Introduction
Soil is naturally occurring
unconsolidated materials on the surface of the earth that has been influenced
by parent material, climate , organisms and relief all acting over the period
of time to produce soil that may be differ from the materials which from
which it was derived in many physical, chemical, & biological ,
mineralogical & morphological properties .
Factors influencing soil
formation
Parent material
- > Climate
- >Biota
The soil profile
The earth surface consists of
particular combination of influences from the five soil forming factors,
causing a different sets of layers k/d “horizons” to form each parts of
landscape, thus slowly giving rise to the natural bodies we called soils. Every
soil has particular sequence of these horizons. A vertical combination of these
sequences is termed as Soil profile.
The main horizons & layers
There are Six major horizons
commonly recognized which are denoted by using the capital letters O,A,E,B, C
& R. Sub horizons may occur within a master horizons and they are
symboled by lowercase letters following
the capital letters noting master horizons (e.g., Bt,Ap or Oi).
Description of Horizons
O horizons
The O group is comprised of
organic matters that generally forms above mineral soil or occur in an organic
soil profile. They derive from dead plant and animal residues. It is generally
absent in grassland regions, O horizons usually occurs in forested area and are
commonly refered as the forest floor. Generally, this horizon consists of three
sub O horizons .
Oi horizons
This layer is an organic horizon
of fibric materials, recognizable plants & animal parts which are slightly
decomposed.
Oe horizons
It consists of hemic materials ,
finely fragmented residues intermediately decomposed, but still with much fiber
evident when rubbed with fingers.
Oa horizons
It consists of sapric materials
that highly decomposed , smooth, amorphous residues that do not retain much
fiber or recognizable tissue structures.
A horizons
It is the topmost mineral
horizons. It generally contains enough partially decomposed (humified) organic
matter to give the soil colour darker than that of lower horizons.A
horizon often coarser in texture, having lost some finer materials by
translocation to the lower horizons and by erosion.
E horizons
This horizon is the zone of
maximum leaching or eluviation of clay, iron , alumunium oxides which leaves a
concentration of resistant materials, such as Quartz, in the sand & silt
sizes.An E horizon is ususally found underneath the A horizon & is
generally lighter in color than either the A horizon above it or the horizons
below . Such E horizons are quite common in soils developed under forests, but
they rarely occur in soils developed under grassland.
B horizons
B horizons form below an O,A or E
horizon and have undergone sufficient changes during soil genesis so that the
original parent material structure is no longer discernable. In many B
horizons, materials have accumulated , typically by washing in from the horizos
above, a process termed Illuviation. In humid regions, B horizons are the
layers of maximum accumulatin of materials such as iron & aluminum oxides,
some of which may have illuviated from above horizons & some of them have
formed in place.
C horizons
The C horizon is the
unconsolidated material underlying the solm (A & B horizons). It may or may
not be the same as the parent material from which the solum forms. The C
horizon is below the zones of greatest biological activity and has not been
sufficiently altered by soil genesis to qualify as a B horizon. In dry
regions, carbonates and gypsum may be concentrated in the C horizon. It is
loose in structure so that it can be easily dug with Showel & also
resembles the structural features of the parent rocks or geological deposits
from which it is formed. Its upper layer may in time become a part of the solum
as weatherin & erosion continue.
R Horizons
These are unconsolidated rock,
with little evidence of weathering.
Sub divisions within master
horizons
There are distinctive divisions
within a master horizons which are denoted by a numerical followed by the
capital letter designation . for e.g in case of B horizon there re sub layers
called B1-B2-B3.
If two different geologic
materials (e.g loess over glacial till) are present with in the soil profile ,
the numeral 2 is placed in front of horizons designated O-A-B-2C if the C
horizon developed in glacial till while upper horizons developed in loess.
Where a layer of mineral organic
soil material was transported by humans (usually using machinery ) from a
source outside the pedon, the “caret” symbol(^) is inserted before the
master horizon designation. For e.g. Suppose a landscaping contractor hauls in
and spreads a layer of sandy fill material over an existing soil in order to
level a site. The resulting soil might eventually ( after enough organic matter
had accumulated to form an A horizon ) have following sequence of
horizons: ^A-^C-2Ab-2Btb,where the 1st two horizons were formed in the human
transported till (hence the ^ prefixes), & the last two horizons were part
of the underlying, now buried soils ( hence the lower case “b” designations).
Transition Horizons:
Transitions layers between master
horizons may be dominated by properties of one horizons but also have
characteristics of another. The two applicable capital letter are used to
designate transition horizons(e.g. AE, EB, BE, & BC), the dominated horizon
being listed before the subordinate one. Letter combinations with a slash such
as E/B, are used to designate transition horizons where distinct parts of the
horizon have properties of E while other parts have properties of B.
Subordinate distinctions:
Since the capital letter
designates the nature of a master horizon in only a very general way, specific
horizon characteristics may be indicated by a lowercase letter following the
master horizon designation. For e.g. there are three types O horizons (Oi, Oe
& Oa) which presents a commonly encountered sequence of horizons. Other
subordinate distinctions include special physical properties and the
accumulation of particular materials such as clay & salts. A list of the
recognized subordinate letter designations and their meanings is given in below
table.
Lowercase letter symbols to
designate Subordinate Distinctions within master horizons
Letter
Distinctions
|
|
a
organic
matter, highly decomposed
b
buried
soil horizon
c
concentrations
or nodules
d
dense
unconsolidated materials
e
organic
matter, intermediate decomposition
f
frozen
soil
ff
dry
permafrost
g
strong
gleying ( mottling)
h
illuvial
accumulation of organic matter
i organic matter, slightly decomposed
J Jarosite (yellow sulphate mineral)
jj Cryoturbation ( frost churning)
k Accumulation of carbonates
m cementation or induration
n accumulation of sodium
o accumulation of Fe & Al oxides
|
References:
Nyle C. Brady & Ray R.Weil. 2008.”The nature and properties
of soils” page no:- 69-72,
Pearson Education publication
https://www.google.com.np/search?q=soil+horizon+and+profile.ppt, images ‘google search’
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