ferralsols and plinthosols peter schad chair of soil science technische universität münchen...
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Ferralsols and Plinthosols
Peter Schad
Chair of Soil Science
Technische Universität München(Munich University of Technology)
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Factors of soil formation
climateparent materialsbiotatopographytime
Which ones differ largely comparing the tropics with Europe?
Ferralsols and Plinthosols
occur in the- permanently humid tropics- summer-humid tropics
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Timemany parts of the old Gondwana continent:
- South American lowlands- most parts of Africa- India- western Australia
no ice in the Pleistocene
(Central and Northern Europe: nearly all soils began their formation after the Pleistocene)
Climate
high temperatures and high precipitation:
1. chemical weathering increased
2. decomposition of organic matter increased
3. ion leaching increased
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Main process of soil formation: ferralitization
1. weathering of silicates, mainly by hydrolysis
2. leaching out of base cations and silicon ions
3. formation of kaolinite and oxides
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Mineral constituents of Ferralsols and Plinthosols
1. quartz (residual): SiO2
2. kaolinite: 1:1 clay mineral
3. iron oxides:
hematite: red, high temperatures: Fe2O3
goethite: brown, everywhere: FeOOH
4. aluminum oxides:
gibbsite: Al(OH)3
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Definition of Ferralsols
1. having a ferralic horizon at some depth between 25 and 200 cm from the soil surface and
2. lacking a nitic horizon within 100 cm from the soil surface and
3. lacking a layer which fulfills the requirements of an argic horizon and which has in the upper 30 cm, 10 percent or more water-dispersible clay (unless the soil material has geric properties or more than 1.4 percent organic carbon)
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Definition of the ferralic horizon (1)
A ferralic horizon must have:
1. a sandy loam or finer particle size and less than 90 percent (by weight) gravel, stones or petroplinthic (iron-manganese) concretions; and
2. a cation exchange capacity (by 1 M NH4OAc) of 16 cmolc kg-1 clay or less and an effective cation exchange capacity (sum of exchangeable bases plus exchangeable acidity in 1 M KCl) of less than 12 cmolc kg-1 clay; and
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Definition of the ferralic horizon (2)
3. less than 10% water-dispersible clay, unless the soil material has geric properties or more than 1.4 percent organic carbon; and
4. less than 10 percent weatherable minerals in the 50 - 200 µm fraction; and
5. no characteristics diagnostic for the andic horizon; and
6. thickness of at least 30 cm.
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Porfile depth
often several meters, up to 100 m
deeper horizons: saprolite
chemical weathering in situ
nothing else happens:
- little physical weathering
- almost no organisms
- no translocations
-> rock structure remains unchanged
-> low bulk density
(ions washed out, but volume remains)
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Physical characteristics (1)
microaggregates:
kaolinite-oxide-complexes
kaolinite: neg. charged (unless the pH is very low)
oxides: pos. charged (pH <6.5)
also called: pseudosand, pseudosilt (-> problems with the field detection of texture by feel)
stable, little erosion risk
fall to pieces if pH rises over 6.5
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Physical characteristics (2)
pseudosand -> many macropores
-> good aeration, good drainage
high clay content -> many micropores
few mesopores
-> low plant available water capacity
permanently humid tropics: no problem
tropics with dry seasons: may be a problem
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Chemical characteristics (1)
organic matter:
1. rapid decomposition and intensive bioturbation
- high rainfall
- high temperature
- good drainage
-> only few organic acids
pH only moderately acid (often ca. 5)
2. high biomass production
-> many plant residues
-> rel. high stocks of org. matter in the min. soil
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Chemical characteristics (2)
mineral constituents:
- low CEC
- little nutrient release by weathering of primary minerals (only a few primary minerals left)
organic constituents:
- high CEC
- high nutrient release by decomposition
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Chemical characteristics (3)
special problem: phosphate:
at pH < 5: bound to oxides
problem: there are many oxides in Ferralsols
available P: by mineralization of organic matter
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Land use
traditional: shifting cultivation: slash and burn:
2 – 4 years of agriculture
10 – 20 years of forest fallow
modern intensive agriculture: possible if:
- large amounts of mineral fertilizers
(-> expensive)
- no tillage
(avoid too fast mineralization and erosion)
modern alternative: agroforestry systems
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Distribution of Ferralsols
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Ferralsol in Burkina Faso under Tectona grandis
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Ferralsol in Brazil under soybean
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Ferralsol in Bolivia with saprolite in greater depth
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the soils are physically stable
savanna (above)forest (below)
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Formation of Plinthosols (1)
1. ferralitization: relative enrichment of Fe in situ
2. some Plinthosols: absolute enrichment of Fe:
in depressions:
- laterally flowing water
- capillary rise of groundwater
3. iron distribution by redox processes:
groundwater (gleyic properties):
Fe oxides accumulate at the aggregate surfaces
rainwater or floodwater (stagnic properties):
Fe oxides accumulate in the centres of the aggregates
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Formation of Plinthosols (2)
groundwater: processes 2 and 3 together
groundwater: under the enriched horizon:pallid zone:
white (pure kaolinite, Fe-depleted)former horizon with permanently reduced cond.
most Plinthosols:redox processes actually not ongoing
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Hardening in Plinthosols (1)first result: soft horizon with special colour pattern: plinthite
then: may harden:
- hard concretions: pisolithespossible if originated by stagnic properties
- continuously cemented horizon: petroplinthitepossible in both cases (originated by stagnic or gleyic properties)
hardening:
if enough crystalline iron oxides accumulate
may harden under water-logged conditions
may harden deep in the soil
but it always hardens :
- when exposed near to the surface
- and subject to alternating drying and wetting over a long time
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Definition of Plinthosols
Soils having either
1. a petroplinthic horizon starting within 50 cm from the soil surface; or
2. a plinthic horizon within 50 cm from the soil surface; or
3. a plinthic horizon starting within 100 cm from the soil surface when underlying either an albic horizon or a horizon with stagnic properties
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Definition of the plinthic horizon (1)
A plinthic horizon must have:
1. 25 percent (by volume) or more of an iron-rich, humus-poor mixture of kaolinitic clay with quartz and other diluents, which changes irreversibly to a hardpan or to irregular aggregates on exposure to repeated wetting and drying with free access of oxygen; and
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Definition of the plinthic horizon (2)
2. a. 2.5 percent (by weight) or more citrate-dithionite extractable iron in the fine earth fraction, especially in the upper part of the horizon, or 10 percent in the mottles or concretions; and
b. ratio between acid oxalate (pH3) extractable iron and citrate-dithionite extractable iron of less than 0.1; and
3. less than 0.6 percent (by weight) organic
carbon; and
4. thickness of 15 cm or more
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Definition of the petroplinthic horizon (1)
A petroplinthic horizon must have:
1. a. 10 percent (by weight) or more citrate-dithionite extractable iron, at least in the upper part of the horizon; and
b. ratio between acid oxalate (pH3) extractable iron and citrate-dithionite extractable iron of less than 0.1; and
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Definition of the petroplinthic horizon (2)
2. less than 0.6 percent (by weight) organic carbon; and
3. cementation to the extent that dry fragments do not slake in water and it cannot be penetrated by roots
4. thickness of 10 cm or more
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We regard as plinthic horizon
- plinthite (soft)
- horizon with pisolithes
- petroplinthite missing the iron oxide requirements (criterion 1) of the petroplinthic horizon
(diagnostic criteria of the plinthic horizon have to be met)
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Characteristics
physical characteristics:
dominated by plinthite, petroplinthite or pisolithes
chemical characteristics:
like Ferralsols
land use:
do not expose the plinthite to the surface
-> risk of hardening
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Distribution of Plinthosols
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Plinthosol in Bolivia with a soft plinthic horizon
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Soil in Bolivia with pisolithes
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Soil in Bolivia with a petroplinthic horizon
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Soil in South Africa with a petroplinthic horizonand a pallid zone underneath
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Petroplinthic horizons at the surface