AGY 240: INTRODUCTORY SOIL SCIENCE

1. AGY 240: INTRODUCTORY SOIL SCIENCE Lesson Title: Soils Origin and Development

2. Colorado Agricultural Education Standards: • Ag. II 10.9: The student will demonstrate an understanding of soil fertility and its effect on crop production. Colorado Science Standards: • 4.1: Students know and understand the composition of Earth, its history, and the natural processes that shape it.

3. Objectives • As a result of this lesson topic, the student will … • Define a soil body • List examples of the five soil-forming factors • Describe how soils develop • Describe the horizons of the soil profile

4. Pedology • Study of soil formation • Also known as soil genesis, and soil classification and mapping • Modern pedology dates to the eighteenth and nineteenth centuries in Germany, the United States, and especially Russia

5. The Soil Body • Pedon • Section of soil extending from the surface to the depth of root penetration, but generally examined to a depth of 5 feet. • Generally, a pedon has dimensions of about 1 meter by 1 meter, and about 1.5 meters deep (about 3 feet × 3 feet × 5 feet). • Polypedon: collections of pedons  that are much the same 

7. Weathering • Physical: the disintegration of rock by temperature, water, wind, and other factors. Frost wedging occurs when water freezes and expands in rocks or in cracks in the rock, causing it to break apart. • Chemical: dissolution, oxidation-reduction, hydrolysis, and hydration

8. Dissolution. Some minerals simply dissolve slowly in water, as in the dissolution of gypsum Hydrolysis, minerals react with the hydrogen that is in the water molecule, splitting the water apart. Hydration also involves water, but here the water molecule itself joins the crystalline structure of the mineral, again creating a softer, more easily weathered material

9. Oxidation-reduction and other reactions are also important in chemical weathering. • Plants also play an important role in rock crumbling. • Roots can exert up to 150 pounds per square inch of pressure when growing into a crack in rock. • Root wedging from the pressure pries apart stone. • Lichens growing on bare rock form mild acids that slowly dissolve rock. 

10. Rocks and Minerals • Original source of most soils is rock • Unweathered material of the earth’s crust • Solid rock breaks into smaller particles (i.e., parent materials) • Rock types • Igneous rock • Sedimentary rock • Metamorphic rock

11. Rocks and Minerals (cont-) • Igneous Rock • The basic material of the earth’s crust is igneous rock, created by the cooling and solidification of molten materials from deep in the earth. Igneous rocks, such as granite, contain minerals that supply 13 of the 17 required plant nutrients  • Granite, a coarsely grained rock that weathers very slowly to sandier soils.

12. Sedimentary Rock • Igneous rock comprises only about one-quarter of the earth’s actual surface, even if most of the crust is igneous. • This is because sedimentary rock overlays about three-quarters of the igneous crust. • Sedimentary rock forms when loose materials such as mud or sand are deposited by water, wind, or other agents, slowly cemented by chemicals or pressure into rock.

13. Metamorphic Rock • If igneous and sedimentary rocks are subjected to great heat and pressure, they change to form metamorphic rock. • For instance, limestone is a fairly soft, gritty rock. When subjected to heat and pressure, it changes to marble, which is harder and can be cut and polished. • Sedimentary rocks tend to become much harder when metamorphosed, becoming slower to weather. • Sandstone, for example, changes to the far harder rock quartzite.

14. Figure below shows gneiss, a form of metamorphosed granite.

15. Parent Material • Soil genesis • This is the process of creating soil from parent material • Residual soils which are formed in place from the residuum of broken-down bedrock, are actually less common than soils of parent materials carried from elsewhere by such pervasive agents of transport as wind, water, ice, or gravity.  • Residual soils form slowly, as solid rock must be weathered first • Transported soils develop from already weathered material, so they develop more quickly.

16. Major parent materials of soils of the United States.

17. Agents of transport and parent materials • Glacial ice: glacial drift • Glacial ice carried parent materials over the northern part of North America during numerous glacial periods over the past 2 million years. • Glaciers that expanded out of Canada crushed and ground the earth; picked up and transported clay, sand, rocks, and other materials; and deposited them elsewhere to become the parent materials of new soil. These deposits are termed glacial drift • Glaciers deposited materials in many ways, so there are several kinds of glacial drift. During the melting process, some debris simply dropped in place to form deposits called glacial till. • Some till dropped at the margins of the glacier, forming hills called moraines.  • Because there was no sorting action in the deposition, glacial till is extremely variable, and so are the soils derived from it. • Till soils often contain pebbles, stones, and even boulders.

18. Wind: eolian deposits are wind deposited soil material , mostly silt and fine sand • The wind-deposited silt  (medium particles) are know as loess soils. — They are important agricultural soils and are found in much of Iowa, Illinois, and neighboring states. Loess often blankets other materials, so often forms the upper parts of a soil. • Water: alluvial soils are soils whose parent materials were carried and deposited in moving freshwater to form sediments 

19. Water-and marine-deposited soils. Floodplains form along rivers from materials deposited during flooding. Alluvial fans are deposited at the base of slopes by running water. Deltas form when smaller particles drop out as a river enters an ocean. River terraces are old floodplains left above a new river level.

20. Gravity: Colluvium are parent materials that move simply by sliding or rolling down a slope • An example of a colluvial material is a talus—sand and rocks that collect at the foot of a slope • Avalanches, mudslides, and landslides are other examples.

21. Volcanic • The ash blown out of a volcano and deposited nearby or carried some distance by wind forms a chemically distinct, dark, and lightweight parent material. • The Pacific Northwest, Hawaii, and Alaska are areas of the United States where such deposits are common. • Organic • Organic soils, containing 20 percent or more organic matter, form underwater as aquatic plants die • Organic soils are extensive in Minnesota, Wisconsin, Florida, Michigan, and Alaska.

22. Climate • Extremely complex • We are speaking primarily of temperature and precipitation • Terms • Arid, semiarid, and humid • Climates which, in order, experience very low, low, and higher rainfall

23. Organisms • Actively affect soil formation • Organisms that live in soil: plants, insects, and microbes • Mineral soils having the highest organic matter content • Form under grasslands • Grassland vegetation, mostly herbaceous, forms a deep, dense mat of fibrous roots

24. Topography • Slope • Position • Length • Moisture retention • Slope aspect – The direction the slope is facing • Solar energy • Wind exposure

26. Time • Important considerations • Aging -Soils change over time • Weathering -Weathering of the young soil continues, and many generations of plants live and die, so the young soil becomes deeper and higher in organic matter. • Biological processes and Leaching -As soils age, biological processes tend to increase the nitrogen content, while leaching tends to reduce phosphorus.  • Soil development is progressive when it becomes deeper and more complex with more and better defined layers. • But if the combination of soil-forming factors changes, soil can also regress, becoming shallower and less complex. • Very old soils may have undergone repeated cycles of progression and regression.

27. Humans • Human activity modifies soil • Land use • Pollutants • Fertilization • Waste disposal • Cultivation

28. The Soil Profile • Soil profile is the vertical section of a soil through all its horizons, ending in the parent material • Soils change over time in response to their environment, represented by the soil-forming factors • Soil forming factors • Additions -Materials may be added to the soil • Losses -Materials may be lost from the soil • Translocations-Materials may be moved within the soil • Transformations -Materials may be altered in the soil

29. The Soil Profile (cont’d.) • Soil horizons • O: organic layer • A: topsoil • E: eluviation • B: subsoil • C: parent material • R: bedrock • The A, B, and C horizons are known as master horizons. They are part of a system for naming soil horizons in which each layer is identified by a code: O, A, E, B, C, and R.

30. The Soil Profile (cont’d.) • Subdivisions of the master horizons • Between master horizons in position and properties • Transitional layers • Identified by the two master letters, with the dominant one written first

33. Summary • This chapter reviewed several topics • Soil body • Rocks and minerals • Parent material • Climate • Topography • Time • Soil-forming factors • Soil profiles