how does scientist discover the layers of the earth?
Main methods --\n(1)studying the patterns of vibrational echoes returned from surface and subsurface man-made explosions;\n(2)studying data gathered by instruments during earthquakes;\n(3)studying volcanic materials
It uses up all of the "nutrients" or whatever in the soil.  Crop rotation and planting things like legumes were done to allow the land to recharge between planting cycles.