d. all of the above.
When at first thought, it would seem that it is erosion. Erosion not only eliminates the soil minerals, it also washes everything else and that includes the soil. In a different manner, Leaching can happen when certain minute particles of the minerals are melted and mixes with water. All that is left in the soil will be the bigger particles that are not soluble by water. In this manner, only a soil that cannot properly nourish remains as it is stripped off of minerals.
