Oxisols are highly weathered tropical soils characterized by a deep subsurface horizon dominated by iron and aluminum oxides. They form under rainforest vegetation in areas with high rainfall and warm temperatures, causing intense chemical weathering that leaches most nutrients from the soil. Oxisols occupy about 8% of the world's land area, mostly in central Africa, northern South America, and southeast Asia. They require careful management and fertilization due to low fertility resulting from the weathering process.

1. OXISOLS
SUMEENA K J
2018-11-109
DEPT.OF SOIL SCIENCE AND
AGRICULTURAL CHEMISTRY

2. OXISOLS
 The word "Oxisol" comes from "oxide" in reference to the
dominance of oxide minerals such as bauxite.
 The central concept of Oxisols is that of soils of the tropical
and subtropical regions. They have gentle slopes on surfaces of
great age.
 They are mixtures of quartz, kaolin, free oxides, and organic
matter. For the most part they are nearly featureless soils
without clearly marked horizons. Differences in properties with
depth are so gradual that horizon boundaries are generally
arbitrary.

3. Properties and Distribution
 Most highly weathered soils found mostly in tropical areas (16-80m
thick).
 Occupy nearly 8% of the world’s land.
 Characterized by a deep oxic subsurface horizon.
 The oxic horizon is dominated by clay-size particles of the hydrous
oxides of iron (Fe2O3) and aluminum (Al2O3.3H2O), with a low
silica-sesquioxide ratio of around 1.5.

4. Properties and Distribution
 Deep red in color due to the presence of iron oxide.
 High clay content, but the clays are of low-activity,
nonsticky type e.g. kaolinite).
 Good physical property, easy to till and drains extremely
well.
 Acidic in reaction, low levels of humus and base content,
infertile.
 Formed under rainforest vegetation in the tropics, hence
most of the nutrients are stored in the over storey layer.

5. Distribution
 Oxisols are dominantly observed in the tropical world ,on
gently sloping,very old landscapes in central Africa, north and
north – east of south America, madagascar and south – east
Asia.
 In tropical India, such soils are localized in Kerala, Tamil nadu,
Karnataka and Odisha.

7. FORMATION
 Intense weathering of rock minerals results in the release
of silica (desilication), alkali and alkaline earths from the
primary aluminosilicates.
 Alkali and alkaline earths are completely leached out of
the profile.
 The released silica (SiO2) either combines with alumina
(Al2O3) to form kaolinite (Al2O3.2SiO2.2H2O) or is
leached by the percolating water. The soil is thus
impoverished in silica, but enriched with iron oxides
(Fe2O3.nH2O) and hydroxides (FeO.OH).

8.  Alternatively, the decrease in silica can also be relative, and caused
by the deposition of sesquioxides (R2O3) transported laterally with
groundwater from the higher surroundings.
 Desilication results in the formation of the oxic mineral horizon
underneath the surface. It consists of weathered mixtures of
sesquioxides, clay and quartz sand. It is poor in humus and is at
least 30cm thick.
 Under high rainfall conditions and where there is no erosion of the
land, this oxic horizon remains soft all the time. Conversely, it
hardens irreversibly to form laterite when exposed to air.

9. CLASSIFICATION
 In the U S soil taxonomy , Oxisols were earlier classified into 5
suborders, such as Aquox, Udox, Ustox, Torrox, and Humox . However ,
their classification was recently been revised by ICOMOX.
 ICOMOX - International Committee on Oxisols
The International Committee on Oxisols (ICOMOX) was formed to
study and discuss the Oxisols and Oxisol-like soils, many of which occur
in tropical areas. family level of soil classification.
The committee was formed in 1978 under the direction of
Dr. Ray Meyer, and concluded activities in 1986 with a final report
issued in 1988 .

10. The changes proposed and adopted by ICOMOX
include,
 1.more coarser soil may qualify for Oxisols, as the limit of 15 %
or more clay content previously required for the oxic horizon
has been deemphasized.
 2. The limit of weatherable minerlas of less than 10% in the
oxic or kandic horizon, fixed earlier, has been waived off in
case of soils containing 40% or more clay in the surface
horizon.
 3.The suborder Humox , which was based on the high humus
content , has been discontinued and another, viz Perox, has
been introduced to take care of the humus rich soil developed
in the perudic soil moisture regime.

11. Key to Suborders
 Keeping in view the considerations and the criteria of soil moisture
regimes, the oxisols have now been subdivided into five suborders,

12. Key to Suborders
 Aqu.ox
 Aquox are the wet Oxisols.
 They remain remain saturated with water (aquic conditions) for
some time in a year within 50 cm of the soil surface and have one or
more of : a histic epipedon, or an epipedon with a colour value (moist)
of ≤ 3 and ,directly below it, a horizons with chromas of ≤ 2 , or
distinct or prominent redox concentrations within 50cm of the soil
surface ,that is an epipedon and a horizon underneath with hues of 2.5
y or yellower or chromas of ≤ 3,or within 50cm enough active ferrous
iron to give a positive reaction to alpha, alpha-dipyridyl,when soil is
not being irrigated.
 Curing many seed crops and storage of produce are difficult on
these soils.

13.  Torr.ox
 Torrox are the Oxisols of arid regions.
 such soils are only occassionally observed and might have
devoloped under more humid, paleoclimatic conditions.
 Ust.ox
 Ustox are the Oxisols that have an ustic or xeric moisture
regime.
 They are moist long enough for one rain-fed crop.
 Crops are not grown continuously on these soils because there
is inadequate moisture for at least 90 days in normal years.

14.  Per.ox
 oxisols that experience perudic soil soil moisture regime,
indicating continous leaching and translocation of the weathering
products.
 Ud.ox
 Udox are well drained Oxisols with a udic soil moisture regime.
 They have a year-round growing season.
 The dry period is short enough for rainfed crops to be grown
continuously in normal years.

16. Key to Great Groups
 Acr .aquox, Acro.torrox, Acr.ustox, Acro.perox, and Acr.udox
(L.acer, acetum) : Aquox, Torrox, Ustox, Perox and Udox that have
strong acidity and low base saturation. Such soils have in one or
more horizon(s)of an oxic or kandic horizon within 150 cm of the
soil surface, an apparent ECEC of <1.5 c mol(+) kg -1 clay and a pH
value (in 1 N KCL) of >5.0
 Eutr.aquox, Eutr.torrex, Eutro.perox and Eutr.udox : Aquox,
Torrox, Ustox , Perox and udox that have a base saturation (by
NH4OAc) of > 35 % in all horizons within 125 cm of the soil surface.

17.  Plinth.aquox : other Aquox that have plinthite (in a continous
phase) within 125 cm of the soil surface.
 Sombri.ustox, Sombri.perox and Sombri.udox : Ustox , Perox, and
Udox that have a sombric horizon within 150 cm of the soil
surface.
 Kandi.ustox, Kandi.perox and Kandi.udox : Ustox , Perox and
Udoxthat have a kandic horizon that has its upper boundary within
150 cm of the soil surface.
 Hapl.aquox, Haplo.torrox, Hapl.perox and hapl.udox : Aquox,
Torrox, Ustox, Perox and Udox that are of simple and common type
and do not meet the criteria of all other great groups.

18. LAND USE
 Oxisols are chemically degraded soils and need careful
management and fertilization for agricultural use and for
restoring their productivity .
 They are mainly used for low intensity grazing and forestry.
 However some are used for growing plantation and other
crops, such as coffee ,rubber, coca, sugarcane by following the
system of shifting agriculture and some for tropical fruit crops,
such as pineapple, coconut, cashew nut, jackfruit etc

19. CONSTRAINTS
 The oxisols, being acidic,face the problem of fixation of
phosphorus by iron and aluminium to form insoluble phosphstes
(of iron and aluminium)
 These soils are prone to surface soil crusting.
 Shifting agriculture is a common practice adopted in such
areas supporting Oxisols. Reduction of shifting cycle from 20-25
years to 3-5 years due to increased population and high
demand for food results in serious problems of water
erosion,nutrient depletion and poor yield of crops .

20. THANK YOU