Topsoil is usually more fertile than the other layers and has the greatest concentration of plant roots. 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. Soils’ properties vary with the soil depth. But these areas are too wet for the plant residues (leaves, branches, roots, trunks, and the like) to efficiently decompose. Such areas are swamps where plants grow and thrive. These organic soils are typically found in areas that just 50,000 years ago were below sea level. In the easternmost NC coastal plain, the dominant parent material is organic matter. 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 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 piedmont and mountains, the parent material is typically weathered bedrock known as saprolite. The predominate parent material varies by location in North Carolina. Collectively, the horizons make up the soil profile. Each layer can have two or more sublayers called horizons. The layers are the topsoil, subsoil, and parent material. Most naturally occurring, undisturbed soils have three distinct layers of variable thicknesses. The Soil Profile Skip to The Soil Profile The distribution of solids and porespace in ideal, compacted, and poorly drained soils is illustrated in Figure 1-1a, Figure 1–1b, and Figure 1–1c. But for plant growth, most soil scientists agree that 50% pore space, 45% mineral matter, and 5% organic matter make up an ideal ratio. The relative amounts of pore space and mineral and organic matter vary greatly among different soil types. The organic matter consists of decaying plant and microbial residues. The mineral materials are typically weathered rock of varying sizes called sand, silt, and clay. Soil solids are a blend of mineral materials and organic matter. For example, tilling increases pore space, while poor drainage and compaction reduce it. This distribution rarely occurs because pore space varies with soil texture and soil management. In this role, soil provides structural stability for plants and retains and relinquishes water and the nutrients necessary for plant growth.Īn ideal soil for plant growth contains 50% pore space and 50% solids, with the pore space filled with equal parts air and water. Soil is a living, breathing, natural entity composed of solids, liquids, and gases. Introduction: What Is Soil? Skip to Introduction: What Is Soil?
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