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Into the Depths

Into the Depths. Introduction to Agriculture, Food, and Natural Resources. Unit 3 – Lesson 3.4 The Whole Soil. Soil Layers. Horizons vary depending upon the make up of Particle sizes How the particles are arranged Color Parent Material Drainage Organic Matter Content.

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Into the Depths

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  1. Into the Depths Introduction to Agriculture, Food, and Natural Resources Unit 3 – Lesson 3.4 The Whole Soil

  2. Soil Layers Horizons vary depending upon the make up of • Particle sizes • How the particles are arranged • Color • Parent Material • Drainage • Organic Matter Content Soil is typically formed over time in distinct layers called horizons.

  3. Soil Profile O – organic A – topsoil B – subsoil C – parent material R – bedrock Vertical section of soil consisting of soil horizons United States Department of Agriculture, Natural Resources Conservation Service. (2010). A soil profile. Retrieved from http://soils.usda.gov/education/resources/lessons/profile/

  4. Horizons • Organic (O) • Also called litter layer • Comprised of whole or partially decayed organic matter • Topsoil (A) • Dark in color • Comprised of decayed organic matter and aged mineral matter • Best environment for root growth

  5. Horizons • Subsoil (B) • Lighter in color • Mostly mineral matter, little organic matter • Tend to be high in clay • Parent material (C) • Parent material, soft, weathered bedrock • Bedrock (R) • Un-weathered

  6. Soil Color • Influenced greatly by organic matter and iron • Dark colors indicate high levels of humus (organic matter), typically fertile soils • Gray indicates wetness or no organic matter • Brown and red indicate presence of iron, typically are well-aerated

  7. Soil Structure When soil particles cling together in an arrangement known as a ped, the shape of the ped is called structure. Two considerations must be made for soil structure: • Structure type • Structure grade

  8. Structure Type The shape of the ped formed is called structure type. Granular – roughly spherical, like grape nuts. Usually 1-10 mm in diameter. Most common in A horizons. Platy – flat peds that lie horizontally in the soil.

  9. More Defined Structure Types Blocky – roughly cube-shaped, with more or less flat surfaces. Blocky structures are typical of B horizons. Prismatic – larger, vertically elongated blocks, often with five sides.

  10. Structureless Types Massive – compact, coherent soil not separated into peds of any kind. Single grain – in very sandy soils, every grain acts independently. Structureless soils are not ideal for most crop use.

  11. Structure Grade • How distinct the shape of the ped is. • How well the ped holds the shape. • Four grades define soils: • Strong • Moderate • Weak • Structureless

  12. Effects of Structure Good structure, or strong structure, means good aeration because of ample pore space. Organic matter is important for forming soil structure. • Increases porosity • Contributes to water holding capacity

  13. Effective Depth Relative to structure is a soil evaluation category called effective depth. Effective depth is the zone in which plant roots can easily grow.

  14. References Huddleston, J. H., & Kling, G. F. (1996). Manual for judging Oregon soils. Corvallis, OR: Oregon State University. Parker, R. (2010). Plant and soil science: Fundamentals and applications. Clifton Park, NY: Delmar. Plaster, E. J. (2003). Soil science & management (4th ed.). Clifton Park, NY: Delmar.

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