(a) Sun : The sun heat expands the rocks and sudden cooling forms cracks and forms rocks.
Water: Water gets freezed between the rocks during winter and forms cracks and resulting in formation of soil. Fast flowing water results in collision of big and small rock pieces and forms soil.
Wind : Strong winds cany the rock pieces from place to place and collide with each other resulting in formation of rocks.
Living organisms like lichens growing on rocks release certain substances that cause the rock surface to powder. Plants like mosses growing on these powdery soil particles and break them further.
(b) Humus : It causes the soil to become more porous and allows water and air to penetrate deep underground.