In fact, if you’re planting in clay soil in the landscape, adding gravel to the bottom of the pot or hole actually worsens the situation. Water moves through the soil in large and small pores. Large ones, such as those made by worms, can move water quickly, and they allow air to move in the soil, too.
Small pores can be microscopic in size, and they allow water to be retained in the soil for plant roots to use. Water has surface tension that connects the molecules together. When water in a pore gets to a larger pore, it doesn’t automatically move into the larger pore. It must build up enough pressure behind it to overcome the surface tension and move into the open space below.
Imagine a large, flat sponge. It has lots of large and small pores just like soil. Submerge it in water to get all the air out of the pores. Take it out of the water, and you will see water flow out of the pores. But once it stops, you can see water hanging from the bottom of the sponge. There is not enough water pressure above it in the sponge to move the water into the open space below. The top of the sponge will be damp, but the bottom of the sponge will remain saturated and waterlogged.
The same thing happens to the soil above the gravel in the pot or hole because the water pressure won’t be strong enough to force the water into the large pores below. The soil above the gravel will be waterlogged and can drown roots.
If the soil is similar in texture, it will allow the water to wick its way through. If the water can move out, then air can move in. Roots need both air and water for healthy growth. If you want to improve clay soil, use organic matter and mix it into the clay.
If you’re just planting some annuals in the half-barrel, they don’t need the whole barrel full of soil. Filling the bottom half of the barrel with Styrofoam peanuts or other lightweight filler can make the barrel easier to move and still give the plants enough soil to grow in. Use a weed-barrier cloth between the soil and the peanuts or put them in plastic bags so they don’t become clogged with dirt.
Use lightweight potting soil that has lots of organic matter in the half-barrel. It will retain moisture in the small pores but still allow for drainage. Plants in pots have less room for roots to grow, so a lack of water is usually the most critical factor for plant health.
