Presentation on theme: "Lecture 4b Soil Forming Factors Parent Material Climate * Vegetation * Topography * Time Soils vary from place to place because the intensity of the factors."— Presentation transcript:
Lecture 4b Soil Forming Factors Parent Material Climate * Vegetation * Topography * Time Soils vary from place to place because the intensity of the factors is different at different locations.
Soil Forming Factors - Biotic (Vegetation) Animal - Soil Mixing~ earthworms, crawfish, scorpions, moles, gophers ~ this mixing can result in the destruction of horizons.
Botic (cont.) – plants - Prairie and Forest Vegetation - Additon of Organic Matter (OM). Prairie ~ OM added to upper 2 ft. of soil due to fibrous root system of grass plants. Ap A AB Bg
Biotic (cont.) Forest ~ OM added to upper 4 “ due to yearly leaf fall to surface of soil.
Prairie - Border Biotic Factor Prairie - Border soils (oak savannahs) have the influence of the prairie and forest ~ due to changes in vegetation over the past 8000 years the soils have been both under prairie and forest. – A horizon is not as dark, not as thick and E may be present or absent Ron E. VanNimwegen
Dyad Describe your experience of being in a native or restored prairie or an old growth forest. What was unique about this experience?
SFF - Topography or Landscape Position – 3 kinds : 1) Catena, 2) landscape position, 3)aspect 1) Catena - A series of soils with different horizons due to differences in their depth to the water table Drainage classes Well-drained Moderately well-drained Somewhat poorly drained Poorly drained
Catena – Natural Soil Drainage Classes NOTE: Natural drainage refers to depth to water table not permeability. n Natural Soil Drainage Classes Well Drained - mottles begin > 4 ft. Moderately well drained - mottles > 3 ft. Somewhat poorly drained - mottles > 2 ft Poorly drained - dark surface - gray colors in subsoil below surface (red mottles)
Summit and Backslope Summit will have minimum erosion and maximum soil development (greatest horizonation). Backslope will be similar to summit unless slope is > 20%.
Shoulder X Greatest erosion - least water infiltration - greatest runoff - minimal soil development. Ap Bw Bk BC C
Footslope Deposition of materials from upslope - may be near water table - may have greatest leaching due to water from upslope and rainfall. Ap A1 A2 A3 AB Btg Water
3. Aspect Direction the slope faces - important when slope is > than 10 %. Noticeable in SE MN. ç North Slope = 1. colder soils, 2. less evaporation, 3. more leaching ~ thus more soil development ç South Slope = 1. warmer soils, 2. more evaporation, 3. less leaching ~ thus less soil development.
Aspect Direction the slope faces Important when slope > 10 % A Bw C A Bt C E
Hillslope in SE Minnesota South Facing Slope (open, fewer trees) North Facing Slope - more trees - more soil develop- ment. Less soil development
Soil Forming Factor - Time Vegetation and Climate act on the Parent Material and Topography over Time. The age of a soil is determined by its development and not the actual number of years it has been developing. How long it takes for a soil to become old depends on the intensity of the soil forming processes or intensity of the other 4 soil forming factors.
Age Sequence Youth = Juvenile = A C A Bw C Mature = A E Bt C Adult A E Bt1 Bt2 Old Age “Senile” A E Btqm Bqm
Factors that retard soil profile development low rainfall high lime content high clay content steep slopes cold temperature severe erosion low humidity high quartz hard rock high water table constant deposition mixing by animals
Factors that Slow Soil Formation Climate Low rainfall Low humidity Cold temperature
Factors that Slow Soil Formation Biota Mixing by animals or man Human made soil Suborder = ARENT anthropogenic factor – human influence on soil formation – Direct = plowing, manuring, liming. Indirect = changing soil forming factors – deforestation, relief, parent material.
The Effects of Management on the Interactions Between Plants and Soils In this example, heavy continuous grazing followed by drought produces positive feedbacks between vegetation and soil properties that enhance physical, chemical, and biological degradation processes. These feedbacks lead to the following: a) a decrease in soil organic matter and an increase in size of bare spaces, b) a decrease in soil aggregate stability and resistance to erosion, c) a loss of topsoil and a decrease in infiltration, d) an additional loss of grass and increase in shrubs, which cause the feedback loop to continue. (Figure modified from Bird et al., 2001.) Arrow represent physical, chemical, and biological degradation * Desert grass displaced by shrubs. Soil OM decreases and Size of bare space increases. * Aggregate stability and resistance to erosion decrease * Topsoil and fertility lost and infiltration decreases.
What happens to a soil with time Loss of nutrients ( bases) = lower pH or soil becomes more acid Increase in concentration of iron or soil becomes redder Increase in clay content or old soils have more clay Deeper weathering into the parent material Oldest soils in US are on high terraces and alluvial fans of the mountains in the western U.S.
Oldest soils in United States Petrocalcic Durixeralf - California Petroargid Arizona A Bk Bkm C
Callanish Standing Stones – Isle of Lewis - Scotland In 3000 BC the early farmers plowed the soil where the stones would be placed. In 1000 BC the stones were forgotten and peat filled in around the stones to a depth of five feet. When the stones were discovered in the 18 th century, the peat was removed.