1. Ferralsols and Plinthosols
Peter Schad
Chair of Soil Science
Technische Universität München
(Munich University of Technology)

2. Factors of soil formation
Ferralsols and Plinthosols
occur in the
- permanently humid tropics
- summer-humid tropics
Factors of soil formation
climate
parent materials
biota
topography
time
Which ones differ largely
comparing the tropics with Europe?

3. Climate
high temperatures and high precipitation:
1. chemical weathering increased
2. 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 ice in the Pleistocene
(Central and Northern Europe: nearly all soils began their formation after the Pleistocene)

4. 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

5. 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

6. 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)

7. 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

8. 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 µm fraction; and
5. no characteristics diagnostic for the andic horizon; and
6. thickness of at least 30 cm.

9. 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)

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. Distribution of Ferralsols

17. Ferralsol in Burkina Faso under Tectona grandis

18. Ferralsol in Brazil under soybean

19. Ferralsol in Bolivia with saprolite in greater depth

20. the soils are physically stable
savanna (above)
forest (below)

21. 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

22. 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

23. Hardening in Plinthosols (1)
first result:
soft horizon with special colour pattern: plinthite
then: may harden:
- hard concretions: pisolithes
possible if originated by stagnic properties
- continuously cemented horizon: petroplinthite
possible 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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)

30. 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

31. Distribution of Plinthosols

32. Plinthosol in Bolivia with a soft plinthic horizon

33. Soil in Bolivia with pisolithes

34. Soil in Bolivia with a petroplinthic horizon

35. Soil in South Africa with a petroplinthic horizon
and a pallid zone underneath

36. Petroplinthic horizons at the surface