Healthy roots are the foundation of sturdy plant growth, so making sure your pots have the right soil for the type of plant you want to grow is your first priority in container gardening. You can buy bags of premixed soil or buy the ingredients and mix your own but, either way, be sure your soil has the right pH and has the right mix of air spaces and water-holding capacity.
Roots and Containers
Containers are small environments that force roots to occupy a limited space, restricting their ability to search out water and nutrients. They often dry out more quickly than the ground around them and offer less protection from winter cold. Air is also more limited inside the confines of a container, so potting soils need to be more fibrous and resistant to compaction than garden soil.
Potting soil needs to hold moisture well but drain freely to avoid root rot. The proportion of moisture-holding material depends on the needs of the plant. Blueberries and fuchsias, for instance, need a soil that stays evenly moist, while succulents and drought-tolerant perennials need one with a smaller proportion of water-absorbing material.
Oxygen is essential for root growth. A light, fibrous mix with plenty of spaces gives quick drainage and plenty of air.
Matching the pH of the soil to the needs of your plant is also important. Use a neutral to slightly acid mix for vegetables and most flowers, and a more acid mix for rhododendrons, blueberries, strawberries and other acid-loving plants.
Organic matter such as peat moss, coir, compost, sawdust and fine bark are common moisture-holding ingredients. The more fibrous ones, such peat moss and coir, are also useful for holding air and holding moisture.
Perlite, vermiculite, calcined clay (sold as kitty litter) and sand add air spaces to the mix and help the soil drain quickly.
Garden soil can be added if you keep it to less than a quarter of the mix and put in plenty of air-space material to keep the soil from compacting. Use only sterilized mixes without soil or compost if you're potting seedlings, since small plants are more susceptible to disease.
Amending Soil Mix
Even if you buy ready-mixed, bagged potting soil, you can add extras such as bone meal for tulips and daffodils, steer manure for heavy feeders such as roses and peonies, or extra perlite or sand for drought-tolerant perennials that dislike wet soil. Know the preferences of the shrubs, trees, vegetables, perennials or annuals that you want to grow, and suit the soil to their needs. You can mix plants with similar needs, but avoid planting perennials that like boggy soil with those that want a dry environment.
If you have a tall shrub in a small container, use sand or calcined clay to add weight to the pot and avoid toppling. Or if you have a large, heavy pot, put a 2- or 3-inch layer of perlite in the bottom to reduce the weight.