Not all soil or good screened loam is the same and based on its components, the suitability for planting can be very different. Dirt with a very high clay content may mean it holds water very well. But, if there is too much clay, it could actually create a real problem and not allow any water to pass, creating ponding. Too high a sand content can cause the opposite effect in that it will not hold enough water to support the plant in hot weather or high wind. Silt is an in-between situation in that its particle size is larger than sand, it is less of a plastic type consistency than clay and often has a higher organic content associated with it.
Proper blending of these components can create a healthy, fertile growing medium for your landscaping needs. The fertility of the soil needs to be considered separately based on the chemical make-up of the components and the organic content in the soil.
Topsoil is composed of three main components in varying amounts: clay, sand and silt. The chart below shows how the description of the soil changes based on the varying components.
Soil classification is based on the components that make up the soil and has no factor on organic content nor nutrient content. There normally is a correlation, but it cannot be assumed. With an understanding of these components, you can look at and be a better consumer of topsoil.