Wk10_Module10A_NatureOfSoil

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School

University of Cincinnati, Main Campus *

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Course

108

Subject

Geography

Date

Oct 30, 2023

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doc

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(Slide #1) Hello Class! In this short module we’ll get started on our topic for this week, which is SOIL. I want to start with some introductory material on the NATURE of the soil: its mineral and organic COMPONENTS, its STRUCTURE, its ACIDITY and ALKALINITY, and so on. (Slide #2) SOIL is the uppermost layer of the LITHOSPHERE, or land surface of the EARTH. As we look at the LAYERS present on the Earth’s surface, we wee first VEGETATION. Beneath vegetation is what we call LITTER, which is the bits of DECAYING vegetation. Litter represents vegetation that is on its way to becoming SOIL, which is the NEXT layer. Next is what we call REGOLITH, which is the ROCKY material from the underlying BEDROCK that is in the process of WEATHERING – breaking down and becoming part of the SOIL. So our VERTICAL layers from surface on down are VEGETATION, LITTER, SOIL, REGOLITH, and BEDROCK. Human life DEPENDS on soil, BECAUSE we depend on the edible material that is PRODUCED from soil. And all PLANTS depend on soil. It is through SOIL that plants get their NUTRIENTS, their WATER, and their PHYSICAL SUPPORT. Soil development hinges on several factors: the cover VEGETATION of the area, the CLIMATE, the PARENT MATERIAL in the LITHOSPHERE of the Earth underlying any particular AREA, and finally, TIME. Soil requires TIME to develop. Soil contains ALL the FORMS of matter: liquid, solid, and gas. In addition to solid particulate material in soil, there is LIQUID water in soil and also GASES contained within soil. Some soil is lighter and more air- filled, and some is more compact with less gases. The solid components in SOIL include BOTH mineral AND organic material. (Slide #3) Soil varies in COLOR and TEXTURE. ORGANIC material in soil is called HUMUS, and soil that is RICH in HUMUS is DARK. For instance, the soil of the tall-grass PRAIRIE is DARK and RICH in HUMUS, from all that decaying PLANT material that accumulates. OTHER colors in soil – reds and browns, for

instance – can be caused by MINERAL components. IRON OXIDE gives soil a REDDISH cast. All soil is made up of PARTICLES of a RANGE OF SIZES. Not counting the LARGEST particles – GRAVEL, which are really OBJECTS not PARTICLES – we divide SOIL particles according to SIZE: from SAND to SILT to CLAY. The smallest particles are the almost microscopic particles called COLLOIDS. SOIL varies widely in TEXTURE. What gives SOILS their various TEXTURES are the proportions of the different sized PARTICLES – sand, silt, and clay. We see on this slide a set of PIE GRAPHS showing the makeup of variously textured SOILS. (Slide #4) I mentioned COLLOIDS, which are the FINEST soil particle. COLLOIDS are important to any discussion of SOIL, because it is COLLOIDS that attract BASE ELEMENTS to soil and it’s from BASE ELEMENTS that soil derives it’s nutrient content that it imparts to PLANTS. So in order to understand how SOIL comes to have NUTRIENTS, we have to talk about COLLOIDS. MINERAL COLLOIDS are FLAT CLAY PARTICLES. These FLAT particles are ENVELOPED in NEGATIVE IONS, and they attract a LAYER of POSITIVELY CHARGED IONS of FOUR key BASE ELEMENTS: CALCIUM, MAGNESIUM, POTASSIUM, and SODIUM. These are the four base elements needed for PLANT GROWTH. It is the presence of COLLOIDS that allow these essential ELEMENTS to BOND to the soil. WITHOUT them, the base nutrients would be carried away and lost. (Slide #5) ACID ions are ALSO present in soil – namely HYDROGEN and ALUMINUM. These ACID ions CAN replace the nutrient base elements, binding to the colloids INSTEAD of the base elements. If that happens, then the BASE ELEMENTS are washed downward BELOW the level of plant roots, placing them out of reach of plants. Acidity of soil CAN be corrected – gardeners and farmers do this all the time by applying special COMPOUNDS. Soil ACIDITY – or the opposite – ALKALINITY, varies from place to place. Moist cool regions with CONIFEROUS

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