The greater the slope in surface features, the longer soil takes to develop because steep slopes are more exposed to erosion that removes soil as it forms. On the other hand, depressions in the land that result in poor drainage and lack of adequate oxygen retard plant growth. Plants and animals also help develop soil. When plants die, water leaches plant food from them and carries it down into the pore spaces. This organic matter, or humus, helps the soil to stay porous and crumbly.
Plant roots help water to drain or percolate into the soil.
In dry times capillary action draws water up the channels made by the roots, bringing with it material that has leached down. Plant rootlets can split rocks by exerting pressure after working into cracks and crevasses. Soil is enriched by the wastes and decayed bodies of animals. Many insects directly enrich the soil by fertilizing flowers, thus aiding the spread of plant life. Composition of Soil Soils are composed of mineral matter and organic matter and contain pore spaces filled with water or air and soluble nutrients.