Soil is loose, aggregate material that forms on the Earth's surface. Plants are completely reliant on soil for water, minerals and nutrients. All animals are either directly or indirectly dependent on plants for food; therefore, soil is figuratively and literally the foundation for life on Earth. Soil is composed of four basic elements that include rock particles, organic material, water and air.
Large sedimentary and igneous rocks are broken down over large periods of time into smaller rock particles through the process of weathering and erosion. Weathered rock particles slowly release vital minerals needed by plants, such as potassium and phosphorus. Different types of soil are composed of different size rock particles. Sand, silt and clay represent the three major classifications of rock particles, sand being the largest particle and clay the smallest. Rock particles make up approximately 45 percent of soil composition.
Approximately five percent of soil is composed of decaying organic matter. Organic material such as leaves, sticks, animal waste and dead animal remains are decomposed into organic matter by fungi, bacteria and microscopic animals within the soil. Organic matter provides vital nutrients to root systems within the soil, moderates diurnal topsoil temperature fluctuations and absorbs and retains moisture within the soil.
Water and Air
Empty space between the rock particles and organic matter is filled with water and air. These pore spaces occur naturally because the rock particles and organic matter do not fit perfectly together. Additional pore space is carved out of the soil by earthworms, ants, moles and plant root systems. The resulting empty space accounts for 50 percent of the soil composition and is filled with 25 percent air and 25 percent water. The air and water in the soil is absorbed by the plant roots, providing vital carbon dioxide and moisture to sustain plant growth.