Soil vs earth

[monica]

In the following exercise, I'm supposed to choose the correct option:
"What do scientists mean by soil/earth erosion and what causes it?"
The answer key gives "soil" as the correct answer, but I seem to be having some problems to see the difference between these two, particularly in this context, so any help would be greatly appreciated. Thank you.

[Molly]

"Soil erosion" is the common collocation. As you see, both words can be synonyms of each other, but earth also has other meanings, such as "the solid part of the earth's surface".

soil - noun: material in the top layer of the surface of the earth in which plants can grow (especially with reference to its quality or use) (Example: "Good agricultural soil")
earth noun: the loose soft material that makes up a large part of the land surface (Example: "They dug into the earth outside the church")