Always use a high-quality potting soil in any type of container and never recycle old potting soil when transplanting plants to new pots or potting up new seedlings. Doing so may pass disease or pests from old containers to new plants. Potting soils vary in makeup depending on their use and brand. The majority of them consistently provide enough of the nutrients plants need to thrive through most of the growing season.
The most abundantly used nutrient in the development of healthy plants is nitrogen. Nitrogen is necessary for the process of photosynthesis, wherein plants create the sugars they need to grow. Nitrogen makes your plant green and full and is used the quickest of all nutrients. Compost added at the time of planting can give an excellent boost of nitrogen as well as a slow-release fertilizer. When purchasing fertilizer or potting soil with a fertilizer added, the amount of nitrogen added is shown as the first number in the ratio equation of nutrients. At the sign of yellowing leaves on your plants, give a boost of nitrogen by using a soluble fertilizer as instructed. Be careful giving flowering vegetables such as tomatoes too much nitrogen though--an abundance of it will cause your plant to grow large and gorgeous but never get to the bloom stage to produce a crop.
Potassium, often referred to as "potash" in potting soil, is the basic building block of cell development in all living things. The second number in the ratio equation on packaging, potassium is used up slower than nitrogen. Plants use potassium for strong stalk and leaf development. In potting soil, potassium seldom needs replenishment past the normal dose of weekly fertilizer.
The last number in the ratio equation is phosphorus. Needed for healthy root growth and seed production, phosphorus is not as vital to annual growth and is often more abundant in potting soil blends mixed specifically for vegetable and fruit gardening.
There are a number of other lesser-important minerals in potting soil that growing plants use. Usually a mixture of peat moss, compost and drainage helpers such as vermiculite or perlite, potting soil mimics the most ideal garden soil conditions possible. Compost helps give the soil composition and provides many of the above-mentioned nutrients as well as trace minerals such as magnesium, calcium, sulfur and carbon, all needed in small doses.