While we might talk about a "patch of soil" or a "bed of soil," there's no specific collective noun like "flock" for birds or "herd" for cattle.
However, you could get creative and use a descriptive term based on the context, such as:
* "A blanket of soil" to describe a layer of soil covering an area.
* "A mound of soil" to describe a pile of soil.
* "A layer of soil" to describe a specific depth of soil.
Ultimately, the best way to describe a collection of soil depends on the specific situation and what you want to convey.