55
2. 10 ml of 1 (N) K 2 Cr 2 O 7 be pipetted into the
conical flask and be swirled a little.
3. The conical flask be kept on an asbestos sheet.
4. Then, 20 ml of H 2 SO 4 (containing 1.25 % of
silver sulphate) be added into the conical flask
and the flask be swirled again 2 or 3 times.
5. The conical flask be now allowed to stand for
30 min.
6. Then, 200 ml of DW be added into it.
7. After adding 10 ml of phosphoric acid and
1 ml of diphenylamine indicator, the contents
in the conical flask be titrated with ferrous
ammonium sulphate till the colour changes
from blue-violet to green.
8. Similarly, a blank be run without soil.
NB: If more than 7 ml of ferrous ammonium
sulphate solution is consumed, the whole procedure of determination be repeated with a smaller
quantity of soil.
Calculation and Result
Organic carbon
weight of soil
%
.
( ) =
-
(
)
é ë
ù û ´
´
-
(
)
10
0 003 100
B T
B
where
B = Volume of ferrous ammonium sulphate solution required for blank (B)
T = Volume of ferrous ammonium sulphate solution required for Test soil sample (T)
4.5.4 Available Phosphorus (AP)
It indicates the amount of phosphorus available
in the soil. AP could be estimated by two methods, namely, (a) by Olsen’s method (Olsen et al.
1954) and (b) by Dickman and Bray’s method
(Dickman and Bray 1940).
The Dickman and Bray’s method is briefly
described below:
4.5.4.1 Principle
The available phosphorus content in the soil is
evaluated by rapidly extracting it with suitable
reagents. In this method, the orthophosphate
and molybdate ions are said to condense in
acidic solution to give molybdophosphoric acid
(phosphomolybdic acid), which, upon selective
reduction (say, with stannous chloride), produce a
blue colour due to molybdenum blue of uncertain
composition.
4.5.4.2 Main Materials Required
1. Klett-Summerson (USA) Photoelectric colorimeter (say model No.800-3)
2. Dickman and Bray’s reagent: 15 g of ammonium molybdate be dissolved in 300 ml of
DW and be warmed to about 60 °C and cooled
and filtered, if necessary. To this, 350 ml of 10
(N) HCl be added followed by an excess 50 ml
of 10 (N) HCl and made to 1 l. The normality
of the HCl used be adjusted correctly by
titration.
3. Stannous chloride solution: 10 g of stannous
chloride (crystalline) be dissolved in 26 ml of
conc. HCl by warming and stored in an ambercoloured bottle. This is the 40 % SnCl 2 stock
solution. Just before use, 0.5 ml be diluted to
66 ml with DW.
4. Glasswares like beakers, conical flasks, volumetric flasks, measuring cylinders, burettes,
pipettes, test tubes.
4.5.4.3 Procedure
1. 1.5 ml of soil extract be pipetted into a 25-ml
volumetric flask.
2. 5 ml of Dickman and Bray’s reagent be added
to it.
3. The neck of the flask be washed down and the
contents in the flask be diluted to c 22 ml.
4. To this, 1 ml of diluted stannous chloride solution is added, shaken and made up to the
25-ml mark.
5. The intensity of the blue colour be measured
just after 10 min. This is important, as the
colour starts to fade away after the stipulated
time.
6. Preparation of standard curve for phosphorus:
0.439 g of AR grade potassium dihydrogen
orthophosphate (KH 2 PO 4 ) be weighed accurately (after keeping it in hot air oven at 40 °C
for 1 h) and be dissolved in about 500 ml of DW.
To this, 25 ml of 7 (N) H 2 SO 4 be added and
made up to 1 l with DW. This gives a 100- ppm
stock solution of phosphorus (100 microgram
4.5 Chemical Characteristics
2. 10 ml of 1 (N) K 2 Cr 2 O 7 be pipetted into the
conical flask and be swirled a little.
3. The conical flask be kept on an asbestos sheet.
4. Then, 20 ml of H 2 SO 4 (containing 1.25 % of
silver sulphate) be added into the conical flask
and the flask be swirled again 2 or 3 times.
5. The conical flask be now allowed to stand for
30 min.
6. Then, 200 ml of DW be added into it.
7. After adding 10 ml of phosphoric acid and
1 ml of diphenylamine indicator, the contents
in the conical flask be titrated with ferrous
ammonium sulphate till the colour changes
from blue-violet to green.
8. Similarly, a blank be run without soil.
NB: If more than 7 ml of ferrous ammonium
sulphate solution is consumed, the whole procedure of determination be repeated with a smaller
quantity of soil.
Calculation and Result
Organic carbon
weight of soil
%
.
( ) =
-
(
)
é ë
ù û ´
´
-
(
)
10
0 003 100
B T
B
where
B = Volume of ferrous ammonium sulphate solution required for blank (B)
T = Volume of ferrous ammonium sulphate solution required for Test soil sample (T)
4.5.4 Available Phosphorus (AP)
It indicates the amount of phosphorus available
in the soil. AP could be estimated by two methods, namely, (a) by Olsen’s method (Olsen et al.
1954) and (b) by Dickman and Bray’s method
(Dickman and Bray 1940).
The Dickman and Bray’s method is briefly
described below:
4.5.4.1 Principle
The available phosphorus content in the soil is
evaluated by rapidly extracting it with suitable
reagents. In this method, the orthophosphate
and molybdate ions are said to condense in
acidic solution to give molybdophosphoric acid
(phosphomolybdic acid), which, upon selective
reduction (say, with stannous chloride), produce a
blue colour due to molybdenum blue of uncertain
composition.
4.5.4.2 Main Materials Required
1. Klett-Summerson (USA) Photoelectric colorimeter (say model No.800-3)
2. Dickman and Bray’s reagent: 15 g of ammonium molybdate be dissolved in 300 ml of
DW and be warmed to about 60 °C and cooled
and filtered, if necessary. To this, 350 ml of 10
(N) HCl be added followed by an excess 50 ml
of 10 (N) HCl and made to 1 l. The normality
of the HCl used be adjusted correctly by
titration.
3. Stannous chloride solution: 10 g of stannous
chloride (crystalline) be dissolved in 26 ml of
conc. HCl by warming and stored in an ambercoloured bottle. This is the 40 % SnCl 2 stock
solution. Just before use, 0.5 ml be diluted to
66 ml with DW.
4. Glasswares like beakers, conical flasks, volumetric flasks, measuring cylinders, burettes,
pipettes, test tubes.
4.5.4.3 Procedure
1. 1.5 ml of soil extract be pipetted into a 25-ml
volumetric flask.
2. 5 ml of Dickman and Bray’s reagent be added
to it.
3. The neck of the flask be washed down and the
contents in the flask be diluted to c 22 ml.
4. To this, 1 ml of diluted stannous chloride solution is added, shaken and made up to the
25-ml mark.
5. The intensity of the blue colour be measured
just after 10 min. This is important, as the
colour starts to fade away after the stipulated
time.
6. Preparation of standard curve for phosphorus:
0.439 g of AR grade potassium dihydrogen
orthophosphate (KH 2 PO 4 ) be weighed accurately (after keeping it in hot air oven at 40 °C
for 1 h) and be dissolved in about 500 ml of DW.
To this, 25 ml of 7 (N) H 2 SO 4 be added and
made up to 1 l with DW. This gives a 100- ppm
stock solution of phosphorus (100 microgram
4.5 Chemical Characteristics