Soils are formed by?

• A. Weathering and erosion of parent material plus decomposition of organic matter adding nutrients
• B. Only weathering of parent material without erosion
• C. Erosion of organic material only
• D. Deposition of sand dunes with no weathering

Answer: (A)

Soils form from the breakdown of rocks and the addition of organic matter. Weathering of the parent material creates the mineral particles and releases nutrients that plants can use. At the same time, organic matter from plants and organisms decomposes, forming humus that improves soil structure, water-holding capacity, and nutrient availability. Erosion can move and redistribute material, helping to build the soil profile, but the essential pieces are weathered material plus decomposed organic matter that supplies nutrients. Without weathering, or without inputs of organic matter, you wouldn’t get a typical soil; simply depositing sand dunes with no weathering would leave loose sediment rather than a developed soil.