Major geological landforms

1. Major geological landforms Distribution of the principal folded belts formed since the Precambrian period (after Kummel 1970)

2. Topography Time Humans • Old deep weathered infertile soils in areas of Precambrian rocks. • Young fertile soils in areas of mountain lifting where new parental material is frequently exposed (and along rivers draining from such areas). • Volcanic soil is usually initially rich in minerals but poor in nitrogen and organic matter, with time it loses minerals but gain more nitrogen and organic matter. • Too frequent and intensive slash and burn cause nutrient loss and hardpan formation (= laterite)

3. Soils influence ecosystems: • Water retention and supply • Nutrient availability (N, P, K, Ca, Mg, etc) • Decay rate of dead organic matters • Local productivity • Species diversity

4. Constituents of Soil Inorganic solids ≈ 50% Water ≈ 25% Air ≈ 20% Soil Organic Matter ≈ 5% bulk density (b) = g soil / cc ≈ 1.0 - 1.6 g/cc

5. Water retention is affected by pore sizes. • Macropores – filled up in soil saturated with water, which drains freely with gravity. • Micopores (diameter < 50 m) retain water against gravitation (capillary water).

6. Soil particle size • sand; 2 - 0.02 mm; do not adhere • silt; 0.002 - 0.02 mm; do not adhere • clay; <0.002 mm; adhere, charged Peds (= Micelle): aggregation of clay minerals with organic matter under influence of plant roots, dry-wet cycle, and animal activities. Micropores inside peds retain water and nutrient.

7. Elemental Composition of Soil Soil elements reflect the Earth’s crust (rock): O 46.6% Fe 5.0% K 2.6% Si 27.7% Ca 3.6% Mg 2.1% Al 8.1% Na 2.8% all other 1.4% What plants need from the soil: Macro nutrients: N, S, K, Ca, P, Mg Micronutrients: Fe, Mn, Co, B, Zn, Si, Na, Cl, Ni, Mo Never needed (only toxic): Al Nitrogen (N) has to come from fixation in the atmosphere. In the soil, N exists in organic form or as nitrate (NO3-), and it is lost when the soil is heated.

8. Soil Weathering = physical and chemical alteration of the inorganic solids and underlying rock The older the bed rock, the deeper and the poorer is the weathered soil layer.

9. Soil Weathering Physical changes by: - temperature (freeze/thaw, fires) - mixing by animals (bioturbation) - abrasion (wind, ice, water) Chemical changes by: - dissolution of elements - change in molecular structures of minerals - enrichment by biotic activity (C,N,S)

10. CO2 from biological activities acidify and accelerate weathering 2CO2 + 2H2O  2H+ + 2HCO3 -(carbonic acid) Ca2+ + 2HCO3-  CaCO3 +CO2 + H2O Calcite precipitates in desert soils in equilibrium with soil CO2 derived from plant root respiration.

11. Minerals dissolution rates Ca2+, Mg2+, Na+, K+ > Si > Fe, Al • Elements and minerals that remain reorganize into secondary minerals • Solubility of Si depends on what form it exists (quartz, abundant in course sands and granite, is less likely to dissolve than Al and Fe)

12. Layered clay minerals Si tetrahedral sheet Al octahedral sheet

13. Layered alminosilicate clays Chemical weathering 2:1 type clay 1:1 type clay Leaching of Si Al octahedral sheet Si tetrahedral sheet

14. Cation Exchange Capacity CEC = total negative charge in a soil [mEq/100 g dry soil] • Sources of CEC: • Carbonate terminals of organic matters –COOH  -COO- + H+ • Edge of Si tetrahedral sheets: -SiOH  -SiO- + H+ • Isomorphous replacement of Si4+ with Al3+ and Al3+ with Al2+ CEC of soil types: Alfisols: 9-12 Ultisols: 3-15 (increase w/depth) Oxisols: 2-7 Sandy Podsols: 0-3 CEC of clay minerals: Illite (2:1 type): 10-40 Kaolinite (1:1 type): 3-15

15. Other clay minerals 1) Non-crystal (amorphous): of volcanic origin; e.g., allophan Al octahedrals Adsorbed water (from Kitagawa 1975) 2) Iron and aluminum sesquioxides (Al2O3, Fe2O3): - giving red color to the soil - particularly abundant in weathered tropical soils - important source of variable charge

16. Variable charge due to H+ ion adsorption • low pH, positive charge • ( anion exchange capacity) • (below pH 7-8 for Al2O3, Fe2O3 ) • intermediate pH, neutral • high pH, negative charge Can be important for adsorption of NO3-, which is highly mobile in temperate soils

17. Soil Development Soil horizons are distinct layers in the soil with particular properties 4 processes contribute to horizon formation: transformations: soil components are modified, destroyed, synthesized 2. translocations: movement of materials 3. additions: materials added to soil from outside 4. losses: materials are removed from soil

18. Soil Master Horizons O -- organic material A-- contains organic material. Minerals are leached from here and moved downward (eluviation) B -- contains little organic material. Minerals and clay from above are deposited here (illuviation) C --unconsolidated rock http://www.mo15.nrcs.usda.gov/

19. Ultisol(clay-enriched B horizon) Oxisol Subtropical, broad-leaf, monsoon forests, impeded drainage, or quartz-rich parental rock

20. Spodosol example: State Soil of Florida: Myakka Series A horizon: White-grey sand (Some consider it “E” horizon, the zone of strong eluviation) B horizon: accumulation of clay and organic matter, and iron oxide in some soils. http://soils.usda.gov http://www.fao.org

21. Soil Order Derivation Formative element Alfisols Nonsense symbol alf Andisols Jap. ando, black soil and Aridisols L. aridus, dry id Entisols Nonsense symbol ent Gelisols Gr. gelid, very cold el Histosols Gr. histos, tissue ist Inceptisols L. inceptum, beginning ept Mollisols L. mollis, soft oll Oxisols Fr. oxide, oxide ox Spodosols Gr. Spodos, wood ash od Ultisols L. ultimus, last ult Vertisols L. verto, turn ert 12 Soil orders

23. Important soil orders in the tropics Oxisols --highly weathered redish soil, Fe, Al oxides. Low CEC Ultisols --Lower base saturation. Weathered soils, but not as much as Oxisols Spodosols --leached E horizon. B horizon with organics, Al, Fe oxides. Most common in cold boreal forests, but in the tropics, from precambrian parental rock rich in quartz (as in Guyana shield) or sandy soil (as in Florida) Vertisols --high shrink/swell clay content. Cracks. Savanna.

24. Soil formation, geomorphology and parental material in equatorial climate oxisols Basic rock (Etherington 1982)

25. Take home message • Large area of the moist-wet tropics are covered by highly weathered soils, especially in the continental shield regions. • The rich soils in tropics are mostly of volcanic origin (rich in allophane) or sedimentary deposit of erosion from relatively new geological substrate (e.g., from the landslide on Andes hillsides).

26. SOIL MOISTURE: WET (equatorial) WET/DRY (tropical) DRIER (subtropical) Organic Carbon Most Intermediate Least (<0.5%) Iron Dissolves Goethite Hematite Aluminum Can dissolve Kaolinite Gibbsite Clay minerals kaolinite quartz kaolinite sesquioxides quartz Kaolinite sesquioxides Bottom Line for the relationship between type of clay minerals and climate (personal communication from Robert Stallard)