The amount of aggregation in a soil is strongly influenced by the amount of organic matter present. The pore spaces in a soil are normally filled with air or water. As the amount of water increases, the amount of air must therefore decrease. The pores of a well-drained soil have certain physical characteristics that, after a period of heavy rainfall, enable water to rapidly drain away and allow air to return to its original percentage. The amount of organic matter in soil is an important factor in soil structure.
Organic matter consists of dead and decomposing plant and animal parts.
Living organisms break down plant debris into organic matter. Clay and organic matter contain predominantly negatively charged sites that attract cations. Applied nutrients such as ammonium nitrogen, potassium, calcium, and magnesium attach themselves to the negatively charged soil particles. This phenomenon is called cation exchange, and it allows the soil to be a reservoir for plant nutrients. Before selecting a site for an orchard, consult a county soil map.