Ferralsols, PlinthosolsAlisols, Acrisols, Lixisols

Nitisols

Peter Schad

Soil ScienceDepartment of Ecology

Technische Universität München

Ferralsols and Plinthosols

occur in the

- permanently humid tropics- summer-humid tropics

Factors of soil formation

climate

parent materialsbiota

topographytime

Which ones differ largely comparing the tropics with Europe?

Climate

high temperatures and high precipitation:

1. chemical weathering increased2. decomposition of organic matter increased

3. ion leaching increased

Time

many parts of the old Gondwana continent:

- South American lowlands- most parts of Africa- India- western Australia

no glaciars and no tundra in the Pleistocene(Central and Northern Europe: nearly all soils began their formation after the Pleistocene)

Main process of soil formation: ferralitization

1. weathering of silicates, mainly by hydrolysis2. leaching out of base cations and silicon ions

3. formation of kaolinite and oxides

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. aluminium oxides:gibbsite: Al(OH)3

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 fulfils the requirements of an argic horizon and which has in the upper 30 cm, 10% or more water-dispersible clay (unless the soil material has gericproperties or more than 1.4% organic carbon)

Definition of the ferralic horizon (1)

1. a sandy loam or finer particle size and less than 90% (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

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% organic carbon; and

4. less than 10% weatherable minerals in the 50 - 200 µm fraction; and

5. no characteristics diagnostic for the andichorizon; and

6. thickness of at least 30 cm

Porfile depth

often several meters, up to 100 m

deeper horizons: saprolitechemical 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)

Physical characteristics (1)

microaggregates:

kaolinite-oxide-complexeskaolinite: 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 riskfall to pieces if pH rises over 6.5

Physical characteristics (2)

pseudosand -> many macropores

-> good aeration, good drainagehigh clay content -> many micropores

few mesopores -> low plant available water capacity

permanently humid tropics: no problem

tropics with dry seasons: may be a problem

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

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

Chemical characteristics (3)

special problem: phosphate:

at pH < 5: bound to oxidesproblem: there are many oxides in Ferralsols

available P: by mineralization of organic matter

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

Ferralsol in Burkina Faso under Tectona grandis

Ferralsol in Brazil under soybean

Ferralsol in Bolivia with saprolite in greater depth

the soils are physically stable

savanna (above)forest (below)

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

Formation of Plinthosols (2)

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

Formation of Plinthosols (3)

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 going on

Hardening in Plinthosols (1)

first result: soft horizon with special colour pattern: plinthite

then: may harden:- hard concretions: pisolithspossible if originated by stagnic properties

- continuously cemented horizon: petroplinthitepossible in both cases (originated by stagnic or gleyic properties)

Hardening in Plinthosols (2)

hardening:

if enough crystalline iron oxides accumulatemay harden under water-logged conditions

may harden deep in the soilbut it always hardens : - when exposed near to the surface

- and subject to alternating drying and wetting over a long time

Definition of Plinthosols

having either1. a petroplinthic horizon starting within 50 cm

from the soil surface; or2. a plinthic horizon starting within 50 cm from

the soil surface; or3. a plinthic horizon starting within 100 cm from

the soil surface when underlying either an albic horizon or a horizon with stagnicproperties

Definition of the plinthic horizon (1)

1. 25% (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

Definition of the plinthic horizon (2)

2. a. 2.5% (by weight) or more citrate-dithionite extractable iron in the fine earth fraction, especially in the upper part of the horizon, or 10% 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% (by weight) organic

carbon; and4. thickness of 15 cm or more

Definition of the petroplinthic horizon (1)

1. a. 10% (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

Definition of the petroplinthic horizon (2)

2. less than 0.6% (by weight) organic carbon; and3. 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

We regard as plinthic horizon

- plinthite (soft)

- horizon with pisoliths- petroplinthite missing the iron oxide requirements (criterion 1) of the petroplinthic horizon

(diagnostic criteria of the plinthic horizon have to be met)

Characteristics

physical characteristics:

dominated by plinthite, petroplinthite or pisoliths

chemical characteristics:

like Ferralsolsland use:

do not expose the plinthite to the surface-> risk of hardening

Plinthosol in Bolivia with a soft plinthic horizon

Soil in Bolivia with pisoliths

Soil in Bolivia with a petroplinthic horizon

Soil in South Africa with a petroplinthic horizonand a pallid zone underneath

Petroplinthic horizons at the surface

Alisols, Lixisols, Acrisols

characterized by an argic horizon

(like Luvisols and Albeluvisols)

Definition of the argic horizon (1)

1. sandy loam or finer and at least 8% clay; and2. more total clay than the overlying horizon:

a. if the overlying horizon has less than 15% clay, the argic horizon must have at least 3% more (abs.); b. if the overlying horizon has 15 - 40% clay, the ratio of clay in the argic horizon to that of the overlying horizon must be at least 1.2;

c. if the overlying horizon has more than 40% clay, the argic horizon must have at least 8% more (abs.); and

Definition of the argic horizon (2)

3. if formed by clay illuviation: increase in clay content within 30 cm;

if formed by any other processes: increase in clay content within 15 cm; and

4. autochthonous rock structure absent in at least half of the volume; and

5. thickness of at least 7.5 cm (shortened)

Genesis of the argic horizon

1. transport of clay minerals from the topsoil to the subsoil

2. in the subsoil: higher weathering of primary silicates and higher clay mineral synthesis

3. in the topsoil: higher weathering of clay minerals

4. selective erosion of clay minerals from the topsoil

Distinguish:Luvisol, Alisol, Lixisol, Acrisol

AcrisolLixisolCEC < 24 cmolc kg-1 clay in the argic in some part

AlisolLuvisolCEC >= 24 cmolc kg-1 clay in the argic throughout

alic properties in 25 – 100 cm in the major part

BS < 50 %,no alic prop. in 25 – 100 cm in the major part

BS >= 50 % in 25 – 100 cm in the major part

Definition of alic properties (1)

1. a cation exchange capacity (by 1 M NH4OAc) of at least 24 cmolc kg-1 clay; and

2. a. a total reserve in bases (TRB = exchangeable plus mineral Ca, Mg, K and Na) of the clay which is 80% or more of the TRB in the soil; orb. a silt / clay ratio of 0.6 or less; and

3. a pH (KCl) of 4.0 or less; and

Definition of alic properties (2)

4. a KCl extractable Al content of 12 cmolckg-1 clay or more, and a KCl extractable Al / CECclay ratio of 0.35 or more; and

5. an aluminium saturation (exch. Al / ECEC * 100) of 60% or more

Distribution

Lixisols: summer-humid (winter-dry) tropics

Alisols: humid tropics and sub-tropics (active weathering state)

Acrisols: humid tropics and sub-tropics (older soils)

Lixisol (Ghana)

Alisol (Perú)

Acrisol (China)

Land use problems

Lixisols: low CEC

Alisols: low BS, high amount of free Al

Acrisols: low CEC low BS

Nitisols

having

• a nitic horizon starting within 100 cm from the soil surface; and

• gradual to diffuse horizon boundaries between the surface and the underlying horizons; and

• no ferric, plinthic or vertic horizon within 100 cm from the soil surface

Definition of the nitic horizon (1)

1. diffuse to gradual transitions to horizons immediately above and below (less than 20% change in clay content, over at least 12 cm, no abrupt colour change); and

2. a. more than 30% clay; andb. water-dispersible clay / total clay ratio less than 0.1 (unless there is more than 0.6% organic carbon); andc. silt / clay ratio is less than 0.4; and

Definition of the nitic horizon (2)

3. moderate to strong, nutty or polyhedric structure, with shiny pedfaces, which cannot or can only partially be associated with illuviation argillans in thin sections; and

4. Munsell colour value of 5 or less, and chroma of 4 or less, but no mottling of hydromorphic nature (gleyic or stagnic); and

Definition of the nitic horizon (3)

5. a. 4.0% or more citrate-dithionite extractable iron („free“ iron) in the fine earth fraction; andb. more than 0.2% acid oxalate (pH 3) extractable iron („active“ iron) in the fine earth fraction; andc. ratio between „active“ and „free“ iron of 0.05 or more; and

6. minimum thickness of 30 cm

Characteristics of the Nitisols

“small brother” of the Vertisols: swelling and shrinking clay minerals, but less

“big brother” of the Ferralsols:

kaolinite,but less

land use: intermediate

Nitisol (Vietnam)