When you grow such robust plants in your garden soil, it would lead you to believe that you could use the same soil with success in your potted planters! Unfortunately that just isn't the case.
Garden soil doesn't offer enough air, water or nutrients to a plant growing in a garden pot to assure success. Potting soils used in planters are a special mix that can overcome these situations. It is so important that the soil provide the root systems of your plants and flowers with access to air so that water can drain away from them and not pool on the bottom of the planter, despite drain holes. The more compact the soil the less water will drain off. The the ultimate problem will exist: root rot!
By adding loose fill like perlite, and sand to garden soil it helps to keep the soil from clumping and forming hard clots. The simpliest way to get the right mix is to purchase potting soil from your local garden center that will contain the right amount of ingredients. But, if you want to mix your own we have a basic formula that works. This step is so important for great pottery growth.
Mix 2 gallons of each:
Peat moss, perlite, garden soil
Add 1/2 cup of each:
Dolomite, green sand, kelp powder or rock phosphate.
Sift the garden soil and peat moss to remove clumps. Then add the remaining ingredients and mix with hands or shovel.
What you are looking for basically, is a positive environment for the plants root system so that you grow the most healthy plants and flowers possible. Keep the soil in the planters loose and crumbly. Once it gets packed down at the end of the season you need to re-sift it or replace it. The health of your garden depends on it.