The Fundamentals Of Soil
Like we said: Don't call it "dirt"
You want to know the real dirt on soil? Don't call it dirt! Technically, soil is a composite of organic matter, microorganisms, minerals, air, and other inorganic materials. To really understand soil you must be familiar with such factors as cation exchange capacity, C:N ratio, pH, electrical conductivity, bulk-density, water-holding capacity, air-filled pore space, friability, mineral element content, and other things. (See "Important Factors Regarding Soil")
Fortunately - you don't need to know everything about soil in order to grow plants. In fact, for most container gardening, you won't even be using soil.
Confused? Soil on the surface of the planet (which we'll refer to as "topsoil" from now on), has a world of nutrients, water, and microorganisms beneath and around it that are available to support plant life. But when you remove topsoil and put it in a container, you lose many of those natural support systems and are left with material that is too dense and heavy for plants to grow in well.
Your job as a container gardener is to simulate the best conditions topsoil can offer - adequate air, beneficial microorganisms, organic matter, and nutrients - in a form that won't compact and be too dense. It's not as hard as it sounds. Read the other articles in the Soil section for more details.