In This Article Soils as an Environmental System

  • Introduction
  • Physical Components of Soil

Environmental Science Soils as an Environmental System
by
Eldor Paul, Sherri Morris
  • LAST MODIFIED: 30 November 2015
  • DOI: 10.1093/obo/9780199363445-0041

Introduction

Soil, the living surface of planet Earth, sustains nearly all life on the terrestrial surface. It is a complex media resulting from parent material, climate, topography, and biota, interacting over time. The complexity of soil lies within its composition. The physical structure of soil, which relates to the size and organization of its particles, determines many qualities such as nutrient and water-holding capacity, productivity, and erodibility. The physical structure results, primarily, from the breakdown products of the parent material, slope position, climate, and organisms present. The soil composition and structure influences the amount of water that can be held within the soil and how tightly that water is held by the soil. Life requires water, whether it be the plants and other autotrophs, or the soil food web. Water also controls soil aeration and is an important control on O2 and CO2 exchange. The importance of soil as an environment is tied to its biota, chemistry, physics, and biochemistry as well as the nutrient transformations that occur within soil. Living organisms are dependent on the elements such as C, H, O, P, K, N, S, Ca, Fe, etc., for the basic building blocks of life, and most of these elements are provided to terrestrial organisms through the soil. Bacteria and fungi weather rock and decompose organic materials, which allows the inorganic building blocks to become available to organisms at higher levels in food webs. The availability of nutrients for other organisms depends on the quality of the habitat that the soil environment provides for the decomposers. The term biogeochemistry, which describes the interactions involved in soil nutrient cycles, emphasizes the intertwined role of the soil biota, geologic components, and chemistry needed for nutrients in soil to become available for use by living organisms. Human impacts have altered the soil as an environmental system. The two main terrestrial fluxes of C with the atmosphere, photosynthesis and decomposition, are intimately tied to soil quality. The balance of these two processes determines the size of the terrestrial sink for C. As such, soil organic matter (SOM) dynamics are of great interest to the global research community as scientists endeavor to develop scenarios to better understand the potential outcomes of global climate change. Agronomic practice and urbanization have changed the basic structure and function of soils as disturbance events (e.g., cultivation, tillage, removals, and additions) alter the composition of the soil. Whereas soils form over hundreds to thousands of years, humans can remove that history in one swipe of a bulldozer. The need to protect the human food supply and ecosystem services (soil formation, agricultural production, flood control, SOM protection, etc.) upon which humans are dependent requires research, restoration, and protection of this essential resource.

The Nature of Soils

Soils are the thin skin of the earth, usually associated in depth with that area penetrated by plant roots, and thus, usually are restricted to the top meter. They are largely characterized by their inorganic and organic constituents and by the role that each of these components plays in providing an environment for living organisms. Soils differ across the globe, having been formed by an interaction of parent materials as acted upon by climate, biota, and topography over time. The characteristics of soils that are essential for living organisms are determined largely by the mineral composition, the plant residues, the organisms decomposing them, and the microbial degradation products that are protected from decomposition by interactions with the physically and chemically weathered mineral solid particles. Soil, as a complex product of geological and biological processes, provides the growth medium necessary for sustaining life on the planet.

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