1. Introduction2. Physical Properties3. Chemical Properties3.1 Chemical Cycles3.2 Chemical Composition3.3 Biological Components of Soil4. Land Use4.1 Agriculture and Ranching4.2 MiningCharacterization of the Land of the Amazon RainforestLast Updated: November 6, 2002Table of Contents1. Introduction......................................................................................................................22. Physical Properties...........................................................................................................23. Chemical Properties.........................................................................................................53.1 Chemical Cycles............................................................................................................53.2 Chemical Composition...................................................................................................73.3 Biological Components of Soil......................................................................................84. Land Use........................................................................................................................104.1 Agriculture and Ranching............................................................................................104.2 Mining..........................................................................................................................131. IntroductionThe Amazonian tropical rainforest is one of the most complex ecological systems on the planet. This complex ecological machine supports countless species of flora and fauna. At the present it is being destroyed at an alarming rate, undoing the work of thousands of years of evolution. This natural development has resulted in a complex ecological machine, with perfectly balanced biological interdependencies and ecological cycles, is unfortunately also sensitive to the drastic changes imposed by human encroachment. The land in the rainforest is a critical component in understand this ecological system. Thus in seeking to preserve the rainforest, one must examine the chemical and physics processes and characteristics of the soil itself. Since the soil characteristics are critical to the ability of the flora to proliferate, soil quality must be monitored. In examining the physical and chemical composition of this region, we will determine the natural tendencies of the Amazon Rainforest as well as some of the human activities which negatively impact the health of the land and soil. 2. Physical PropertiesThe Amazon Rainforest is situated in a region known as the Amazon Basin. This is a depression in the Earth’s crust resulting from plate tectonic activity where sediments from erosion and river deposition accumulate (Schlumberger Oilfield Glossary, http://www.glossary.oilfield.slb.com/MainAbout.cfm, 2002). The resulting shape is a bowl, with continued deposition causing further depression. Naturally, the Amazon Basinis in a state of net deposition, amassing more deposition during each rainy season than is eroded during the rest of the year. In many areas, this deposition is beneficial, bringing fresh minerals from the Andes Mountains and surrounding uplands; however, this processcan also lead to an imbalance in the constituents of the soil, depositing too much sand andclay, while creating a deficiency in mud. This has repercussions for the porosity of the soil - it’s ability to retain water. Soils consisting primarily of sand have low porosity, and therefore nutrients held within water cannot be effectively stored for plant useage (Hillel, Daniel, Introduction to Soil Physics, New York, 1982, pp. 9-10)). As particle size decreases the permeability of the soil decreases while its porosity increases. As a result, clay does not effectively let water pass through its particles, but the water that does seep through is strongly retained (Hillel,Daniel, p. 192). Water and nutrients can be held within the clay layer, to an extent, but roots that penetrate into the clay layer will have difficulty trying to obtain enough water on a consistent basis to survive. As a result, the optimal medium for agriculture is a mixture of sand, silt and clay. Besides striking a harmony between porosity and permeability, a mixture also allows for greater stability within the soil than can otherwise be maintained by a single particle group. For example, sand held within your hand breaks form at the slightest pressure, while clay on riverbanks is easily and rapidly eroded. Soils of the Amazon Rainforest generally consist of fine sands, silt and clay (i.e. Particles under 0.1mm in diameter). These particle sizes decrease drastically with an increase in soil depth so that inches under the soil a predominantly clay layer will be reached (Negreiros, Gustavo, Perfis de solos da Amazônia (RADAM, EMBRAPA, SUDAM e FAO), 1997). As a result only the top few inches of soil in the Amazon (the top soil) is effectively capable of maintaining nutrients and supporting root structure. One final consequence of the particle size distribution in the Amazon Basin is the presence of “leaching.” This is the process bywhich nutrients from the upper soil layers seep down into the clay layers below. The nutrients in the clay layer are effectively “trapped” by the clay particles and are difficult to retrieve by roots.Over the course of thousands of years, natural processes of erosion and depositionhave defined the current soil profile in the Amazon Basin. The soil is characterized in terms of four layers. The top layer, or O Horizon, consists of fresh organic matter, mostlyfallen biological debris. This layer is quite thin due to high microbial activity. The next layer is the A Horizon, comprised of mineral nutrients and organic compounds. Compared to temperate soils, the tropical A Horizon is incredible small, extending down only 2-5 cm. In other soil types, the next layer, the B Horizon, is composed primarily of rocks, however, in the Amazon, few rocks actually survive the journey down from the Andes, and the resulting B Horizon is mostly soft sedimentary clay, continuing down as deep as four kilometers. The final layer, the C Horizon, is Precambrian bedrock.--- Diagram of Tropical Soil Horizons ---In examining issues concerning the physical structure of the Amazon Basin, B Horizon is of particular concern. The high clay composition makes this layer unstable, allowing
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