The term "soil impoverishment" refers to option 3, which is the soil getting very deficient in plant nutrients. This means that the soil lacks essential nutrients that are necessary for plant growth and development. When soil becomes impoverished, it becomes less fertile and unable to provide adequate nutrients for plants to thrive.

Option 1, soil erosion, refers to the process where the top layer of soil is carried away by natural forces such as wind or water. While soil erosion can contribute to soil impoverishment, it is not the same as the term itself.

Option 2, soil deposition, refers to the process where sediments and organic matter are added or deposited onto the soil surface. This can contribute to soil fertility, rather than impoverishment.

Option 4, soil getting enriched with plant nutrients, is the opposite of soil impoverishment. When soil is enriched, it means that it has an abundance of necessary nutrients for plants to grow and flourish. This is not the same as soil impoverishment.

Therefore, the correct answer is option 3 - soil getting very deficient in plant nutrients.