There are two main types of potting soils, 'loam-based' and 'peat-based'.
Clay agregates used in hydroponics are becoming more common,
but require more maintenance than traditional soils.
Loam - Based soils are heavier than peat based and thus
give more stability to larger plants. They dry out slower and
less completely than peat based soils. They contain many trace
elements that plants need for healthy growth. Most plants will
grow in loam soils.
Peat - Based soils are more uniform in quality than loam
soils. Peat is light and easily carried and stored in a dry cool
place. Most house plants will grow in peat, but feeding is needed
sooner than with loam soils. They dry out quicker than loam based
soils and are more difficult to remoisten if they become too dry.
Clay agregates are types of porous clay that replace traditional
soils for rooting medium. The main problem with clay agregates
is that constant feeding is required. A diluted fertilizer solution
is needed to bathe the roots so the plant can grow.