The growing medium used in containers can be soil alone, it can be soil plus amendments, or it can be completely soil-less. Whatever you choose, the medium must drain well. This is best accomplished by using a combination of soil amendments of various particle sizes to provide small air spaces, drainage, and adequate water-holding capacity.
If you use soil, only use commercially prepared potting mixes instead of native soil for your container plants.
Another problem with free soil is Weeds. Weeds proliferate in garden soil and their seeds will inevitably be found in any soil that you acquire. When placed in a container, they will outcompete whatever is planted in the container. We cannot use herbicides and expect desirable plants to thrive, so sterile potting soil is mandatory as it is certified to be free of nuisance plants. Native soils often contain insects and harmful pathogens.
The only life we want growing in our containers are the desired plants and earthworms. The various bugs found in garden soil do eat our plants. Likewise, the bacteria, fungi, and viruses that can attack our containerized plants are delivered directly to the plants in concentrated form if we use garden soil.
Potting soil is key to the dedicated container gardener. Choose the type that will work best with the plants you desire. The labels will list the ingredients and describe which plants are suitable for the mixture. There are many formulae and your garden center will be happy to offer advice. We suggest purchasing the soil with time release plant food and moisture control. Miracle-Gro brand has an excellent potting mix of this variety.
Or you can use a wide variety of soil amendments to custom-blend your own specific soil mix. Keep it simple! Start with peat moss or good top soil as a base; this will constitute the bulk of your mix. Add perlite, pine bark, or sand to provide drainage and increase aeration. The addition of vermiculite will increase the water-holding capacity. The addition of peat moss to any mix will make the soil very acid, so always add ground limestone to maintain an adequate pH level. The addition of a granular-type fertilizer at low rates can be beneficial as a starter feed to new plants.
A suggested mix:
2 cu.ft peat - 1 cu.ft. perlite - 1 cu.ft vermiculite - 1 cu.ft. pine
Remember - Your soil is your plant's home, and like every home it requires a strong foundation.