Composition

Soil Composition

Soils are complex and dynamic systems consisting of mixtures of solids, pores, water and gases, and are often seen in several different horizons or layers (Fisher and Binkley 2000; Brady and Weil 2004). Horizons will vary in thickness, boundaries, and composition and generally are more or less parallel to the land surface. These layers contain organic material, pore space, water, and minerals. In an "ideal" soil, pore space makes up approximately 50% of soil volume (at right). Depending on the water content in a soil, different proportions of pore space are filled with air and water.

Soil Critters

Surface horizons are typically referred to as "topsoil." In forest ecosytems, this layer may be high in organic matter due to litter fall and incorporation of organic matter into mineral soil following litter decomposition and soil organism activity. Plants and animals (at left), including bacteria, tree roots, reptiles, and small mammals, live within this soil layer. Topsoil can be altered by adding amendments through either chemical or physical means. Soil amendments such as fertilizers, bedding or tilling can positively affect the fertility and productivity of a soil.

Soil Layers and Horizons

Lower horizons are called subsoil. These horizons (at right), not seen from the soil surface, can greatly influence land use. Some soils differ drastically between the topsoil and subsoil, while other soils have a gradual change from one layer to another. Although much of the water needed for plant growth is stored in the subsoil, the topsoil is generally more conducive to root proliferation. Therefore, the depth of the topsoil often correlates to a soils productivity.

These soil layers and associated physical, biological, and chemical properties are the result of five major factors that affect soil development: parent material, time since soil development was initiated, climatic factors such as temperature and rainfall, topography and landscape position, and vegetation and associated organisms. More detailed information related to Southern forest soils can be found in the introductory module under Soils.

The composition of a soil influences many soil properties that, in turn, affect the potential productivity of forest lands and should be considered for management objectives and operations. For example, water and nutrient movement and storage are affected by soil composition, and affect relatively basic forest management decisions such as crop tree species selection, site preparation and planting operations, fertilizer requirements, and timing of thinning and final harvest operations. Therefore, it is important to understand the composition of the soil on a particular site in order to plan forest management operations that will achieve short- and long-term landowner objectives. Much excellent information about soils is available in the various soil survey reports of the USDA-NRCS (Natural Resources Conservation Service) (http://www.nrcs.usda.gov).