Soil Types

Mature Zonal Soils - Based on Climate and Vegetation

This section covers soil types categorized by climate and vegetation zones.

1. Tropical Soils - Laterites (Latosols)

Formation:

  • Develops under tropical forest vegetation.

  • Formation is influenced by climate and vegetation.

  • Key processes: eluviation, leaching, illuviation, hardpan formation.

Climate and Vegetation:

  • Formed in tropical areas with:

    • High temperatures: Average 27°C

    • Heavy and abundant rainfall

    • Natural forest vegetation

  • Soil is typically well-drained, crumbly, and loose.

Soil Profile, Depth, and Horizons:

  • Deep profile: Extends for 2-3 meters.

  • Three distinct horizons: A, B, and C.

Horizons:

  • A Horizon (Topsoil):

    • Thick leaf litter above it.

    • Depth: 0-10 cm.

    • Thin layer of humus on the surface from decayed vegetation and animal matter.

    • Humus is thin despite rapid decomposition.

    • Color: Light reddish.

    • Zone of Eluviation (Leaching): Organic matter and minerals leached to B horizon; iron oxides remain.

  • B Horizon (Subsoil):

    • Depth: 15-180 cm.

    • Zone of Illuviation (Enriched Layer): Deposition of leached materials.

    • Formation of hardpan by iron oxides.

    • Lost of silica from clay minerals.

  • C Horizon (Parent Material):

    • Fragmented or unweathered bedrock.

Humus:

  • Organic matter mainly on the surface due to leaching.

  • Humus layer is thin.

  • Humus is constantly replenished by decay and bacterial activity (due to high temperature and rainfall).

  • Subsoil (B Horizon) contains little to no humus.

  • High rate of chemical weathering and bacterial activity.

  • Significant leaching occurs.

  • Rapid runoff reduces top humus layer.

Decomposition:

  • Soils are not acidic.

  • High temperatures cause decomposition of minerals and clay particles.

  • Rapid decomposition of dead plants and animals due to high temperatures and rainfall.

  • High rate of bacterial activity.

Leaching (Eluviation & Illuviation):

  • Dominant where precipitation exceeds evaporation.

  • Driven by heavy and copious rainfall.

  • Eluviation: Water leaches soil constituents from the A horizon downwards.

  • Illuviation: Deposition of leached materials in the B horizon.

  • Soluble compounds from chemical weathering are leached and deposited.

  • Leaching is the downward percolation of water and minerals through soil pores from A to B horizon.

  • Dominant in permeable soils with heavy rainfall.

  • Friable mineral particles (silica and alkalis) leached from A to B horizon.

  • Sesquioxides of iron and aluminum are not leached and remain in A horizon.

  • Results in high sesquioxides of iron and aluminum in tropical soils, giving a reddish color to the A horizon.

  • High clay content.

  • Soils are usually poor as they quickly lose fertility and become impoverished.

Colour:

  • Reddish color due to iron and aluminum content (sesquioxides) remaining after leaching.

  • Mature soils: Formed over thousands of years.

  • Latosols: Widely weathered tropical soils.

  • Weathering of rocks in tropics forms a layer of red to yellowish-brown clayey soil.

Minerals:

  • Rich in iron and aluminum oxides.

  • Silica and kaolin clay increase towards the B horizon.

  • Other minerals: titanium oxides and manganese oxides.

Pans (Hardpan & Claypan):

  • Hardpan formation: Soil horizon becomes hardened or compact.

  • Iron or Clay pan: Mineral elements (iron oxides, silica) cemented together.

  • Clay pan: High clay proportion, iron and aluminum oxides cemented.

  • Formed in the B horizon (deposition zone).

  • Hardpan inhibits root and water penetration.

  • Leached soils with hardpan are poor and lose fertility quickly when cultivated.

  • Exposed hardpan makes cultivation difficult.

Summary Diagram (Laterite Soils - Tropical Soil Profile):

  • Hot and wet climate

  • Tropical forest vegetation, rich litter layer.

  • A<sub>0</sub> Humus Layer: Decomposed organic and inorganic matter. Dark leaf litter, decomposes rapidly.

  • A Horizon - Topsoil: Light reddish color. Zone of Eluviation - leaching of organic matter, minerals leached to B horizon - iron oxides remain.

  • B Horizon - Subsoil: Formation of hardpan by iron oxides. Zone of Illuviation - enriched layer, leached materials. Loss of silica from clay minerals.

  • C Horizon - Subsoil: Parent material - fragmented/unweathered bedrock.

2. Formation Processes (Podzols) - Temperate Regions

(Podzols)

Formation:

  • Develops under evergreen coniferous vegetation.

  • Key processes: eluviation, leaching, illuviation, hardpan formation.

  • Formation influenced by climate and vegetation.

Location:

  • Temperate regions: e.g., Ontario, Canada.

  • Lowland and highland areas with parent material underlain by glacial drift.

Climate and Vegetation:

  • Cool and wet climate: Rainfall 500-800 mm per annum.

  • Influenced by short summers and long winters.

  • Developed under evergreen coniferous forest vegetation and broadleaf forest vegetation (taiga/boreal forest regions).

Infertility - Little Humus:

  • Generally infertile due to excessive leaching.

  • Crops like potatoes can be cultivated.

  • Thin layer of raw humus.

  • Residuous pine needles from coniferous trees do not form rich humus when decayed.

  • Bacterial activity is restricted due to long, cold winters and short, warm summers.

  • In pine/heath/moorland areas, raw humus (mor) is formed due to slow bacterial activity in cold conditions.

  • Slow rate of chemical weathering due to low temperatures.

Sub-Bacterial Action:

  • Low temperatures slow down chemical weathering.

  • Slow bacterial action.

  • Slow decomposition/decay of organic matter.

  • High degree of acidity due to slow decomposition, resulting in little bacterial action.

Soil Profile - Horizons:

  • Arranged in horizons: A<sub>0</sub>, A<sub>1</sub>, A<sub>2</sub>, B, and C.

  • A<sub>0</sub> Horizon (Litter Layer):

    • Depth: 10-20 cm.

    • Layer of litter of pine needles.

    • Undecomposed organic matter.

    • Dark color.

    • Organic matter partly decomposed.

  • A<sub>1</sub> Horizon:

    • Depth: 30-75 mm.

    • Color: Grey to reddish-brown.

    • Little humus.

  • A<sub>2</sub> Horizon:

    • Depth: 2-5 cm.

    • Appearance: Ash grey to whitish.

    • Eluviation Zone: Leaching of organic matter and minerals - sesquioxides to the B horizon.

    • Bleached horizon.

  • B Horizon:

    • Depth: Reaches to about 20 cm.

    • Illuviation Zone: Leached materials deposited here.

    • Iron, aluminum, and humus leached and deposited.

    • Color: Reddish-brown.

    • Color due to acid nature of the soil.

    • Sticky and clayey due to colloids (tiny mineral and organic particles).

    • Upper layer rich in silicon.

    • Lacking mineral elements for plants due to leaching.

  • C Horizon:

    • Parent material, solid bedrock.

Leaching and Colour:

  • Leaching from A horizon to B horizon (similar to laterites).

  • A<sub>2</sub> horizon is ash-grey to whitish due to acid leaching.

  • Iron and clays leached to B horizon due to high concentration of acid solutions (contrast to laterites).

  • Eluviation in A horizon, Illuviation in B horizon.

  • Iron, aluminum, and humus leached and deposited in B horizon giving it a reddish-brown color.

Iron Pans:

  • Iron and aluminum compounds leached from A to B horizon cemented together to form a thin hard pan.

  • Hardpan limits root and water penetration.

  • Iron sometimes deposited at the top of the B horizon in a thin layer.

  • Hindrance to drainage leading to bogs and swamps.

Summary Diagram (Podsols Soil Profile):

  • Cold winters and cool summers

  • Coniferous vegetation

  • Thin humus layer - decayed vegetation. Slow decomposition of cones and needles to raw acid humus.

  • A<sub>0</sub> Litter Layer: Undecomposed organic matter. Layer of pine needles. Organic matter partly decomposed. Dark color.

  • A Horizon - Eluviation: Ash-light grey color - Leaching of organic matter and minerals - sesquioxides to the B horizon. Bleached horizon. Formation of iron pan - iron oxides.

  • B Horizon - Illuviation: Leached materials in this zone. Iron and aluminum redeposited from A horizon. Reddish-brown color. Clay, sand, stones.

  • C Horizon: Parent material, solid bedrock.

3. Chernozems - Temperate Grassland Vegetation

Formation:

  • Developed under temperate grassland vegetation.

  • Formation influenced by climate and vegetation.

  • Key process: capillarity.

Location:

  • Grassland areas in temperate regions:

    • American and Canadian Prairies

    • Corn Belt in the USA

    • Steppes in Russia

    • Pampas in Argentina

  • Chernozem is a Russian word meaning "black earth."

Climate:

  • Rainfall under 500 mm per annum, mainly in summer.

  • Warm summers and cold winters.

  • Temperature extremes favor development of this soil type.

Capillarity:

  • High summer temperatures mean plants get water largely by capillary action as rainwater percolation is limited.

  • Light rainfall and high evaporation cause upward movement of water and minerals (calcification).

  • Calcium is concentrated in the C horizon.

  • Small air spaces between hydrophilic soil particles (attract water).

  • Water moves upwards by capillary action.

  • In low rainfall areas, plant roots get water this way.

Structure:

  • Black in color.

  • Crumbles very easily.

  • Granular structure.

  • Loose and pervious, dark and friable.

Humus:

  • Humus continuously supplied from grasses, stems, and roots.

  • Decayed material is quickly replaced by new material.

  • High organic and mineral content.

  • Rich in humus and mineral nutrients like calcium.

  • Well mixed by earthworm activity.

Colour:

  • Black color due to abundant organic matter.

  • Thickness: About 1 meter.

  • Grades gradually into a lighter colored layer.

Vegetation & Soil Erosion:

  • High density of grass root network penetrates deep into the soil.

  • Very effective in preventing soil erosion.

  • Decayed vegetation is returned to the soil in a fine structural form.

Horizons:

  • Horizons are not as distinct as in laterite soils.

  • A Horizon:

    • Dark, extending deep downwards (about 1m).

    • Covered with a rich layer of debris undergoing decomposition.

    • Well mixed by earthworms.

    • Fine and loose structure.

    • Color: Grey-brown

  • B Horizon:

    • Not distinct.

    • Marked by limit of lime accumulation.

    • Color: Reddish to brown.

    • Rich calcium nodules.

    • Concentration of lime (calcium carbonate).

    • Pale brown - alkaline.

  • C Horizon:

    • Parent material.

Limited Leaching - High Calcium Level:

  • Leaching is minimized due to low rainfall.

  • Rich in soluble salts like calcium.

  • No distinctive horizons like podsols or laterites due to restricted leaching.

  • Water in summer obtained by capillary action.

  • Eluviation and illuviation are not widespread.

  • Leaching determined by precipitation amount.

Lime Accumulation:

  • Layer of lime accumulation prevails.

  • Found at depths under 1 meter.

  • Located near the base of the B horizon.

Summary Diagram (Chernozem Soil Profile):

  • Hot summers/Cool winters

  • Grassland vegetation - Light rainfall.

  • Humus - Decayed vegetation. Deep, penetrative roots.

  • Thick layer of humus-rich soil, Black, dark and brown - 1m. Grades gradually into light colored layer.

  • Limited leaching due to low rainfall and high evaporation in summer.

  • Mineral and organic matter is mixed.

  • A Horizon: Grey-brown

  • B Horizon: Not distinct. Rich calcium nodules. Concentration of lime (calcium carbonate). Pale brown - alkaline.

  • C Horizon: Parent material.

4. Rendzina Soils - Limestone Parent Material (Intrazonal)

Rendzina Soils:

  • Intrazonal or immature soils: Horizons are not well-developed.

  • Soil structure is mainly influenced by parent material, not climate and vegetation.

  • Found in both temperate and tropical regions.

  • Generally in moist/humid climates and semi-arid regions.

  • Vegetation: grassland or grass-tree vegetation.

  • In tropical regions, termed tropical black earth.

Parent Rock:

  • Develops on calcareous rocks:

    • Marl

    • Chalk

    • Limestone

  • Clay sometimes found beneath.

Colour:

  • Varies in color:

    • Reddish brown to dark grey and yellow (temperate countries).

    • Black where limestone is soft (marl, coral) - due to mix of humus and limestone.

    • Reddish color (terra rossa) if developed on hard limestone rocks.

Depth:

  • Thin, light, and shallow soils on hard limestone.

  • Shallower where rainfall is low.

  • Hard limestone crust hinders root penetration.

  • Water percolates to a low level.

  • Deeper and heavier soil if limestone is soft and impure.

Figure 23.9 Rendzina soil profile (Note: Figure not provided in text, likely a visual representation of a Rendzina soil profile).




Why are latosol soils typically red in colour?

  • Latosol soils are reddish in color because of their high content of sesquioxides of iron and aluminum. During the soil formation process, silica and alkalis are leached out, but iron and aluminum compounds are not, and these red iron compounds remain in the soil, contributing to the reddish hue, particularly in the A horizon.  This distinct color is often associated with tropical and subtropical climates, where high temperatures and rainfall facilitate the weathering of parent materials and the accumulation of these iron-rich compounds.


What are key characteristics of latosol soil profile?

  • Deep profile: Extending to 2-3 meters.

  • Three distinct horizons: A, B, and C.

  • A Horizon (Topsoil): Thin humus layer, light reddish color, zone of eluviation.

  • B Horizon (Subsoil): Zone of illuviation, formation of hardpan by iron oxides, loss of silica from clay minerals.

  • C Horizon (Parent Material): Fragmented or unweathered bedrock.

  • Well-drained, crumbly, and loose texture.


How does weathering impact the nutrient content of latosol soil profile?

  • Weathering in tropical climates is intense and primarily chemical. This high rate of chemical weathering, combined with heavy leaching, leads to the removal of many soluble nutrients from the topsoil (A horizon). While bacterial activity and decomposition are rapid due to the climate, the leaching process is dominant, washing away many nutrients and resulting in soils that are typically poor in nutrients and quickly lose their fertility when cultivated. The notes mention that mineral elements for plants are lacking due to leaching.  


How does tropical climate contribute to the formation of latosol soil?

  • High temperatures (27°C) in tropical climates accelerate the rate of chemical weathering and bacterial activity, which are key processes in soil formation. Additionally, abundant rainfall in these regions enhances leaching, further depleting the soil of essential minerals and leading to the characteristic reddish color of latosol due to iron oxides. This combination of factors results in a soil profile that is highly weathered and often lacks critical nutrients, making it challenging for agricultural practices without the addition of fertilizers.