Composition of Soils

• Soil consists of inorganic and organic material, water and air. Lulu/Photodisc/Getty Images

  Soil covers the earth and has gone through physical and biological processes to become what it is. As the planet evolves, soil changes with it. According to the University of Arizona, soil consists of 1 to 5 percent organic matter, 45 to 49 percent inorganic matter, 25 percent air and 25 percent water.

Organic Matter

• According to the University of Arizona, organic matter makes up 1 to 5 percent of soil. Organic matter usually comes from dead plant material, which decays, breaks down and decomposed by animals an microorganisms in the soil. The top layer of soil usually consists of organic matter. This layer usually exists in forest soils, where dead leaves and other organic debris can accumulate.

Inorganic Matter

• Inorganic material or minerals come from the parent material of soil and the combination of substances in the soil solution. The minerals fall into three categories: sand (diameter 0.05 to 2.0 mm), silt (diameter 0.002 to 0.05 mm) and clay (diameter less than 0.002 mm). Of the three, clay minerals have the highest impact on the properties of soil because of their small size and large surface area. As such, they have more physical reactions and interactions with other soil particles.

Air

• Air and water fill the empty spaces in the soil. Compared with the air in the atmosphere, the air in the soil generally has high water vapor saturation level, more carbon dioxide and less oxygen. The activity of microorganisms in the soil and gaseous exchange between the soil and the atmosphere determine the exact composition of gases. A higher amount of decaying plant matter results in higher microbial activity, which depletes oxygen and produces carbon dioxide. The air in the soil conducts gas exchange with the atmosphere by diffusion. This exchange occurs at a lower rate if few soil voids exist, eventually depleting oxygen and creating anaerobic conditions.

Water

• Air and water complement each other in these voids, one replacing the other continuously. When water saturates the soil, the voids become almost full of water. If the voids in the soil contain water, oxygen cannot easily diffuse into the soil, eventually resulting in anaerobic conditions. Water in the soil contains soluble nutrients that plants need to survive. It flows through saturated soil by gravity and capillary forces. In unsaturated soil, water moves slower by capillary forces. It can also move from warm areas to cool areas as vapor, condensing into liquid in cool soil.