What is the main process by which soil is formed?

Soil formation primarily occurs through the weathering of rocks. This process involves the physical breakdown and chemical alteration of rocks over time due to various environmental factors such as temperature changes, water, wind, and biological activity. As rocks break down, they release minerals and organic materials, which combine to create the rich, nutrient-filled layers of soil that support plant life.

Weathering contributes to soil formation in two main ways: mechanical and chemical weathering. Mechanical weathering physically disintegrates rocks into smaller particles without changing their chemical composition. In contrast, chemical weathering involves the alteration of the minerals within rocks, which contributes to the development of soil horizons, in turn enhancing soil fertility.

The other processes listed do not contribute primarily to soil formation. Evaporation of water is related to the hydrologic cycle and does not contribute directly to the creation of soil. Compaction of sediments may occur after soil has already formed, compressing existing layers rather than forming new soil. Photosynthesis is essential for plant life and the development of organic matter in soil but is not a direct process of soil formation itself. Thus, the correct understanding of soil formation points to weathering of rocks as the main process involved.