Use a premium commercial container soil mix.
These mixes are usually high in peat moss for good water retention, resistance to compaction and good air circulation to the roots. They are also treated with a wetting agent to allow the peat to absorb water more efficiently. There are two types of soil mixes for containers—soil-based and soil-less. Soil-based container mixes consist of sterilized loam, peat moss, sand and nutrients. Soil-based mixes are more water-retentive and less likely to shrink away from the edge of the pot as they dry out than soil-less mixes. The clay particles in the soil have a charge that attracts fertilizer molecules, preventing them from leaching away in water. Soil-based mixes are heavier than soil-less blends. Soil-less mixtures are peat-based formulations.
They are lightweight, making them ideal for containers where weight is a concern, such as hanging baskets. A disadvantage is the lack of nutrients and the mixture’s tendency to dry out quickly. Adding a little compost to soil-less mixtures helps improve fertility and water retention. Basic container mix: 1 part perlite; 1 part basic sterilized potting soil; 1 part peat moss or well-rotted compost. Poly acrylamide copolymers are a useful addition to container plantings. These non-toxic crystals act like tiny sponges, absorbing and retaining up to 400 times their weight in water, which is then released to the roots when the soil is dry. The general guide to their use is about 10 mL (2 tsp) of copolymer per 4.5 L (1 gal) of soil. They are especially useful for container plantings where the prevailing climate is hot and dry. Pre-moisten your soil mixture using lukewarm water. Warm water is better absorbed by the peat moss in the mix than cold water.