I'm taking Soil Science at NYBG this semester - its a fascinating combination of chemistry, biology and physics - and I just want to let you know that soil is not dirt. It's a complex mixture of minerals, organic matter, air and water with well-defined structure that varies with the exact components.
Our instructor defined soil as: "the natural loose covering over most of the earth's land surface that serves as the medium for plant growth".
To support plant growth, soil needs to provide
- both root penetration and anchorage - it's physical consistency needs to be loose enough for root penetration but solid enough to anchor the plant
- needed and essential nutrients