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  1. Soils

  2. What is soil? • Thin layer on the earth’s surface that is made by the interaction of five factors: rocks, sunlight, water, air, and living organisms

  3. Soil Composition The average soil is mainly composed of mineral particles that come from rocks. Below is the composition of an average soil. Taken from

  4. Soil Formation/Composition • The formation of 1 meter of soil can take from 100 – 100,000 years to form (depending on the conditions present) • Climate affects rate • Higher temp. and more rain = faster soil formation • Rain provides water for chemical reactions to occur and warmer temp. increases speed of reactions

  5. Soil formation/Composition • Rocks are the “parents” of the soil • Parent material affects the kind of soil that is formed • Different rocks contain different minerals • Color of soils is partially dependent on the minerals the soils contain

  6. Soil formation/Composition • Weathering – the process of breaking rocks apart or removing minerals from them. • Weathering agents can be physical and chemical • Weathering agents include water, ice, wind, temperature changes, sand, glaciers, and plant roots

  7. Soil formation/CompositionWeathering • Temp. changes cause rocks to expand and contract cracking rocks and releasing minerals • Water in cracks can turn to ice further splitting rocks • Roots can act as physical wedges. They can also produce chemicals that dissolve minerals from the rocks • Sand and rocks carried by moving water scour the soil and rocks beneath • Glaciers that carry much bigger rocks can file and scrape rocks below • Wind-blown sand acts like a sandblaster on the surfaces of rocks

  8. Soil formation/CompositionChemical Changes in Rocks • Equation for ferric oxide reaction (rust) Significance of reaction:

  9. Soil formation/CompositionChemical Changes in Rocks • Equations for Carbonic acid formation and calcium and bicarbonate ion formation: • Significance of reactions:

  10. Soil Composition • The average soil consists of 25% air and 25% water. • Water and air are extremely important -for plant roots -for many of the bacteria, protists, fungi, and animals that live in the soil -All of the organisms that live in the soil interact together to form the soil ecosystem

  11. Soil CompositionOrganic Matter • An average soil contains 5% of organic matter • Although organic matter is only a small percentage of the overall soil it is extremely important • In an average soil, 80% of the organic matter is humus, 10 % is roots, and 10% is organisms

  12. Humus • Humus-partly decomposed organic matter that was once living or was produced by a living thing. • Provides nutrients to many organisms • Increases water-holding capacity of soil • Provides nutrients to plants

  13. Image taken from

  14. Taken from

  15. Soil texture • Mineral particles in soil are classified by size • Size of particles determines soil texture • See soil texture chart on page 213

  16. Soil Texture

  17. Soil Triangle

  18. Spaces in soil • Normally 40-60% of volume of soil is pore space • Important for water and air to travel through soil • Determine infiltration and percolation rates • Size of spaces is dependent on texture • Smaller particles=smaller spaces • Larger particles=larger spaces

  19. Key terms • Soil texture – size of mineral particles • Infiltration – the ability of water to soak into the soil • Water holding capacity – the ability of soil to store water • Aeration – the ability of air to move through the soil

  20. Soil Comparison *Loam is considered an ideal soil for many crops due to its mix of different soil particles (sand, silt, and clay)

  21. Soil Structure • Structure-when individual particles are “glued” together to form larger pieces • The “glue” is produced by organisms in the soil • Granular structure-rounded clumps with a diameter of less than 1.5 cm • Crumb structure-irregular shaped clumps • Platy structure-soil particles glued together into thin horizontal plates

  22. Soil Profile (in descending order) • Topsoil-top layer of humus rich soil • Subsoil-layer of soil beneath the topsoil (does not contain humus) • Parent material –the pieces of rock that lie on top of solid rock • Bedrock-solid rock

  23. Soil Conservation • Erosion-the process in which wind or water move soil to new locations • Erosion occurs naturally but can also be caused by human activities • To much erosion is bad for farming • Displaced soil can also be a major pollutant of aquatic ecosystems

  24. Some causes of erosion • Overgrazing by livestock • Repeated years of monocultures • Plowing land • Construction • Surface mining • logging

  25. Preventing erosion • Contour planting-planting across a slope (rather than up and down it) • Strip cropping-strips of close growing plants are planted next to crops that are planted in rows • Diversion terraces-ridges of soil that are constructed along the contours • Waterways-wide ditches that are planted with a permanent grass cover

  26. Contour planting

  27. Strip Cropping

  28. Diversion Terraces

  29. Waterways

  30. Preventing erosion • Crop rotation • Windbreaks • Conservation tillage-methods to reducing the amount of tilling and avoid the use of the moldboard plow • Cover crops-grasses or legumes planted to hold soil in place

  31. Crop Rotation

  32. Windbreaks

  33. Conservation Tillage

  34. Cover Crops

  35. The dust bowl • Dust bowl- Area in the southern part of the great plains which experienced massive amounts of soil erosion during the 1930s • Caused by poor farming practices combined with drought and winds • As a response the federal government created several agencies and policies to prevent soil erosion