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. Organic matter serves as a binder for mineral particles, contributing to good soil structure and tilth, which refers to the behavior of soil under cultivation. The organic matter content of mineral surface soils ranges from less than 0.5 percent in highly weathered, sandy soils to more than 6 percent in poorly drained prairie soils. Soil organic matter undergoes continual breakdown from fresh plant residue to relatively stable humus. Initially plant residues are attacked by soil animals such as insects or worms.