Lecture 4b Soil Forming Factors

1. Lecture 4bSoil 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.

2. Soil Forming Factors - Biotic (Vegetation) • Animal - Soil Mixing~ earthworms, crawfish, scorpions, moles, gophers ~ this mixing can result in the destruction of horizons.

3. 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

4. Biotic (cont.) • Forest ~ OM added to upper 4 “ due to yearly leaf fall to surface of soil. A

5. 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

6. Dyad • Describe your experience of being in a native or restored prairie or an old growth forest. What was unique about this experience?

7. 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

8. Catena – Natural Soil Drainage Classes NOTE: Natural drainage refers to depth to water table not permeability. • 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)

10. Drainage Classes (depth to water table) gray mottles gray colors below A gray mottles in brown B Mod.well Dr. mottles > 4 feet Well Drained Somewhat Poorly Poorly Drained Drained

11. 2) Topography – erosion & deposition

12. Summit and Backslope • Summit will have minimum erosion and maximum soil development (greatest horizonation). • Backslope will be similar to summit unless slope is > 20%. summit backslope

13. Shoulder • Greatest erosion - least water infiltration - greatest runoff - minimal soil development. Ap Bw Bk BC C shoulder

14. 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 Upslope Sediments footslope Water

15. 3. Aspect • Direction the slope faces - important when slope is > than 10 %. Noticeable in SE MN. • North Slope = 1. colder soils, 2. less evaporation , 3. more leaching ~ thus more soil development • South Slope = 1. warmer soils, 2. more evaporation, 3. less leaching ~ thus less soil development.

16. A E Bt C A Bw C Aspect • Direction the slope faces • Important when slope > 10 %

17. Hillslope in SE Minnesota North Facing Slope - more trees - more soil develop- ment. South Facing Slope (open, fewer trees) Less soil development

18. 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.

19. Youth = Juvenile = Age Sequence A A Bw C C Old Age “Senile” Mature = Adult A E Bt1 Bt2 A E Bt C A E Btqm Bqm

20. 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 retard soil profile development

21. Factors that Slow Soil Formation • Climate • Low rainfall • Low humidity • Cold temperature

22. 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.

23. 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.

24. 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.

25. Oldest soils in United States Durixeralf - California Petroargid Arizona Petrocalcic A Bk Bkm C

26. Petroargid - New Mexico A BA Bt Btk Bkm

27. 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 18th century, the peat was removed.