Project Amazonia: Characterization - Abiotic - Land

Soil

The soil is often characterized in terms of four layers. The top layer, or O Horizon, consists of fresh organic matter, mostly fallen biological debris, and is quite thin because of high microbial activity. The next layer, or A Horizon, is composed of mineral nutrients and organic compounds. The tropical A Horizon has a depth of 2-5cm, far less than that of the A Horizon of temperate soils. In other soil types, the next layer, or B Horizon, is composed primarily of rocks.  Since there are few rocks in the Amazon, the B Horizon is mostly soft sedimentary clay.  This layer may extend to 4km below the surface.  The deepest layer of soil, or the C Horizon, is Precambrian bedrock.
 

In examining issues concerning the physical structure of the Amazon Basin, the 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 difficult¹. In addition, the clay acts as 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. When exposed to rain, such as after clearing of a forested area, nutrients are further leached from the soil. This process is called eluviation, meaning “driven by the downward movement of soil water”². Without the roots of the flora to hold the soil in place and the canopy to shield the soil from heavy rainfall, the A Horizon would be washed away by surface erosion.

The current soil profile in the Amazon Basin is the result of thousands of years of erosion and sediment deposition, where more sediment is amassed during the 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 may also lead to an imbalance in the soil makeup as too much sand and clay and too little mud is deposited

This imbalance has repercussions on the porosity of the soil. Soils consisting primarily of sand have low porosity, and therefore nutrients held within water cannot be stored effectively for plant usage³. As particle size decreases, the permeability of the soil decreases and its porosity increases. Consequently, water does not effectively permeate through clay⁴. Water and nutrients can be held within the clay layer, to an extent, but roots that penetrate into the clay layer do not obtain sufficient water consistently.

The optimal medium for agriculture is a mixture of sand, silt, and clay. Aside striking a harmony between porosity and permeability, this mixture yields greater soil stability than that of homogenous soil.  Soils of the Amazon Rainforest generally consist of fine sands, silt and clay. In the Amazon particle size decreases significantly with increasing in soil depth, such that a predominantly clay layer is within inches of the topsoil⁵.   Hence, only the top few inches of soil in the Amazon are capable of maintaining nutrients and supporting root structure.

One final consequence of the particle size distribution in the Amazon Basin is the leaching of nutrients from the topsoil. Leaching causes nutrients from the upper soil layers to seep down into the clay layers below. Soil in the Amazon Basin is thus deficient in many nutrients, including K⁺, Ca²⁺, Mg²⁺, and inorganic phosphate. However, this does not imply that the Amazon ecosystem as a whole is nutrient-deficient. Unlike many other ecosystems, the nutrients of the rainforest can be found primarily in the plants.  This condition evolved from the high demand of nutrients by the biomass.  Excluding quartz, the minerals in the region’s soils have been weathered to a level which constitutes low activity. Deposition also has caused an abundance of labile aluminum to be present. Weathering and the leaching of basic compounds has resulted in acidic soils; the average pH for the Amazon Basin is between 4.17 – 4.94⁶. Although the pH remains relatively constant over time, we can see that it has effects on the biological and chemical characteristics of the soil. Because native rainforest species are adapted to these conditions, the effects of acidic pHs only become pronounced in areas where the land is adapted to other conditions.

Another important factor in characterizing soil chemical composition is Effective Cation Exchangeability (ECE). This is a measure of a soil’s ability to exchange positively charged ions with plant roots. In soils with high ECE, plants can easily exchange H⁺ for important nutrients, such as Ca, Mg, and K ions. Because the concentrations of these ions are low in tropical soils, their exchangeability is low.  For example, the exchangeability K⁺ of in the Cerado region of Brazil is 0.14 milliequivalents per 100g in the A Horizon⁷.  For comparison, an exchangeability of 0.2 milliequivalents per 100g of K+ exchangeability is considered to be a critically low level by agricultural standards⁸.
 

Sedimentology^9,10,11

Each year, the Amazon transports suspended sediment to the delta plain. On average, the sediment is composed of 1240 metric tons from Andean erosion and 3200 metric tons from flood reworked plain sediments. Sediment exchange between the flood plain and channel also deposits sediment in the rivers. The main methods of this exchange are: band erosion, bar deposition, settling from diffuse overbank flow, and sedimentation in flood plain channels. Different parts of the river exhibit different erosion and deposition patterns. In general, upstream there is sediment erosion in the main channel and deposition in the flood plain channels. This leads to what is known as "scroll bar topography," -- terrain characterized by hundreds of long narrow lakes. Oxbow lakes in such areas quickly vanish as a consequence of this process. In contrast, in further downstream areas, channels are restricted by long-term, stabilizing levee building and flood plain construction, dominated by overbank deposition. This process buries scroll bar topography, producing a flat flood plain covered by a patch work of large, shallow lakes. Such flood plains are recycled in less than 5000 years, and at even faster rates further upstream.

 

Next: Energy Flow->

1: “Where are the Rocks?” Tropical Rainforests: The Understory http://www.monogabay.com/05where_are_the_rocks.htm, 1996-2002

2: Ritter, Michael, http://www.uwsp.edu/geo/faculty/ritter/geog101/modules/soils/soil_development_profiles.html, Copyright 2001

3: Hillel, Daniel, Introduction to Soil Physics, New York, 1982, pp. 9-10

4: Hillel, Daniel, p. 192

5: Negreiros, Gustavo, Perfis de solos da Amazônia (RADAM, EMBRAPA, SUDAM e FAO), 1997

6: Negreiros, G. H. de; Nepstad, D. C. 1994, Mapping deeply rooting forests of Brazilian Amazonia with GIS, Proceedings of ISPRS Commission VII Symposium - Resource and Environmental Monitoring, Rio de Janeiro. 7(a):334-338.

7: Mendonca, Eduardo S. & Rowell, David L. (1996), "Mineral and Organic Fractions of Two Oxisols and Their Influence on Effective Cation-Exchange Capacity" Soil Science Society of America Journal, 60(6)

8: Jones, Benton J., Laboratory Guide for Conducting Soil Tests and Plant Analysis, 2001, CRC Press, Appendix E

9: “Channel floodplain geomorphology along the Solimoes-Amazon River in Brazil.”, by Leal A. K. Mertes, Thomas Dunne, Luiz A. Martinelli. From Geological Society of America Bulletin, September 1996

10: (“Exchanges of sediment between the flood plain and channel of the Amazon River in Brazil” by Thomas Dunne, Leal A. K. Mertes, Robert H. Meade, Jefferey E. Richey, Bruce R. Fursberg. From Geological Society of America Bulletin, April 1998)

11: “Channel floodplain geomorphology along the Solimoes-Amazon River in Brazil.”, by Leal A. K. Mertes, Thomas Dunne, Luiz A. Martinelli. From Geological Society of America Bulletin, September 1996.