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Understanding Soil Layers and Structure in Agriculture

Explore the composition and characteristics of soil horizons, from organic matter to bedrock, affecting plant growth. Learn about soil color, structure types, and the importance of good soil structure for crop productivity.

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Understanding Soil Layers and Structure in Agriculture

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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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