53
Volume of the soil clod (V 2 − V 1 = say X ml) = 27 ml
Weight of the soil (W 2 − W 1 = say Y g) = 55.3 g
Bulk density = weight of the soil/volume of the
soil = Y/X = 55.3/27 = 2.05 g/cc.
4.4.6 Water Infiltration Rate of Soil
Main materials required:
1. Two iron rings: height 25 cm, thickness 2 mm
diameter: 1st ring 30 cm, 2nd
ring 60 cm
2. Scale
3. Soil sample
4.4.6.1 Procedure
1. The two rings be inserted (hammered in)
about 10 cm inside the soil
2. Both the rings be filled with water up to the brim.
3. The system be left as such for c 10 min.
4. The height of water within the inner ring be
measured after c 10 min.
5. The system be left as such again for c 10 min.
6. The height of water within the inner ring be
measured again after c 10 min.
7. In this way, the height of the water column
within the inner ring be measured repeatedly
at an interval of c 10 min till constant height
(depth) of the water column inside the inner
ring is obtained.
8. A graph be plotted with depth (height) of the
water column in the Y-axis and time interval
in the X-axis.
The data be recorded in the following table:
No. of obs.
Height (cm) of the water column inside the
inner ring after each 10 min
At 10-min
interval
1st 10
mins
2nd 10
mins
3rd 10
mins
4th 10
mins
5th 10
min
4.5
Chemical Characteristics
Organic matters may fail to oxidise and may
accumulate at the soil surface, while the deeper
mineral soil is wholly or partly anoxic. Thus,
microbial oxidative metabolism has to depend on
electron acceptors other than molecular oxygen.
The most frequent of these are NO 3
−1
, Mn
3+
,
Mn
4+
, Fe
3+
, SO 4
-2
and CO 3
−2
. The characteristic
grey-green colour of the gley horizon is due to
the conversion of Fe
3+
to Fe
2+
by this process with
loss of the normal red or brown oxide colouration. Fe
2+
compounds are more water soluble than
Fe
3+
and may be washed out by percolating water,
thus bleaching the mineral soil. Concentration of
iron and manganese increases in soil solution due
to electron acceptance by these high valency
ions. The divalent compounds are more soluble
in water than those of the oxidised forms.
In soils rich in organic compounds and especially at high temperatures, sulphate may be used
as an electron acceptor by sulphate-reducing bacteria, producing free H 2 S.
In Fe
2+
-rich soils, the sulphide is immediately
removed from solution by the precipitation of
very insoluble FeS which gives the characteristic
black colour to the sapropel sediments of eutrophic freshwaters and coastal mudflats.
In many flooded soils, dissolved iron and
manganese reach phytotoxic concentrations.
Sulphide is less commonly toxic, primarily
because it is scavenged as FeS. However, in soils
which have been leached of Fe
2+
by laterally percolating water, plant growth may be affected. The
hot organic-rich circumstances of rice culture are
particularly conducive to sulphide formation.
Hence, a range of diseases in rice are attributable
to sulphide toxicity, although Oryza sativa is an
extremely waterlogging-tolerant species.
Anaerobic microorganisms in waterlogged
soil also liberate organic products which influence plant growth. When the soil first becomes
anoxic, some fungi produce ethylene (C 2 H 4 ), a
potent plant growth regulator. Organic acids,
such as acetic and butyric, may also act as phytotoxins in anaerobic soils. Conversely, oxidisable
organic compounds may compete with plant
respiratory oxygen need. Extremely reduced
soils and sediments generate copious amount of
inflammable methane microbially and lesser
amount of hydrogen, so that the soil atmosphere
above the water table of some paddy soils may
contain equal volumes of methane and nitrogen.
4.5 Chemical Characteristics
Volume of the soil clod (V 2 − V 1 = say X ml) = 27 ml
Weight of the soil (W 2 − W 1 = say Y g) = 55.3 g
Bulk density = weight of the soil/volume of the
soil = Y/X = 55.3/27 = 2.05 g/cc.
4.4.6 Water Infiltration Rate of Soil
Main materials required:
1. Two iron rings: height 25 cm, thickness 2 mm
diameter: 1st ring 30 cm, 2nd
ring 60 cm
2. Scale
3. Soil sample
4.4.6.1 Procedure
1. The two rings be inserted (hammered in)
about 10 cm inside the soil
2. Both the rings be filled with water up to the brim.
3. The system be left as such for c 10 min.
4. The height of water within the inner ring be
measured after c 10 min.
5. The system be left as such again for c 10 min.
6. The height of water within the inner ring be
measured again after c 10 min.
7. In this way, the height of the water column
within the inner ring be measured repeatedly
at an interval of c 10 min till constant height
(depth) of the water column inside the inner
ring is obtained.
8. A graph be plotted with depth (height) of the
water column in the Y-axis and time interval
in the X-axis.
The data be recorded in the following table:
No. of obs.
Height (cm) of the water column inside the
inner ring after each 10 min
At 10-min
interval
1st 10
mins
2nd 10
mins
3rd 10
mins
4th 10
mins
5th 10
min
4.5
Chemical Characteristics
Organic matters may fail to oxidise and may
accumulate at the soil surface, while the deeper
mineral soil is wholly or partly anoxic. Thus,
microbial oxidative metabolism has to depend on
electron acceptors other than molecular oxygen.
The most frequent of these are NO 3
−1
, Mn
3+
,
Mn
4+
, Fe
3+
, SO 4
-2
and CO 3
−2
. The characteristic
grey-green colour of the gley horizon is due to
the conversion of Fe
3+
to Fe
2+
by this process with
loss of the normal red or brown oxide colouration. Fe
2+
compounds are more water soluble than
Fe
3+
and may be washed out by percolating water,
thus bleaching the mineral soil. Concentration of
iron and manganese increases in soil solution due
to electron acceptance by these high valency
ions. The divalent compounds are more soluble
in water than those of the oxidised forms.
In soils rich in organic compounds and especially at high temperatures, sulphate may be used
as an electron acceptor by sulphate-reducing bacteria, producing free H 2 S.
In Fe
2+
-rich soils, the sulphide is immediately
removed from solution by the precipitation of
very insoluble FeS which gives the characteristic
black colour to the sapropel sediments of eutrophic freshwaters and coastal mudflats.
In many flooded soils, dissolved iron and
manganese reach phytotoxic concentrations.
Sulphide is less commonly toxic, primarily
because it is scavenged as FeS. However, in soils
which have been leached of Fe
2+
by laterally percolating water, plant growth may be affected. The
hot organic-rich circumstances of rice culture are
particularly conducive to sulphide formation.
Hence, a range of diseases in rice are attributable
to sulphide toxicity, although Oryza sativa is an
extremely waterlogging-tolerant species.
Anaerobic microorganisms in waterlogged
soil also liberate organic products which influence plant growth. When the soil first becomes
anoxic, some fungi produce ethylene (C 2 H 4 ), a
potent plant growth regulator. Organic acids,
such as acetic and butyric, may also act as phytotoxins in anaerobic soils. Conversely, oxidisable
organic compounds may compete with plant
respiratory oxygen need. Extremely reduced
soils and sediments generate copious amount of
inflammable methane microbially and lesser
amount of hydrogen, so that the soil atmosphere
above the water table of some paddy soils may
contain equal volumes of methane and nitrogen.
4.5 Chemical Characteristics