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. As breakdown proceeds, soil microorganisms begin their work. Carbon is changed to carbon dioxide, and complex nitrogen compounds are transformed to soluble forms that plants can use.
Humus is also an important storehouse of phosphorus and sulfur.
Soil water and gases fill the spaces between mineral and organic matter. A total pore space that is 50 percent air and 50 percent water offers an ideal growth medium for most plants. The amount of space that is filled with water and the extent to which the water is held within the soil depend on the degree of saturation. As the soil dries, water is held more tightly in thin films that coat the particles of soil, and this water is not available for use by plants. Minerals and nutrients that have been dissolved in the soil water contribute to the soil solution that is the nutrient lifeline for plants.