Geoscience Reference
In-Depth Information
Table 2.8 Percentage of organic matter, nitrogen, and p H of the A O horizon under varying
altitudinal extent
Altitude
(ft)
p H
content
Mean
annual
rainfall
(in.)
Drainage
condition
Vegetation
type
%of
organic
matter
(A O layer)
%ofN 2
in A O
horizon
6,000
120
Well-
drained
Virgin high
forest
7 - 7.5
22.32
1.14
5,000
150
Well-
drained
Virgin high
forest
6.5
19.23
0.97
4,000
120
Well-
drained
Virgin high
forest
6.5
18.76
0.77
3,000
150
Well-
drained
Virgin high
forest
6
24.9
1.05
2,000
150
Well-
drained
Virgin high
forest
6.5
10.29
0.35
1,000
150
Well-
drained
Virgin high
forest
6.5
7.05
0.31
500
150
Not well-
drained
Virgin high
forest
7
7.5
7.57
0.36
-
Source Darjiling District Gazetteers, O ' Malley ( 1999 )
2.3.1 Textural Characteristics of the Soil
Soil texture is de
ned by size distribution or mass fractions of primary particles in soil
(individual grains and particles). Primary mineral particles formed through physical
and chemical weathering of parent material and refractory organic substances make up
the solid phase. Particle size distribution and shape are the most important character-
istics affecting: pore geometry, total pore volume (porosity), and pore size distribution.
The presence of sand, silt and clay within a speci
c amount of soil are con-
sidered as the soil texture. On the basis of the amount of sand, silt and clay present
in the soil the porosity, water holding capacity of the soil varies and consequently
influence the cohesive nature of the soil. The higher amount of sand present in soil
is characterized by the higher rate of permeability and porosity and decreasing
tendency of the soil cohesiveness. The
finer particles present in soil sometime
becomes mobile in presence of seepage water and disturbs the soil cohesiveness.
The soil becomes more compact in presence of higher amount of clay particles. All
these three attributes of soil texture generally control the seepage, permeability,
porosity, cohesiveness, water holding capacity etc. Hence, the stability condition of
the slope is governed by the existence of the sand, silt and clay. United State
Department of Agriculture (USDA) propounded a soil textural classes which states
that particles with <0.002 mm diameter in size is recognized as clay, the size class
0.002 and <0.05 mm as silt and Sizes between
0.05 and <2 mm and sand and
sizes of more
2 mm in diameter is treated as gravel. The present work is dealt with
USDA System and textural properties are determined applying sieving method.
 
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