Below the topsoil is a layer of clay and small pieces of rock.This layer contains a lot of nutreints.It also contains minerals like iron,which is important for plants because its deficiency causes yellowing of leaves.
Soil is a living, breathing, natural entity composed of solids, liquids, and gases. Soil has five major functions:
Provides a habitat for organisms
Recycles waste products
Filters water
Serves as an engineering material
Provides a medium for plant growth1
Our focus will be on the fifth function. In this role, soil provides structural stability for plants and retains and relinquishes water and the nutrients necessary for plant growth.
An ideal soil for plant growth contains 50% porespace and 50% solids, with the porespace filled with equal parts air and water. This distribution rarely occurs because porespace varies with soil texture and soil management. For example, tilling increases porespace, while poor drainage and compaction reduce it.
Soil solids are a blend of mineral materials and organic matter. The mineral materials are typically weathered rock of varying sizes called sand, silt, and clay. The organic matter consists of decaying plant and microbial residues. The relative amounts of porespace and mineral and organic matter vary greatly among different soil types. But for plant growth, most soil scientists agree that 50% porespace, 45% mineral matter, and 5% organic matter make up an ideal ratio (Figure 1–1a). The distribution of soils and porespace in compacted and poorly drained soil is illustrated in Figure 1–1b and Figure 1–1c.
Most naturally occurring, undisturbed soils have three distinct layers of variable thicknesses. The layers are the topsoil, subsoil, and parent material. Each layer can have two or more sublayers called horizons. Collectively, the horizons make up the soil profile. The predominate parent material varies by location in North Carolina. In the NC piedmont and mountains, the parent material is typically weathered bedrock known as saprolite. In the river bottoms and stream terraces of the NC piedmont and mountains, the parent materials are the floodplain sediments delivered from upstream where erosion has occurred. In the NC coastal plain, the parent materials are marine sediments deposited over eons as the oceans go through the natural cycles of advance and retreat. In the easternmost NC coastal plain, the dominant parent material is organic matter. These organic soils are typically found in areas that just 50,000 years ago were below sea level. Such areas are swamps where plants grow and thrive. But these areas are too wet for the plant residues (leaves, branches, roots, trunks, and the like) to efficiently decompose.
Soils’ properties vary with the soil depth. The surface soil, or topsoil layer (O and A horizon in Figure 1–2), usually contains less clay, but more organic matter and air, than the lower soil layers. Topsoil is usually more fertile than the other layers and has the greatest concentration of plant roots.
The subsurface layer (B and C horizon in Figure 1–2), known as subsoil, usually has a higher clay content and lower organic matter content than the topsoil.
Soil properties often limit the depth to which plant roots can penetrate. For example, roots will not grow through an impenetrable layer. That layer may be bedrock (Figure 1–3), compacted soil, or a chemical barrier, such as an acidic (very low) pH. A high water table can also restrict root growth due to poor soil aeration. Few big trees grow in shallow soils because big trees are unable to develop a root system strong enough to prevent them from toppling over. Shallow soils also tend to be more drought-prone because they hold less water and thus dry out faster than deeper soils. Water lost to runoff on shallow soils would instead be absorbed by a deeper soil. In addition, deep soils allow the roots to explore a greater volume, which means the roots can retain more water and plant nutrients.
Soils change in three dimensions. The first dimension is from the top to the bottom of the soil profile. The other two dimensions are north to south and east to west. The practical meaning of this three-dimensional variability is that as you move across a state, a county, or even a field, the soils change. Five factors of soil formation account for this variation:
Parent material
Biological activity
Climate
Topography
Time
Differences in even one of these factors will result in a different soil type. Soils forming from different parent materials differ. Soils forming from the same parent material in varying climates differ. Soils at the top of a hill differ from soils at the bottom. The top of the hill loses material due to natural erosion; the bottom gains the material from above. Considering the number of possible combinations of these five factors, it is not surprising that more than 450 unique soil series are currently mapped in North Carolina. Globally, more than 20,000 different soil series occur. Neighborhood level soil series can be found by typing “Web Soil Survey” into any Internet search engine.