Major geological landforms
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Major geological landforms. Distribution of the principal folded belts formed since the Precambrian period (after Kummel 1970). Topography Time Humans. Old deep weathered infertile soils in areas of Precambrian rocks.

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Major geological landforms
Major geological landforms

Distribution of the principal folded belts formed since the Precambrian period (after Kummel 1970)




  • 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)

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

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

Water retention is affected by pore sizes
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).

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.

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.

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.

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)

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.

Minerals dissolution rates
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)

Layered clay minerals

Si tetrahedral sheet

Al octahedral sheet

Layered alminosilicate clays
Layered alminosilicate clays

Chemical weathering

2:1 type clay

1:1 type clay

Leaching of Si

Al octahedral sheet

Si tetrahedral sheet

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

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

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

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

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

Ultisol(clay-enriched B horizon)


Subtropical, broad-leaf, monsoon forests, impeded drainage, or quartz-rich parental rock

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.

12 soil orders

Soil Order


Formative element


Nonsense symbol



Jap. ando, black soil



L. aridus, dry



Nonsense symbol



Gr. gelid, very cold



Gr. histos, tissue



L. inceptum, beginning



L. mollis, soft



Fr. oxide, oxide



Gr. Spodos, wood ash



L. ultimus, last



L. verto, turn


12 Soil orders

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.

Soil formation geomorphology and parental material in equatorial climate
Soil formation, geomorphology and parental material in equatorial climate


Basic rock

(Etherington 1982)

Take home message
Take home message equatorial climate

  • 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).

Bottom line for the relationship between type of clay minerals and climate

SOIL MOISTURE: equatorial climate







Organic Carbon



Least (<0.5%)






Can dissolve



Clay minerals








Bottom Line for the relationship between type of clay minerals and climate

(personal communication from Robert Stallard)