Soil Lessons:Soil Science Society America

Color Changes in Soil Instructional Value Soil chemistry - Soil Morphology Dig Grade Level: 6+ grade Deeper Type of Lesson Summary If air (O2) is in the soil the soil is aerobic a. Initial demonstration The students will learn that soil color is only • 4e- + O2 + 4H+ à 2H2O – rusty or oxi- b. experiment with different skin deep so to speak. The major color- dized color persist materials ing agent is Fe-oxides or rust. It occurs on If all O2 is removed soil becomes anaerobic Materials Needed the surface of particle just as the color on (saturation occurs) an M&M is only on the surface. The color • Denitrification Red, yellow or brown soil can be removed or dissolved off in water. o 10e- + 12H+ + 2NO3 à N2 + 6H2O – Some moist topsoil or wetland soil Once removed it is unlikely for the color no color change 2 or more Pint Mason Jars with lids to return. So the particles that have no • Iron (Manganese) Reduction Water coatings help to tell a soil scientist where o 2e- + 6H+ + Fe2O3 à 2Fe(II) + 3H2O Sugar the soil has been saturated. The rate of the – soil turns gray Additional carbon sources (hay, saw reaction varies due to the type of organic • Sulfate Reduction dust, leaves). All should be coarsely materials present, the type of materials and o 8e- + 10H+ + SO4 à H2S + 4H2O – ground. the amount of Fe-oxide coatings. rotten egg odor Autoclave or water bath Why is this subject/activity important You should see little to no changes in the au- – Soil color is used to identify where the Time toclaved sample. This is due to the microbes water table is in the soil. This is used for being killed by the heat. In the other jars Teacher prep time – 30-60 minutes many types of building projects. Student class time – 30-60 minutes bubbles will start to form and over time the initially and then observation over days soil will turn gray. This may take longer in to weeks. Background Information some soils due to the amount of iron, food, and microbes present in the soil. Soil color can tell us a lot about the soil Student Learning Objectives and the environment. Dark black top soil Other variables that can be tested are a. To explain why soils are colored indicates high organic mater contents as exposure to sunlight (little affect), placing b. To be able to show that soil color compare to light brown colors. The pres- some jars in a refrigerator (slower reac- is a coating ence of gray colors in the soil is used to de- tion), or placing jars in a warmer location c. To explain that water can change termine the water table depth. This is done (faster reaction). the color of the soil when assessing the site for many land uses Go outside and dig in the soil. From the d. To determine how fast color can related both to agriculture and well as ur- colors you observe, can you make a predic- changes based on several factors ban/suburban development. Often the gray colors are referred to as wetness mottles or tion on where the water table is (saturated and reduced – anaerobic conditions)? Keywords redoximorphic features (formed from reduc- tion and oxidation chemical reactions in the Once the gray colors due to Fe3+ reduction soil color, iron oxides, rust, quartz, soil). Many land use decision are based on and removal have formed it is unlikely that feldspar, oxidation, reduction, redox, these colors and the fact that they do not the particles will become coated again. anaerobic, aerobic change season to season. Thus in summer Since this reaction occurs in saturated and when water tables are deep gray colors reduced or anaerobic conditions the pres- indicate how high the water table will rise ence of gray colors indicates where the during the wettest time of the year. water table is in that soil. This helps soil The color change from red (rusty) to gray scientist identify wetland or hydric soils and observe in the soil is due to reduction and locate suitable soils for septic systems and oxidation of Fe. This process occurs in soil related land use. This reaction only occurs but in saturated soil this occurs when Fe3+ if there are sufficient numbers of microbes is reduced to Fe2+ due to a microbial medi- and a food source (carbon) present to cause ated redox reaction. anaerobic conditions to occur. 11.  10.  Methods/Procedure dation reactions Redox Reduction –gainofelectrons Oxidation Aerobic –oxygenpresent Anaerobic –nooxygenpresent saturated soil water willflowintoaholedugin Saturation (saturated) – waterinallpore; soil issaturated Water table–depthinthesoilwhere Fe-oxide –rust Vocabulary Color ChangesinSoil Soil Lessons:ScienceSociety America 4. 9. 8. 7. Shaketomixwell. 6. Putlidsonjars. 5.  3. Closeandshaketomix 2. Add a½teaspoontopsoil 1. www.soils4teachers.org Additional Resources are tight. Allow thejartocool–makesurelids experimental variable constitute anadditionaltreatmentor etc. aswell.Eachoneofthesewould – sawdust,cornsyrup,grassclippings could alsousedifferentcarbonsources but itisnotnecessaryatthispoint. You spoons ofsugar. You maywanttomix Remove lidandaddatleast2table USDA methodologyforhomecanning. canning fruitorvegetablesfollowingthe process thejarinawaterbathasif Note: Ifanautoclaveisnotavailable the instructionsforautoclavingliquids. Take 1jarandautoclaveitfollowing inch oftop. Fill eachjartowithinabout¼1/8 Fill eachjarapproximately½waywithsoil. changes andbubbles. Record observation,particularlycolor over thecourseofafewweeks. Let thejarssitandobservechanges –combinationofreductionandoxi –lossofelectrons

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sample) tocomparecolors. variable. Usethecontrol(autoclaved The degreeofcolorchangewillbe hours orcouldtakeseveralweeks. The reactionscouldoccurinaslittle72 is compromised. prevent oxygenfromenteringifjarseal The jarscanbeplacedinawaterbathto tion wheretheywillnotbedisturbed. reactions occur. Keep alljarsinaloca There willbesomegasleakageasthe good placeto colors nearthesurfacewouldthisbea So ifyoudugaholeandsawgray ately Why doesthechangenotoccurimmedi rate ofchange How doesthetypeofcarbonaffect likely toreturn – yes,oncethecolorisremoveditnot gray evenwhenthesoilisnotsaturated Will thegraycolors inthesoilstillbe get saturatedtothatdepth soil whatcanthattellyou–themay If youseegray(uncoated)colorsinthe change back–itisnotlikely Discuss ifitislikelythatthecolorwill washed offordissolved Discuss whythecolorchanges–itis under pressure. of theexperimentascontentscouldbe Open thejarsverycarefullyatend i.  ii. digapond–yesitmaybewet build ahouse–Noitmaybewet - - www.soils.org Soil ScienceSocietyof America

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