perlite is that white, lightweight rock common in high-quality potting soils. Because perlite is blended into the mixture, it will increase drainage. High-quality, well-drained potting soils for interior use are about 70 percent coarse-fibred peat moss and 30 percent perlite.

Second, if you've had drainage problems choose tall pots over short pots where you can. When the height of your potting soil is tall, gravity wins the battle against wicking. However, when the height of a container is short — like my algae-laden seedling flats — wicking dominates gravity. Tall soils will still have a layer of saturated soil at their bases, but the vast majority of the mix will have a good balance of water and air, which is critical for healthy roots and healthy plants overall.

Rocks aren't all bad. Rocks added to the bottom of large pots will reduce the volume of potting soil required to fill a container. So, while they won't increase drainage they will help to save a few bucks on soil. Of course, the money saved on soil might be much less than the money spent on physiotherapy.

Jim Hole is the owner of Hole's Greenouses in St. Albert and a certified professional horticulturist with the American Society for Horticultural Science.

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