Characterization of the Land of the Amazon Rainforest Last Updated November 6 2002 Table of Contents 1 Introduction 2 2 Physical Properties 2 3 Chemical Properties 5 3 1 Chemical Cycles 5 3 2 Chemical Composition 7 3 3 Biological Components of Soil 8 4 Land Use 10 4 1 Agriculture and Ranching 10 4 2 Mining 13 1 Introduction The 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 Properties The 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 Basin is 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 process can also lead to an imbalance in the constituents of the soil depositing too much sand and clay 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 Amaz nia 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 by which 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 deposition have 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 mostly fallen 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 sediment to undergo compression in severely irregular patterns causing large scale construction projects in the Amazon to be extremely difficult Where are the Rocks Tropical Rainforests The Understory http www monogabay com 05where are the rocks htm 1996 2002 In addition the clay is a sealant resulting in virtually no osmosis of soluble ions or nutrients Thus no nutrients can be stored below the thin A Horizon making the preservation of the topsoil extremely critical in the continued viability of the soil Because of these factors erosional effects on tropical soils are more pronounced When exposed to rain such as after clearing of a forested area nutrients are leached from the soil This process is called eluviation and is driven by the downward movement of soil water Ritter Michael http www uwsp edu geo faculty ritter geog101 modules soils soil development profiles html Copyright 2001 Without the roots of the flora to hold the soil physically in place and the canopy to shield the soil from heavy rainfall the A Horizon can also be washed away by surface erosion Thus even with agricultural plant cover nutrients are depleted by these physical forces after only a few seasons 3 Chemical Properties 3 1 Chemical Cycles To maintain a chemical equilibrium in any soils there must
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