The layer of soil at the surface of the ground.
Solum is a scientific word used by people who study the earth and plants.
Examples:
The solum is where plants grow.
The solum is very important for farming.
The solum is full of tiny living things.
I like to play in the solum.
The solum is like a big, brown blanket for the earth.
The solum is where all the good stuff for plants is.