1. LECTURE 3 Factors Influencing Soil Formation – Biota & Time Processes of soil formation

2. Definitions… Percolation (J.B. Thompson) Truncated soil (A.I. Bennett)

3. The 5 factors influencing soil formation Parent materials Climate Biota Topography Time

4. Sequences that develop when one of these factors is dominant in determining differences Parent material – lithosequence Climate – climosequence Biota – biosequence Topography – toposequence Time - chronosequence

5. Biota (living organisms) Contribute towards: Organic matter accumulation Biochemical weathering Profile mixing Nutrient cycling Aggregate stability Can be divided into Natural vegetation Animals

6. Natural Vegetation Reduces natural soil erosion rates. Slows down rate of mineral surface soil removal. Certain leaves produce organic acids that bring iron and aluminium into solution. Accelerate the downward movement and accumulation of these metals in the B-horizon.

7. Development of the “A” horizon: Comparison of soils under grassland and forest vegetation. –Thickness of A-horizon –Structural stability of mineral soil aggregates Cation cycling by trees: Ability of vegetation to take up mineral elements from the soil influences the characteristics of soils that develop –NB acidity! –Comparison between coniferous forest and deciduous forest

8. Heterogeneity in rangelands In arid and semi-arid landscapes, there is competition for limited soil water –Does not permit vegetation dense enough to cover soil surface –This alters soil properties in several ways: »Trapping of windblown dust »Scavenging of nutrients form interplant areas »Deposition of nutrients under plant canopies »Addition of organic acids »Development of “islands” of enhanced fertility, thicker A-horizons etc.

9. Animals Role of large animals, e.g. moles, rabbits, gophers Bring materials from deep in the profile to the surface Open the profile up to air and water Role of smaller creatures, e.g. earthworms, ants, termites Soil mixing (bioturbation) Aeration of soil Increase in stability of soil aggregates (assuring infiltration of water) Activities of animals decrease the differences among soil horizons.

10. Human influences? It is believed that Native Americans regularly set fires to maintain prairie grasslands in Indiana and Michigan. Human destruction of natural vegetation. Tillage of soils. Irrigation, addition of fertilizers. Mining, urbanization.

11. Time “Stopwatch” starts ticking when parent materials are deposited or exposed. Rates of weathering “young” vs. “mature” soils? Time spans generally in the range of hundreds to hundreds of thousands of years. Very rare that soil formation can be observed in a human lifetime.

12. Chronosequences: Methods such as carbon dating, analysis of fossils and human artifacts etc. used as evidence regarding the time spans involved in soil formation, but another tool is to analyse a chronosequence. –E.g. on alluvial terraces of differing age. Interaction with parent materials: Residual parent materials vs. transported parent materials. Interaction with other soil forming factors.

13. S = ƒ(c, p, r, v, o, t)

14. Processes of soil formation

15. Transformations Translocations Additions Losses

16. Transformations Mineral weathering. Organic matter breakdown. Some soil constituents are modified or destroyed, others are synthesized.

17. Translocations Movements of organic and inorganic materials from one horizon up or down to another. Material moved by water and soil organisms.

18. Additions Organic matter from leaves Dust from atmosphere Minerals from groundwater Others?

19. Losses Leaching to groundwater. Erosion. Others?