Let’s talk dirt. It is the stuff you walk through outdoors and track indoors. “Take off those dirty shoes!” is a refrain most of our mothers yelled from across the house. Dirt is soil out of place.
Soil is the loose, natural surface outdoors that supports plant growth. It is made up of parent materials: mineral and organic matter. The minerals vary, depending on where you are in the world and come from aged, weathered minerals like granite, limestone, chalk, sandstone, etc. A typical handful of dirt is mostly minerals and a little bit of organic matter, from 3-5%.
Organic matter is anything that is or was alive. Worms, beetles, centipedes, pill bugs are the big organisms. Fungi and bacteria are the microorganisms. These live critters eat and break down the remaining organic matter of leaves, clippings, dead and dying stuff.
The rest of the handful is called pore space. These pores contain air and water. They allow water to drain and roots to breathe. A very dense soil (or concrete for that matter) has very little pore space. Water does not drain and roots will not grow. That leads me to the topic of Bad Soil.
Normal, natural outdoor soil is perfect for supporting plant life…but it makes terrible potting soil. In the endless horizontal setting outdoors it has good drainage. But put it in a pot and the drainage profile dramatically changes. The small soil particles do not drain well. Even very sandy soil, poured into a pot, can hold too much water and have very low oxygen levels, leading to root rot. Dirt in a pot is out of place.
Compost can have a similar effect. It is an excellent soil amendment, outdoors, in the ground or in raised beds. But compost in a pot can continue to decompose and appear loose and airy on top, but dense and mucky at the bottom, again leading to root rot.
Most garden centers offer bagged “soil” under various names: Top Soil, Garden Soil, Raised Bed Mix, Organic Compost, and catchy brand names like Black Kow and BaBaDoo (composted sheep manure.) They all serve the purpose of amending and improving existing soil by being incorporated before planting, but should not be mistaken for potting media.
“Growing media” is a technical term for a soil-less mix. It does not contain sand, silt or clay. Most commercial blends are made up of various types of peat moss, composted pine bark, perlite and other regionally available components like coir (coconut fiber), rice hulls, hardwood bark, etc. The ideal media is sterile and it holds just enough water but it drains well and can have more than 50% pore space. And, the larger the pot, the larger the pore space.
If a pot is 2′ wide or deep you should use a mix that is up to 50% pine bark so that the water drains well all the way through. Perlite is another component that can improve pore space and enhance drainage.
Lastly, don’t follow the old advice of putting a layer of rocks or broken clay pots in the bottom of your pot before you fill it with soil. It will not improve drainage. The different drainage profile of that layer can actually slow down drainage and interfere with healthy root growth. As long as the pot has drain holes and a well-drained growing medium, you will get excellent results.