Why might soil weather and erode?

• There are a number of factors…

• 1. Climate2. Vegetation3. Topography

4. Parent Material 5. Time 6. Human Activities

How does climate cause soil erosion?

• Energy and precipitation strongly influence physical and chemical reactions on parent material in the soil.

• Climate also determines vegetation cover, which in turn influences soil development.

• Precipitation also affects horizon development factors like the translocation of dissolved ions through the soil.

• How does hot dry climates impact the soil?

• Hot, dry desert regions- sparse vegetation and hence limited organic material available for the soil.

• Lack of precipitation inhibits chemical weathering leading to coarse textured soil in arid (dry) regions.

• In cold regions- bacterial activity is limited.

• In the warm and wet tropics- bacterial activity proceeds at a rapid rate, thoroughly decomposing leaf litter. (Tress take up the nutrients from the soil, and high rain fall flushes away some organic material in the soil). As a result the upper horizons (layers) of the soil lack organic matter.

Vegetation's role in impacting soil…

• Pine forests- Decomposing pine needles in the presence of water creates a weak acid that strips soluble bases from the soil leaving it in an acidic state.

• pine trees have low nutrient demands so few soil nutrients are taken back up by the trees to be later recycled by decaying needle litter.

• Deciduous forests- Broadleaf deciduous trees like oak and maple have higher nutrient demand and thus continually recycle soil nutrients keeping soils high in soluble bases.

How might topography affect soil?

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• Water moving across the surface strips parent material away impeding soil development.

• Water erosion is more effective on steeper, unvegetated slopes.

• The amount and velocity of water, and rate of erosion increases as you near the base of the slope. (Parent material is striped away and can’t develop into a soil).

• Soil parent material is the material that soil develops from.

• Which of the following is more prone to weathering?

• Soils developed on parent material that is coarse grained and composed of minerals.

• Soils developed on parent material that is fine grained unstable minerals.

Parent material composition has a direct impact on soil chemistry and fertility:

• Parent materials rich in soluble ions -calcium, magnesium, potassium, and sodium, are easily dissolved in water and made available to plants.

• Limestone and basaltic lava- both have a high content of soluble bases and produce fertile soil in humid climates.

• Parent materials low in soluble ion- water moving through the soil removes the bases and substitutes them with hydrogen ions making the soil acidic and unsuitable for agriculture.

• Soils developed over sandstone- are low in soluble bases and coarse in texture which facilitates leaching (the loss of water-soluble plant nutrients from the soil due to rain).

Time:

• As time passes, the weathering processes continue to act on soil parent material to break it down and decompose it.

• Horizon development processes continue to differentiate layers in the soil profile by their physical and chemical properties.

• older more mature soils have well-developed sequence of horizons.

• Some geological processes keep soils from developing by constantly altering the surface and not allowing parent material to weather over a significant period of time. (ex: Hillside erosion, river deposition of sediment).

• Organism, both plant and animal, play an important role in the development and composition of soil.

• Biotic elements of the environment need life-sustaining nutrients that find their origin in the soil. Upon their death, organisms return these nutrients to the soil to be taken up again by other plants and animals.

• This cycling refreshes and maintains the nutrient status of soils (this enables soils to support life).

• What types of trees contribute little nutrients to the forest floor?