Soils

1. Soils • Soil is formed through the interaction of climate, parent material, topography, vegetation, living organisms and time. • Solid portion of soil consists of minerals and organic matter. • Pore spaces between solid particles filled with air or water.

2. Soils • Soils divided into horizons: • Topsoil (10-20cm) • A horizon - Dark, with more organic material than lower layers • E horizon - Light • B Horizon (0.3-0.9m) - Subsoil • More clay, lighter in color than topsoil • C Horizon (varies) - Parent material which extends to bedrock

3. SoilsParent Material • Parent material - Rock that has not been broken down into smaller particles • Rock types: • Igneous – Volcanic • Sedimentary - Deposited by glaciers, water or wind • Metamorphic - Changes in igneous or sedimentary rocks from pressure or heat

4. SoilsClimate • Climatevaries throughout the globe, as does its role in weathering of rocks • Deserts - Little weathering by rain, and soils poorly developed • In areas of moderate rainfall - Well-developed soils • Areas of high rainfall - Excessive water flow through soil leaches out important minerals.

5. SoilsLiving Organisms and Organic Composition • In soil there are many kinds of organisms, roots and other plant parts. • Bacteria and fungi decompose organic material from dead leaves, plants and animals. • Roots and other living organisms produce carbon dioxide, which combines with water and forms acid that increases the rate at which minerals dissolve. • Small animals alter soil by their activities and by their wastes. • Humus - Partially decomposed organic matter, gives soil a dark color

6. SoilsTopography • Topography- Surface features • Steep areas: • Soil may erode via wind, water or ice. • Flat, poorly drained areas: • Pools and ponds may appear. • Development of soil arrested. • Ideal topography permits drainage without erosion.

7. SoilsSoil Texture and Mineral Composition • Soil Texture - Relative proportion of sand, silt and clay in soil • Sand - Many small particles bound together chemically • Silt - Particles too small to be seen without microscope • Clay - Only seen with electron microscope • Individual clay particles called micelles • Negatively charged and attract, exchange or retain positively charged ions, such as Mg++ and K+

8. Soils • Best agricultural soils - loams composed of 40% silt, 40% sand and 20% clay • Coarse soils drain water too quickly. • Clay soils allow little water to pass. • Soil Structure - Arrangement of soil particles into aggregates • Productive agricultural soils are granular with pore spaces occupying between 40-60% of the total soil volume. • Particle size is more important than total volume.

9. SoilsWater in the Soil • Hygroscopic Water - Physically bound to soil particles and unavailable to plants • Gravitational Water - Drains out of pore spaces after a rain • Capillary Water - Water held against the force of gravity in soil pores • Determined by structure and organic matter, by density and type of vegetation, and by the location of underground water tables • Plants mostly dependent upon this type.

10. SoilsWater in the Soil • Field capacity - Water remaining in soil after water drains away by gravity • Determined by texture, structure and organic content of soil • Permanent Wilting Point - Rate of water absorption insufficient for plant needs • Plant permanently wilts. • Available Water - Soil water between field capacity and the permanent wilting point

11. SoilsSoil pH • Affects nutrient availability • Alkalinitycauses some minerals, such as copper, iron and manganese to become less available. • Counteract by adding sulfur, which is converted to sulfuric acid by bacteria, or by adding nitrogenous fertilizers • Acidityinhibits growth of nitrogen-fixing bacteria. • Counteract by adding calcium or magnesium compounds = liming