The plants they grow and products they harvest from it exhaust the ground, because the plants need minerals to grow but the minerals dont return to the ground when they die. Instead, (part of) the plant is harvested and the ground is left exhausted from minerals. Farmers add fertiliser to the soil to artificially make it rich of nutrients for the plants.
What is a "structure"? a building or other object constructed from several parts. "the station is a magnificent structure and should not be demolished"
The depth in a multilayer rock formation with the oldest rock most likely to be found in the igneous rock because almost anywhere is where the volcanoes have formed the earth.