Forests | Geology | Rangeland | Soils | Water | Wildlife

Course Goals

Students will gain understanding of the complexity of soils as a physical and biological system providing a basic resource for above ground biological systems. Students will learn a variety of characteristics used to describe soils and differentiate different soil types. Students will visit several different soil types and learn how each have unique characteristics that strongly influence a wide variety of management decisions.

Topics Covered

Soils as complex living Systems

  • Soil is not lifeless dirt but rather is an incredibly complex, living, breathing system!
  • Soils take in oxygen and expel carbon dioxide.
  • Water passes into, through and out of soil
  • Soil hosts thousands of organisms, both micro and macro-organsims

Humans would have serious Problems without Soils

  • Soils are the main medium for plant growth, especially crops. These crops include forests and rangeland as well as fruits, vegetables and grains.
  • Soils support our structures - our homes and other buildings, roads, and other structure
  • Soils also dispose of our wastes in landfills and with onsite sewage treatment systems such as septic tank drainfields.

Soil Properties (Terms)

Soils have several main components including: mineral, water, air, organic. We will point out some important features of soils throughout this class including:

  • Horizons - these are the horizontal layers of soil which you can see in a soil profile or roadcut , and which change with depth.
  • Color - dark brown or black usually means high organic matter content, bright colors (red, yellow) usually mean high iron and clay content.
  • Texture - the percentage of sand, silt, clay, and rocks
  • Sand - feels gritty
  • Silt - feels smooth but doesn't form a ribbon
  • Clay - feels smooth and sticky, will form a ribbon
  • Structure - (dirt clods) - lumps of soil formed by root growth, clay movement, wetting and drying, shrink-swell
  • Organic matter content - mostly in the surface from roots and litter. Main source of nitrogen, phosphorus, and sulfur.
  • Roots - root distribution tells you a lot about where the plants are getting their water and nutrients. Bedrock, dense layers (glacial till), or loose sand and gravel can limit roots.PH - acidity/alkalinity. A measure of how many hydrogen ions are around.
  • Grassland soils are often more alkaline and conifer forest soils are often more acidic.
  • Available water holding capacity - how much water will the soil hold and make available to plants. Sands hold about 0.5 - 1 inch/foot of soil and silty or clayey soils hold about 2 inches/foot.
  • Mottles - (redoximorphic features) are spots of rust in the soil which indicate saturation. Usually from a seasonal high ground water table.

Soil Formation

There are five main factors of soil formation. When you look at soils, think of these factors and try to figure out which have had the most effect. By knowing about soil forming factors, you can figure out how the land developed throughout geologic history.

  • Parent Material - what the soil is made of. This includes the kind of rock and the processes that acted on the rock and soil material, including streams, glaciers, lakes, wind, gravity.
    • Streams move quickly, carry gravels and make rocks rounded, most rocks are about the same size. Stratify layers or deposit layers of about the same size material.
    • Glaciers pick up all sizes of rocks, including gravels, stones, and boulders as well as soil and mix these up into a jumbled deposit.
    • Lakes deposit mainly silts and clays with no rocks, may have varves (layers of sediment).
    • Wind deposits silts and fine sand, no rocks.
    • Gravity breaks rocks off outcrops and moves them downhill slowly over time, these rocks are usually angular.
  • Topography - slope steepness, aspect, slope length, landscape position (valley bottom, ridgetop, mid-slope).
  • Vegetation - forests, grassland, wetlands.
  • Climate - rainfall, temperature, freeze-thaw cycles, wet-dry cycles.
  • Time - how long has all this been going on.
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