Most garden soil is rich with nutrients but it does not provide enough drainage and normally carries bacteria and weed seed. It also tends to form a heavy mass, which prevents root respiration and fertilizer absorption and results in low performing plants. Even if you feel you have very good garden soil, don’t use it for container gardening.
Containers perform best when the potting soil provides fast drainage of water while still holding moisture, air and fertilizer that is suspended in a clean and lightweight growing medium. Commercial potting mixes are composed of organic materials such as peat, fir bark or composted redwood. Fertilizers, compost and trace materials are sometimes added as well.
One of the particular advantages of container gardening is that different mixes can be used for different plants, giving each plant a growing environment it thrive in.
Pre-moistening the potting soil before filling the containers to make it damp will help settle air pockets and protect the plants. Fill the containers to about one inch below the rim of the pot and press down firmly to settle the air pockets then place the container in a well-protected spot overnight before planting.
Potting mix in an enclosed container cannot replenish itself in the same way garden soil does so it needs to be replenished. It is also a good practice to re-pot containers at least every two years with fresh potting mix. If the containers are filled with annuals, use new potting mix each time the plants are changed.