Soil is a mixture of mineral and organic components, which are in solid, gaseous, and aqueous states. In the United States, the mineral parts may make up to about 95 percent of soil, with organic matter making up the remaining 5 percent. However, some soils have a higher organic content while some, such as desert soils, have less or even none.
The arrangement of mineral and organic components of soil is known as soil structure. Some components may clump or stick together while others are more widely separated. The sizes and shapes of the pores, the open spaces between soil particles, are of major importance. Water and air move through these spaces, carrying plant nutrients that can be stored in plant-available form by soil organic matter. The ability of plants to send out healthy root systems, and the ease with which seedlings can germinate and grow, are determined largely by soil structure.
The quantities of -- and relative balance between -- mineral and organic particles in the soil is called soil texture. The classification of soil textures is determined by the fractions of sand, silt, and clay in a soil. Loam is used to describe a soil that has a roughly equal mixture of sand, silt, and clay. Thus we get soil descriptions such as sandy loam, silty clay, or silt loam.
The Organic Component
Organic matter in soils is vital for natural plant health. In a totally natural environment, such as the floor of a forest, leaves, twigs, and dead branches decompose gradually and are constantly incorporated (through the activities of insects and animals) into the topsoil. In an agricultural, managed environment, such as a farm field, organic matter consists of plant residues left behind or deliberately added to the soil.
Manures and Compost
The incorporation of animal manures and compost is the most common method for increasing the organic-matter content of soils. In addition, special crops (called cover crops, or green manures) are grown that benefit the soil, by "fixing" nitrogen via nodules on their roots, and holding the soil in place to prevent erosion by wind or water. When the top growth of these crops is incorporated into the soil, it rapidly decomposes and adds to the total organic-matter content of the soil. In the United States, a soil organic-matter content of 5 percent is considered good.
Benefits of Organic Content
Organic matter in the soil holds nutrients and makes them available to plants as needed. It also holds water, preventing soils from drying out too rapidly. Finally, it helps hold the pores in the soil open, allowing good drainage, encouraging beneficial microbiological activity, and permitting the passage of oxygen needed by plant roots.