Mid-Atlantic Gardening: Soils, Soil Mixes, and Drainage Issuesby Tapla (Al)., 1 by tapla

tapla wrote:
When I approach the question from the perspective of what is best for the plant, I can say that soils like regular Miracle-Gro, comprised of primarily small particles (peat, compost, coir, composted forest products ....), already hold too much water to be the best choice for our plants. Adding additional water retention to soils that already hold more water than desirable (for best plant performance) only amplifies the issue. Having to water more frequently is an inconvenience to the grower, but a benefaction for the plant.

Drainage layers don't work unless the size of the particles in the drainage layer are less than about 2.1X the size of the particles in the soil that sits atop the drainage layer. I'll offer some illustrations to explain. If a soil supports perched water, that is a layer of saturated soil at the bottom of the pot from which the water won't drain by the force of gravity alone, the height of the perched water table (PWT) will be the same in any pot - no matter what it's size or shape. Fresh Miracle-Gro soil usually supports from 4-6" of perched water, so let's say it supports 4" for the illustration/. In a pot that is 4" deep, after a thorough watering, the soil will be 100% saturated from bottom to top. In a 12" deep container, the PWT height will still be 4" tall, so about 67% of the soil will be free from perched water, so that set-up would be easier to grow in, though far from ideal. Now, let's imagine a 4" layer of peastone on the pots bottom as a "drainage layer". What will happen is this: There will be 4" of well-aerated peastone at the bottom of the pot, then 4" of soil that contains perched water, then 4" of soil with no perched water. Instead of the layer of soggy soil being at the bottom of the pot, the excess water "perches", just like a bird, in the soil immediately above the drainage layer, so in effect, you've simply reduced the volume of soil in the pot that provides a healthy root environment.

Best is to use a coarse mixture that supports little or no perched water and forget the drainage layer. If you think you have more water retention than you should have, use a wick or other technique that will reduce water retention immediately after watering. Even tipping the pot at a 45* angle after you water can eliminate a LOT of excess water - see the illustration below to see how pot shape impacts how much perched water a planting can hold.

Al