Define a "sustainable" rate of soil loss. Describe how you might determine whether a given farm was practicing sustainable use of soil.

Rate of soil loss that is not higher than the local rate of soil formation

Explanation:

A "sustainable" rate of soil loss means "the rate at which soil is lost from a surface is not higher than the local rate at which soil is formed at an area."

We can determine whether a given farm was practicing a sustainable use of soil or not by merely measuring the amount of topsoil at the given farm.