R P Voroney

Introduction

Soil Genesis and Formation of the Soil Habitat Physical Aspects of Soil Soil Habitat Scale and Observation References and Suggested Reading

At first sight nothing seems more obvious than that everything has a beginning and an end and that everything can be subdivided into smaller parts. Nevertheless, for entirely speculative reasons the philosophers of Antiquity, especially the Stoics, concluded this concept to be quite unnecessary. The prodigious development of physics has now reached the same conclusion as those philosophers, Empedocles and Democritus in particular, who lived around 500 BCE and for whom even ancient man had a lively admiration. (Svante Arrhenius, Nobel Lecture, 1903)

introduction

Soil is the naturally occurring, unconsolidated mineral and organic material at the earth's surface that provides an environment for living organisms. Recently, it has been referred to as the earth's "critical zone" and as deserving special status, because of its role in controlling the earth's environment and thus affecting the sustainability of life on the planet. This concept, that the earth's physicochemical properties are tightly coupled to the activity of the living organisms it supports, was proposed in the early 1970s by James Lovelock as the Gaia hypothesis. He theorized that the Earth behaved as a superorganism, with an intrinsic ability to control its own climate and chemistry and thus maintain an environment favorable for life. However, it is only microorganisms that have proven they can sustain the biosphere and can do so even without larger organisms.

The soil is where living organisms, or the biosphere, interact with rocks and minerals (geosphere), water (hydrosphere), atmosphere, and dead organic matter (detritosphere). Scientists study soil because of the fundamental need to understand the dynamics of geochemical-biochemical-biophysical interactions at the earth's surface, especially in light of recent and ongoing changes in global climate. What complicates this study is that while geochemical fluxes between the hydrosphere, atmosphere, and geosphere may take place over the time span of hundreds to millions of years, biologically induced fluxes between the geosphere, atmosphere, biosphere, and detritosphere take place over a much shorter time frame, hours and days to months.

The soil habitat is defined as the totality of living organisms inhabiting soil, including plants, animals, and microorganisms, and their abiotic environment. The exact nature of the habitat in which the community of organisms is living is determined by a complex interplay of geology, climate, and plant vegetation. This interaction of rock and parent material with temperature, rainfall, elevation, latitude, exposure to sun and wind, and many more factors, over broad geographical regions with similar environmental conditions and characteristic plant communities, has evolved into the current terrestrial biomes with their associated soils (Fig. 2.1).

Because soils provide such a tremendous range of habitats, they support an enormous biomass, with an estimated 2.6 X 1029 prokaryotic cells alone, and harbor much of the earth's genetic diversity—a single gram of soil contains kilometers of fungal hyphae and more than 109 bacterial cells, organisms belonging to tens of thousands of different species. Micro-zones of good aeration may be only millimeters from areas poorly aerated. Areas near the soil surface may be enriched with decaying organic matter and other nutrients, whereas the subsoil may be nutrient poor; the soil solution in some pores may be highly acidic, others more basic, depending on soil mineralogy and biological activity. Temperature and water contents of surface soil can vary widely from that of subsoils; and the microenvironment of the surfaces of soil particles, where nutrients are concentrated and water films vary in thickness, is very different from that of soil pores.

soil genesis and formation of the soil habitat

By definition, soils are greater than 10 cm thick if formed from minerals and extend from the earth's surface into the underlying parent material from which they are formed. Soil may even be covered with water to a depth of up to 0.5 m as in coastal tidal marshes or inland water areas where bodies of periodically submerged soils merge into bodies of water in the natural landscape.

When plant residues are submerged in water for prolonged periods of the year and availability of O2 is limited, biological decay is slowed and organic matter in various stages of decomposition accumulates. Deposits containing >30% organic matter and extending to depths of ~0.5 m or more form organic soils, and they include peatland, muck or bog, and fen soils.

Steppes Semidesert and desert Savannas Tropic forest zone Savannas

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