Wk10_Module10B_SoilStructureMoistureFormation

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University of Cincinnati, Main Campus *

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108

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Geography

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Oct 30, 2023

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(Slide #1) Hello Class! In this short module we’ll talk about soil structure, moisture, layering, and formation. As you hopefully started to realize with the previous module, in which we introduced SOIL, there really is a LOT going on under the SURFACE of our Earth! (Slide #2) The term “Structure” refers to the way that soil GRAINS CLUMP together. The CLUMPS in soil are called PEDS. Particles in Soil are bound together by the colloids in the soil to create PEDS. Some soils have small granular peds, other soil has large blocky peds. Small peds make soil grainy or crumbly and large peds give a blocky structure to the soil, which is good for cultivation of crops. Soil with a high content of CLAY lacks peds. (Slide #3) Now soil and moisture go hand-in-hand. The amount of moisture in soil is KEY to whether the soil is good for vegetation and for agriculture. Soil gets moisture directly from rain, and in some regions it gets moisture by flood waters or by melting snow. In Egypt’s Nile Valley, for instance, rain seldom falls, but the Nile River regularly floods out of its banks due to rains and snow in a high mountainous plateau region of eastern Africa, far to the south, which FEED the Nile River. So it is this flood water that brings sufficient MOISTURE to the soil to make agriculture possible. We can ARTIFICIALLY deliver moisture to the soil in the form of irrigation canals. Water sinks into the soil, or it runs off if the soil can’t absorb it. Either way, it eventually gets back to the Earth’s HYDROSPHERE – ground water, streams, rivers, and eventually back to the sea. How does moisture STAY in the soil? What prevents it from all draining down by gravity? Well, moisture clings to soil particles by a force called CAPILLARY ACTION. That’s the same force that allows water droplets to remain on the side of a glass without running down. Soil texture gives it its STORAGE CAPACITY, in other words, its ability to retain water even after the excess water has drained away.

(Slide #4) Plants depend on soil’s ability to hold moisture, especially in conditions of drought. As soil is less able to hold moisture, plants WILT more quickly in conditions of drought. In soil that can hold MORE moisture, plants have a higher WILTING POINT. In the area of southwest Ohio where I live, the soil has a lot of CLAY, which holds a lot of water and holds it very TIGHTLY. So a few summers ago when there was a bad drought and it barely rained at all from May to October, most plants did alright because the soil was able to hold on to moisture. (Slide #5) In agricultural areas, soil scientists actively monitor the BALANCE between soil and water, because it is so important in determining the success of agriculture. Soil loses water through EVAPOTRANSPIRATION, which refers to the sort of EXHALATION of moisture through the LEAVES of PLANTS. Plants DRAW water up through their root systems, then some of that moisture is RELEASED back into the atmosphere by the plant – and we all that EVAPOTRANSPIRATION. Also soil loses moisture when water PERCOLATES down through the soil and back into the GROUND WATER ZONE. (Slide #6) OK, let’s move on to the LAYERS found in SOIL. Soil forms in DISTINCT LAYERS, which we call HORIZONS. It’s easy to see why they’re called HORIZONS, since they lie in HORIZONTAL form. Soil HORIZONS are layers that DIFFER in physical or chemical composition, in organic content, or in structure. (Slide #7) In soil science, we talk about a SOIL PROFILE, which refers to a CROSS SECTION of the horizons present in the soil. Taking a SOIL PROFILE allows us to better analyze the soil. Soil scientists identify the specific HORIZONS using these LETTERS. The topmost “O” horizons are organic debris, in various stages of decomposition. The next “A” horizon is the layer of a lot of mineral and organic mixing, and a very ACTIVE layer for biological processes. The fact that it is ACTIVE is why it’s called the “A” horizon. The “E” horizon is a lighter colored layer from which clay, organic matter, and mineral oxides have been removed to the

horizons above and below. The next horizon is the “B” horizon, where a lot of mixed material has accumulated. Beneath the B horizon is the “C” horizon, which is the PARENT MATERIAL of the soil – the WEATHERING ROCK or REGOLITH. Finally, we have solid BEDROCK at the bottom. The DEPTH of all these layers varies greatly, from a few inches in some areas to many feet in others. (Slide #8) This image from your book shows a soil profile from a sandy area on the New England coast. The “A” horizon is very sandy, and you can see the pale E horizon beneath it, and the reddish B horizon on down under that. (Slide #9) So now I want to talk about the processes of SOIL FORMATION. There are FOUR types of soil-forming PROCESSES. The first is ENRICHMENT – in which either ORGANIC or MINERAL components are INCORPORATED into the soil. The second is REMOVAL, in which material is taken OUT of the soil, by LEACHING or by EROSION. The third is TRANSLOCATION, in which particles are transported DOWNWARD through the soil horizons to accumulate in the LOWER horizons. And the fourth is TRANSFORMATION, in which components in the soil CHANGE composition – so minerals oxidize from PRIMARY to SECONDARY, or organic material DECAYS. (Slide #10) This diagram from your book is meant to illustrate two of the those processes, namely, TRANSLOCATION and TRANSFORMATION. You can see how TRANSLOCATION occurs, the movement of particles DOWNWARD towards the LOWER soil horizons. Also, the material in soil is actively under TRANSFORMATION, both mineral and organic. (Slide #11) This diagram indicates to us the ways in which soil is MOVED AROUND on the surface of the Earth. This happens by WIND, by overland FLOW, and by FLOODING. Wind can carry fine material from one place to another and plays a role in soil formation. We know that the Earth’s surface is seldom exactly LEVEL, so GRAVITY plays a role in transporting soil from higher areas to lower

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ones. And FLOODING, as I have already mentioned, plays an important role in soil formation, eroding soil from one area and depositing it in another. (Slide #12) Finally, the TEMPERATURE of soil plays a big role in how soil develops. Why? Because of the process of how ORGANIC MATERIAL DECOMPOSES. As we know, warmer temperatures HASTEN decomposition. So in COLD CLIMATES, organic matter decomposes very SLOWLY, so organic debris ACCUMULATES without a lot of DECOMPOSITION. But in WARMER climates, organic matter decomposes RAPIDLY. In the WARMEST climates, such as EQUATORIAL climates, organic matter decomposes SO rapidly that there is very little organic matter left in the soil at all, so it is less RICH for agriculture. In a lot of warmer climates, such as in the tropics, agricultural traditions have developed by which ORGANIC matter is ADDED to the soil in the form of ORGANIC fertilizers – such as MANURE from various animals, or even from human waste. This adds organic matter to the soil and makes it more productive for cultivation. I like this image because it gives you a sense of just how ACTIVE the soil layer can be – with living creatures of various kinds, along with plant roots and bacteria and fungi, all in the active “A” horizon of the soil. MIDLATITUDE soils are so productive precisely because the range of temperature throughout the year helps REGULATE the decay of ORGANIC material. (Slide #13) In 1972 when I was looking across this expanse of dry eroded soil in the short-grass prairie region of western South Dakota, I don’t think I was thinking about the structure, moisture, layering, or formation of the soil. But maybe the man on the RIGHT, the man in the cowboy hat, WAS thinking about those things! That guy was my “COUSIN WALTER,” a real-live COWBOY who homesteaded that land with his family in 1912 when he was just a small boy, and lived on it for the rest of his life. He had a ranch. You saw an image of his CATTLE in an earlier module. When settlers first came to that region, they didn’t know what the soil would be like.

But they quickly learned that the soil was not suitable for growing crops and that it didn’t hold moisture very well. The nature of the SOIL determined how they made a living on the land. My “COUSIN WALTER” struggled to make a living on the land, and raising cattle was really his only option. It was the SOIL that made all the difference. So that’s it for this short module! In the next module, we’ll look at some different specific soil TYPES around the world. Thanks for your attention!