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APPENDIX Methods of Soil Analysis

Soil Texture:

In the dispersed soil samples, the soil particles settle down at a differential settling rate according to their size. In the estimation of soil texture, particles below 2mm diameter are separately determined which constitute sand, silt and clay. Each one of them is characterized as below:

Coarse sand: 2.0 – 0.2 mm diameter

Fine sand: 0.2 – 0.02 mm diameter

Silt: 0.02 – 0.002 mm diameter

Clay: < 0.002 mm diameter

Soil Moisture:

Gravimetric method of moisture estimation is most widely used where the soil sample is placed in an oven at 105̊C and dried to a constant weight. The difference in weight is considered to be water present in the soil sample.

Moisture (%) = Oven - dry weight of soil

Loss in weight

X 100

PH:

The procedure for measurement of soil pH is given below.

Apparatus:

• pH meter with a range of 0-14 pH, Pipette/dispenser, Beaker, Glass rod

Reagent

• Buffer solutions of pH 4, 7 and 9

• Calcium chloride solution (0.01M): Dissolve 14.7 g CaCl2.2H2O in 10 litre of water to obtain 0.01M solution.

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Procedure:

1. Calibrate the pH meter, using 2 buffer solutions, one should be the buffer with neutral pH (7.0) and the other should be chosen based on the range of pH in the soil. Take the buffer solution in the beaker. Insert the electrode alternately in the beakers containing 2 buffer solutions and adjust the pH. The instrument indicating pH as per the buffers is ready to test the samples

2. Weigh10.0g of soil sample into 50 or 100 ml beaker, add 20ml of CaCl2 solution (use water instead of CaCl2 solution throughout the procedure if water is used as a suspension medium).

3. Allow the soil to absorb CaCl2 solution without stirring and then thoroughly stir for 10 second using a glass rod.

4. Stir the suspension for 30 minutes and record the pH on the calibrated pH meter.

Based on soil pH values, following types of soil reactions are distinguished:

<4.6 ----- Extremely acid

4.6 - 5.5 --------- Strongly acid

5.6 - 6.5 --------- Moderately acid

6.6 - 6.9 --------- Slightly acid

7.0 -------- Neutral

7.1 - 8.5 ------- Moderately alkaline

>8.5 ---------- Strongly alkaline

Electrical conductivity:

Generally a 1:2 soil/water suspension is used.

Apparatus

• EC meter, Beakers (25 ml), Erlenmeyer flasks (250 ml) and pipettes, Filter paper

Reagent

0.01M Potassium chloride solution: Dry a small quantity of AR grade potassium chloride at 600 C for two hours. Weight 0.7456 g of it and dissolve in freshly prepared distilled water and make the volume to one litre. This solution gives an electrical conductivity of 1411.8x10-3 i.e. 1.412 mS/cm at 250C. For best result, select a conductivity standard (KCl solution) close to the sample value.

186

Procedure

1. Take 40 g soil into 250 ml Erlenmeyer flask, add 80 ml of distilled water, stopper the flask and shake on reciprocating shaker for one hour. Filter through Whatman No.1 filter paper. The filtrate is ready for measurement of conductivity.

2. Wash the conductivity electrode with distilled water and rinse with standard KCl solution.

3. Pour some KCl solution into a 25 ml beaker and dip the electrode in the solution. Adjust the conductivity meter to read 1.412 mS/cm, corrected to 250 C.

4. Wash the electrode and dip it in the soil extract.

5. Record the digital display corrected to 250 C. The reading in mS/cm of electrical conductivity is a measure of the soluble salt content in the extract, and an indication of salinity status of this soil (Table 18). The conductivity can also be expressed as mmhos/cm.

Organic Carbon: Volumetric method (Walkley and Black, 1934)

Apparatus:

Conical flask - 500 ml, Pipettes - 2, 10 and 20 ml, Burette - 50 ml

Reagents

• Phosphoric acid – 85%, Sodium fluoride solution – 2%, Sulphuric acid – 96 % containing 1.25% Ag2SO4, Standard 0.1667M K2Cr2O7: Dissolve 49.04 g of K2Cr2O7 in water and dilute it to 1 litre.

• Standard 0.5M FeSO4 solution: Dissolve 140 g Ferrous Sulphate in 800 ml water, add 20 ml concentrated H2SO4 and make up the volume to 1 litre

• Diphenylamine indicator: Dissolve 0.5 g reagent grade diphenylamine in 20 ml water and 100 ml concentrated H2SO4.

Procedure

1. Weigh 1.0 g of the prepared soil sample in 500 ml conical flask.

2. Add 10 ml of 0.1667M K2Cr2O7 solution and 20 ml concentrated H2SO4 containing Ag2SO4.

3. Mix thoroughly and allow the reaction to complete for 30 minutes.

4. Dilute the reaction mixture with 200 ml water and 10 ml H3PO4.

5. Add 10 ml of NaF solution and 2 ml of diphenylamine indicator.

187

6. Titrate the solution with standard 0.5M FeSO4 solution to a brilliant green colour.

7. A blank without sample is run simultaneously.

Calculation

Percent organic Carbon (X) = 10 (S - T) x 0.003 X

S ˟ Wt. of soil

100

Where,

S = ml FeSO4 solution required for blank

T = ml FeSO4 solution required for soil sample

3 = Eq W of C (weight of C is 12, valency is 4, hence Eq W is 12÷4 = 3.0)

0.003 = weight of C (1 000 ml 0.1667M K2Cr2O7 = 3 g C. Thus, 1 ml 0.1667M

K2Cr2O7 = 0.003 g C).

Organic Carbon recovery is estimated to be about 77%. Therefore, actual amount of organic carbon (Y) will be: Percent value of organic carbon obtained X

77

100

Total Nitrogen: (Subbia And Asija, 1956):

Apparatus

• .Nitrogen distillation unit, preferably with six regulating heating elements.

• Conical flasks, pipettes, burette, etc.

Reagents

• 0.32% KMnO4: Dissolve 3.2 g of KMnO4 in water and make the volume to one litre.

• 2.5% NaOH: Dissolve 25 g of sodium hydroxide pellets in water and make the volume to one litre.

• 2% Boric acid: Dissolve 20 g of boric acid powder in warm water by stirring and dilute to one litre.

• Mixed Indicator: Dissolve 0.066 g of methyl red and 0.099 g of bromocresol green in 100 ml of ethyl alcohol. Add 20 ml of this mixed indicator to each litre of 2% boric acid solution.

• 0.1M Potassium Hydrogen Phthalate: Dissolve 20.422 g of the salt in water and dilute to one litre. This is a primary standard and does not require standardization.

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• 0.02M H2SO4: Prepare approximately 0.1M H2SO4 by adding 5.6 ml of conc. H2SO4 to about 1 litre of distilled water. From this, prepare 0.02M H2SO4 by diluting a suitable volume (20 ml made to 100 ml) with distilled water. Standardize it against 0.1M NaOH solution.

• 0.1M NaOH. Dissolve 4gm NaOH in 100 ml distilled water. Standardize against potassium hydrogen phthalate.

Procedure

1. Weigh 20 g of soil sample in a 800 ml Kjeldahl flask.

2. Moisten the soil with about 10 ml of distilled water, wash down the soil, if any, adhering to the neck of the flask.

3. Add 100 ml of 0.32% of KMnO4 solution.

4. Add a few glass beads or broken pieces of glass rod.

5. Add 2-3 ml of paraffin liquid, avoiding contact with upper part of the neck of the flask.

6. Measure 20 ml of 2% boric acid containing mixed indicator in a 250 ml conical flask and place it under the receiver tube. Dip the receiver tube in the boric acid.

7. Run tap water through the condenser.

8. Add 100 ml of 2.5% NaOH solution and immediately attach to the rubber stopper fitted in the alkali trap.

9. Switch the heaters on and continue distillation until about 100 ml of distillate is collected.

10. First remove the conical flask containing distillate and then switch of the heater to avoid back suction.

11. Titrate the distillate against 0.02M H2SO4 taken in burette until pink colour starts appearing.

12. Run a blank without soil.

13. Carefully remove the Kjeldahl flask after cooling and drain the contents in the sink.

Calculation:

Volume of acid used to neutralize ammonia in the sample = A – B ml

N content in the test sample = (A – B) x 0.56mg

Percent Nitrogen (A – B) x 0.56 x 5

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Where,

A = Volume of 0.02M H2SO4 used in titration against ammonia absorbed in boric acid.

B = Volume of 0.02M sulphuric acid used in blank titration. 1 ml of 0.02M sulphuric acid = 0.56 mg N (1 000 ml of 1M H2SO4 = 14 g Nitrogen).

Wt. of soil sample = 20 g. Thus, factor for converting into % Nitrogen = 100/20 = 5

Available Phosphorus: Olsen’s Method

In these methods, specific coloured compounds are formed with the addition of appropriate reagents in the solution, the intensity of which is proportionate to the concentration of the element being estimated. The colour intensity is measured spectro -photometrically.

Apparatus

• Spectrophotometer, Pipette - 2 ml, 5 ml, 10 ml and 20 ml, Bearkers/flasks - 25 ml, 50 ml, 100 ml and 500 ml

Reagents

• Bicarbonate extractant: Dissolve 42 g Sodium bicarbonate in 1 litre of distilled water and adjust the pH to 8.5 by addition of dilute NaOH or HCl. Filter, if necessary.

• Activated carbon – Darco G 60.

• Molybdate reagent

• Stannous chloride solutionProcedure

1. Preparation of the standard curve:.

2. Extraction: Add 50 ml of the bicarbonate extractant to 100 ml conical flask, containing

2.5 g soil sample. Add 1 g activated carbon. Shake for 30 minutes on the mechanical shaker and filter.

Calculation

P (kg/ha) = A X 50 X

1000000 5 5

2000000 = 4A

Where,

Weight of the soil taken = 5 g

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Volume of the extract = 50 ml

Volume of the extract taken for estimation = 5 ml

Volume made for estimation (dilution = 5 times) = 25 ml

Amount of P observed in the sample on the standard curve = A (μg).

Wt. of 1 ha of soil upto a depth of 22 cm is taken as 2 million kg.

Available Potassium (Flame photometeric method)

Potassium present in the soil is extracted with neutral ammonium acetate of 1 molarity. This is considered as plant available K in the soils. It is estimated with the help of flame photometer. This is a well-accepted method.

Apparatus

• Multiple Dispenser or automatic pipette – 25 ml, Flasks and beakers - 100 ml, Flame Photometer

Reagents

• Molar neutral ammonium acetate solution: Dissolve 77 g of ammonium acetate (NH4C2H3O2) in 1 litre of water. Check the pH with bromothymol blue or with a pH meter. If not neutral, add either ammonium hydroxide or acetic acid as per the need to neutralize it to pH 7.0.

• Standard potassium solution: Dissolve 1.908 g pure KCl in 1 litre of distilled water. This solution contains 1 mg K/ml . Take 100 ml of this solution and dilute to 1 litre with ammonium acetate solution. This gives 0.1 mg K/ml as stock solution.

• Working potassium standard solutions: Take 0, 5, 10, 15 and 20 ml of the stock solution separately and dilute each to 100 ml with the M ammonium acetate solution. These solutions contain 0, 5, 10, 15 and 20 μg K/ml, respectively.

Procedure

1. Preparation of the Standard Curve: Set up the flame photometer by atomizing 0 and 20 μg K/ml solutions alternatively to 0 and 100 reading. Atomize intermediate working standard solutions and record the readings. Plot these readings against the respective potassium contents and connect the points with a straight line to obtain a standard curve.

2. Extraction: Add 25 ml of the ammonium acetate extractant to conical flask fixed in a wooden rack containing 5 g soil sample. Shake for 5 minutes and filter.

3. Determine potash in the filtrate with the flame photometer.

191

Calculation:

K (kg/ha) = A X 25 X

5 1000000

2000000

Where,

A = content of K (μg) in the sample, as read from the standard curve:

Weight of 1 ha of soil up to a plough depth of 22 cm is approx. 2 million kg.

Calcium by Versenate (EDTA) method

Apparatus

• Shaker, Porcelain dish, Beakers, Volumetric/conical flask.

Reagents

• Ammonium chloride – ammonium hydroxide buffer solution: Dissolve 67.5 g ammonium chloride in 570 ml of conc. ammonium hydroxide and make to 1 litre.

• Standard 0.01N calcium solution: Take accurately 0.5 g of pure calcium carbonate and dissolve it in 10 ml of 3N HCl. Boil to expel CO2 and then make the volume to 1 litre with distilled water.

• EDTA solution (0.01N): Take 2.0 g of versenate, dissolve in distilled water and make the volume to 1 litre. Titrate it with 0.01N calcium solution and make necessary dilution so that its normality is exactly equal to 0.01N.

• Muroxide indicator powder: Take 0.2 g of muroxide (also known as ammonium purpurate) and mix it with 40 g of powdered potassium sulphate. This indicator should not be stored in the form of solution, otherwise it gets oxidized.

• Sodium diethyl dithiocarbamate crystals: It is used to remove the interference of other metal ions.

• Sodium hydroxide 4N: Prepare 16% soda solution by dissolving 160 g of pure sodium hydroxide in water and make the volume to 1 litre. This will give pH 12.

Procedure

1. Take 5 g air dried soil sample in 150 ml conical flask and add 25 ml of neutral normal ammonium acetate. Shake on mechanical shaker for 5 minutes and filter through Whatman filter paper No.1.

2. Take a suitable aliquot (5 or 10 ml) and add 2-3 crystals of carbamate and 5 ml of 16% NaOH solution.

192

3. Add 40-50 mg of the indicator powder. Titrate it with 0.01N EDTA solution till the colour gradually changes from orange red to reddish violet (purple). It is advised to add a drop of EDTA solution at an interval of 5 to 10 seconds, as the change of colour is not instantaneous.

4. The end point must be compared with a blank reading. If the solution is over titrated, it should be back titrated with standard calcium solution and exact volume used is thus found. Note the volume of EDTA used for titration.

Calculation

If N1 is normality of Ca++ and V1 is volume of aliquot taken and N2V2 are the

normality and volume of EDTA used, respectively, then,

N1V1 = N2V2

N1 =

V1 ml of aliquot taken

N2V2 = Normality of EDTA X Vol of EDTA

Here N1 (Normality) = equivalent of Ca2+ present in one litre of aliquot

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