SPADA Draft Documents

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6.4 Soil Organic Matter (SOM) 149 animal decomposition, material synthesized by organisms, and cells/debris from soil 150 organisms. SOM impacts the physical and chemical properties of soils, including soil 151 quality and function. 155 (bedrock and deposited sediments), 3) Organisms (micro and macro), 4) Relief 156 (topography), and 5) Time. Time as a forming factor refers to the time of active 157 weathering versus the standard linear time scale. For example, a soil in Hawaii can be 158 considered older than a soil found in North America due to active weathering. These 5 159 soil-forming factors lead to unique characteristics in each soil and the formation of soil 160 horizons along a vertical profile (Figure 2). There are 5 identified horizons, called O, A, 161 E, B, and C horizons layered above the unweathered parent material. Each horizon has 162 distinct properties as defined in the Terms and Definitions. A soil may contain all or just 163 a few of these horizons. The top horizon may be an O horizon of loose, partly decayed 164 organic matter or an A horizon consisting of mineral matter mixed with organic material. 165 For most experimental purposes related to the very upper portion of the Earth’s crust, the 166 O, A and B horizons tend to dominate sample collection and handling practices. Figure 2 167 is an actual soil profile showing the boundaries and variability of depth of horizons. The 168 soil in Figure 2 has a 4-inch A horizon above a 20-inch B horizon. The C horizon is of an 169 unknown depth. Soil horizons can vary in depth from a few inches to 40 feet or more. 170 The organic matter component of soil is comprised of substances from plant and 152 153 154 6.5 Soil Formation and Horizons Soils are formed by 5 main soil-forming factors: 1) Climate, 2) Parent Material

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