The soil that covers the Earth is composed of varying levels of clay, silt and sand. But unless you're dealing with lawn care, that's not the soil you'll be working with. Potting soil is the most commonly used dirt for household planting, from little potted plants to growing your own tomatoes.
Composition
Very little actual soil can be found in potting soil, as it's too heavy for most garden plants. Instead, you get varying levels of peat moss, sand, broken down organic matter, limestone and perlite. The peat moss helps hold moisture, while the sand helps keep the soil from getting too compacted for the roots to breathe or get water. The limestone and perlite also aid in aeration, drainage, and pH balancing. The organic matter adds nutrients and texture to soil.
Plant Needs
Know your plant's needs. Just like a rabbit and a dog have different nutritional requirements, so do plants; this is sometimes called a soil's food value. Carnivorous plants like the infamous Venus fly trap prefer less nutrients in their soil, for example. More popular garden plants have specific soil mixtures available that are geared to their nutritional requirements.
Another issue is moisture: peat moss potting soils are better for plants that require more moisture, so they wouldn't be good for your favorite cactus plant. Location is another consideration. Out-door plants have different water and drainage issues than indoor plants, which can be prone to mold. A soil that drains well while still allowing air to get to the roots is vital for all plants, but indoor plants in particular. Soil erosion is more of an issue for outdoor plants; try covering the soil with mulch, which will add protection and nutrients.
Be careful adding supplements to soil, as you can throw the pH balance off. Some plants prefer more acidic soils, while others need alkaline-heavy soil. This is especially problematic in environments where plants have been imported from other areas.
