Our recommendation for containers is ALWAYS purchase potting soil specifically for containers. Do not reuse last years dirt or add dirt from your garden. The reason is that container mixes are specifically made to provide nutrients, give exceptional drainage and have added ingredients that help to retain moisture.
Gardening soil in general should be well draining and loose. You can achieve this by adding sand, clay or compost depending on what type of soil you are starting with. Compost enhances the physical aspects of soil which makes it healthier. The organic materials in compost break down in the soil and encourage the presence of earthworms.
The following is a description of soils so you can figure out what type you are starting with prior to enhancing.
Sand is the largest particle in soil. It feels rough and gritty. It doesn’t contain many nutrients but drains well.
Silt falls between sand and clay. It feels smooth and powdery when dry and is not sticky when wet.
Clay is the largest particle in soil. It is smooth when dry but sticky when it gets wet. It holds many nutrients but does not allow enough air and water passage. Too much clay makes soil too heavy and unsuitable for growing plants.
Loam is a good balance of all three. It breaks up easily, encourages organic activity and retains moisture while allowing for drainage and aeration.
Loam is the soil you are trying to create by adding other additives.