In this article we will discuss about how to determine the nutrients present in soil.
The real value of soil testing is in finding out the information about the available nutrients present in the soil. The collection, preparation and storage of the sample represent the most significant steps in entire analytical operation.
The soil samples, received through the farmers or KPS or ADO, now ADA of respective area, are ground with wooden pestle and mortar. After grinding the soil, it is sieved through 0.2.mm sieve. The particles which are left on the sieve are stone and gravel.
Usually chemical analysis of soil is done to assess the available amounts of major nutrients present in the soil, mainly Nitrogen (N), Phosphorus (P) and Potassium and other properties of soil, say soil reaction (i.e. pH of the soil).
1. Soil pH:
(ii) Wooden Rack
(iii) Beckman or photo volt glass electrode pH meter.
(iv) 20 ml automatic pipettes.
(i) Buffer solution
(ii) Distilled water
50 gm air dry soil is taken in 150 ml beaker and 50 ml water is added in the beaker. It is then stirred occasionally for an hour after placing in a wooden rack. The soil pH is then determined by a pH meter as per directions of the manufactures of the instrument (i.e. pH meter).
2. Organic Carbon:
A. Volumetric Method:
(i) Conical flask – 500 ml.
(ii) Burette – 50 ml.
(iii) Pipettes – 2 ml, 10 ml, and 20 ml
(i) Standard 1 N Potassium dicromate (K2 Cr2O7) solution (Dissolve 49.09 gm K2Cr2O7 in water and dilute it to one litre).
(ii) Standard 0.5 N FeSO4 solution (Dissolve 196.1 gm Fe(NH4)2. (SO4)2 6H2O in 800 ml water containing 20 ml conc. H2SO4 containing 1.25% Ag2SO4 and dilute it to one litre).
(iii) Phosphoric acid (H3PO4) – 85 per cent.
(iv) Sodium flouride (NaF) solution – 2 per cent.
(v) Diphenylamine indicator (0.5 gm. diphenylamine reagent grade is dissolved in 20 ml water and 100 ml concentrated sulphuric acid.
2.0 gm of prepared soil sample is measured in 500 ml conical flask. 10 ml of 1 N K2Cr2O7 and 20 ml concentrated H2SO4 are added to it. The ingredients are mixed thoroughly and allowed the reaction to complete for 30 minutes. 200 ml distilled water, 10 ml Phosphoric acid (H3PO4), 10 ml Sodium fluoride solution (NaF) and 2 ml of Diphenylamine indicator are added. Then the titration was made by adding N/2 Ferrous sulphate until a brilliant green colour is developed. A blank experiment is also done simultaneously in the similar manner without taking the soil.
Volume of Potassium dichromate (K2Cr2O7) = 10 cc
Volume of Ferrous sulphate = 12.4 ml
Weight of soil taken = 2 gm
(B) Calorimetric Method:
(i) Photoelectric calorimeter.
(ii) Reaction Test Tubes – 50 ml or 100 ml flasks.
(iii) Automatic pipette or automatic multiple dispenser – 10 ml or 20 ml.
(i) Standard 1 N K2Cr2O7 solution (Dissolve 49.04 gm K2Cr2O7 in water and dilute it to one litre).
(ii) Conc. sulphuric acid (H2SO4) [AR. containing 1.25 per cent Ag2SO4]
(iii) Sucrose (A.R. quantity).
Weight out accurately on a microbalance, several different amounts of anhydrous A.R. quantity sucrose in the range of 1 to 25 milligrams. Put the weighted sucrose in 50 ml. test tubes, add 10 ml 1 N K2Cr2O7 Solution and 20 ml of cone. H2SO4, The mixture in the test tubes is stirred and is allowed to cool for half an hour.
A blank solution is similarly prepared without sucrose. Read the green colour of reaction of the liquid on the photoelectric calorimeter using 660 mμ. (millimicron) red filter, adjusting the blank solution to zero. Plot the calorimeter reading against carbon values. Calculated from the weights of sucrose by multiplying with 0.42.
Add to each test tube or flask in the wooden rack containing 1 gram measured soil sample, 10 ml of 1 N K2Cr2O7 solution and 20 ml of cone. H2SO4 containing 1.25% Ag2SO4 with automatic pipettes or with multiple dispenser. Stir the reaction mixture and allow to stand overnight. The green chromium sulphate colour of the clear supernatant liquid is read on the photoelectric calorimeter using 660 mμ, red filter, after setting the blanks prepared in the same manner to zero. Read the carbon from the standard curve. Express it as per cent of the soil by multiplying the curve reading with 1.00.
Carry the reaction of soil and K2Cr2O7 reagent in a 100 ml measuring flask. Allow about one hour to complete the reaction. Dilute the mixture with distilled water. Cool to room temperature and make up the volume. Shake and keep overnight for soil to settle. Read the green colour of the clean supernatant liquid on the photoelectric calorimeter. Using is negligible in comparison with organic nitrogen.
To include the nitrate Nitrogen in the final result, Ulsch’s method is to be followed. In this method, the nitrate Nitrogen is first reduced with dilute H2SO4 and finely divada’s alloy (i.e. mixture of Cu, Zn and Al) before digestion. Bal has proposed to soak the heavy soil (due to low value of nitrogen) with water before digestion with H2SO4. This helps in breaking down of soil aggregates, thus leaving more surface contact with H2SO4.
Exactly 10 gms of soil (Powdered) are weighed and it is transferred in Kjeldahl flask with a care that no particle ticks to the side of the flask. 10 ml of water is added to the soil in the flask, then it is shaken and allowed to stand for half hours. Then 35 ml (30-35 ml) of conc. H2SO4 is added. Digestion is started over small flames (at 50°C for 15 minutes). Then the heat is increased until the white flames of H2SO4 is produced. The flask is removed and 0.5 gm Potassium sulphate (K2SO4) are added to the content of Kjeldahl flask.
The digestion procedure is continued (1-1.5 hours at 100°C) till the solution turned pale green. The content of the flask is allowed to cool. K2SO4 and CuSO4 are used to raise the temperature of the digest and to shorten the time of digestion respectively. The content of the flask is diluted with 100 ml of distilled water and the fluid part is transferred to 1000 ml. distillation flask leaving as much of as possible 600 mμ red filter. Prepare the standard curve in the same manner.
K as K n Na3 – n Co(NO2)2 x H2O
(i) Kjeldahl flask
(i) Sulphuric acid (H2SO4)
(ii) Ammonium sulphate [(NH4)2SO4]
(iii) Potassium sulphate (K2SO4)
(iv) Copper sulphate (CuSO4)
(v) Granulated zinc
(vi) Glass bits
(vii) Caustic soda (NaOH)
Nitrogen is determined in soil by Kjeldahl flask method in which organic matter is oxidised by sulphuric acid (H2SO4) and the nitrogen is converted to ammonia which at once fixed by acid present in Sulphuric acid. Most of soil nitrogen present in organic combination.
Ammonium sulphate is then distilled with Sodium hydroxide (NaOH) and ammonia (NH3) evolved is absorbed in standard acid. As the reaction between H2SO4 and organic matter is slow, the oxidation process is hastened by addition of catalytic agent like CuSO4 and K2SO4 so that the loss of acid by volatalization is prevented.
The method does not include the whole of the Nitrogen of the soil. For most of nitrate nitrogen is lost. Since the amount of nitrate nitrogen of sand behind. The sandy residue is washed with 4 or 5 lots of 50-60 ml. of water, decanting the washing into distillation flask after allowing the sandy residue to settle for a few second each time.
A few pieces of granulated zinc and glass bits are added to the distillation flask. Zinc promotes smooth boiling and little amount of hydrogen produced prevent back sucking of distillate. Glass bits prevent bumping of content in the distillation flask. 120 ml of 40 per cent NaOH solution or sufficient amount is added to make the content of the flask alkaline to phenolphthalein. The caustic soda (NaOH) is poured down the side of the flask so that it forms a heavy layer at the bottom. The flask is connected with a condenser and the content is mixed by shaking.
The distillation was commenced collecting the ammonia (NH3) evolved in a receiver containing 25 ml of N/10 H2SO4 to which 2-3 drops of methyl red indicator have been added. The distillation continued until about one third of liquid has passed over when the distillation is completed. The receiver is taken out washing the end of condenser inside the receiver. The excess of acid (H2SO4) back titrated with standard NaOH. The end point is indicated by the disappearance of the colour.
Volume of H2SO4 = 25 ml
Normality of H2SO4 = 0.107
Volume of NaOH = 24.2
Normality of NaOH = 0.0973
Weight of Soil Sample taken = 10 gm
1 ml of normal H2SO4 or alkali = 0.014 gm of Nitrogen.
There are two methods for the determination of available phosphorus in soil as follow:
(A) Bray’s method No. 1 for acidic Soils.
(B) Olsen’s method for neutral and alkaline soils.
(i) Photoelectric Calorimeter.
(ii) Multiple dispenser or a 50 ml of automatic Pipette.
(iii) Automatic Shield Pipette – (5 ml and 10 ml)
(i) Bray’s extraction No. 1 (0.03 N Ammonium fluoride (NH4F) in 0.025 N HCl), 22.2 gm of NH4F is dissolved in 200 ml distilled water. The solution is filtered. 18 litres water containing 40 ml concentrated Hydrochloric acid (HCl) is added to the filtrate. The volume is made up to 20 litres with distilled water.
(ii) Molybdate reagent – (1.5 gm of Ammonium molybdate [(NH4)2 MoO4] is dissolved in 300 ml of distilled water. This solution is gradually added to 350 ml of 10 N HCl. Solution with constant stirring. The volume is made to 1 litre with distilled water. The concentrated HCl should be 10 N. (If the Conc. HCl used is not exactly 19 N, Calculated volume of acid equivalent to 350 ml of 10 N HCl is added).
Stannous Chloride (Sn Cl2) Solution (Stock solution) (10 gm of Sn Cl2, 2H2O is dissolved in 25 ml of Concentrated HCl. A piece of Pure metallic tin is added to it and the solution is stored in a glass stoppered bottle).
1 ml of the stock solution is diluted to 66 ml with distilled water before use. Fresh dilute solution is prepared every day.
Preparation of the Standard Curve:
0.1916 gm pure dry KH2PO4 in one litre of distilled water. This is equivalent to 0.10 mg P2O5 per ml. It is preserved by adding a drop of toluene as a stock standard solution of Phosphate. 10 ml of this solution is diluted to one litre with distilled water. This solution is equivalent to 1 microgram (0.001 mg) of P2O5 per ml (p.p.m). 1, 2, 4, 6 and 10 ml of this solution are transferred by burette very carefully to separate 25 ml volumetric flasks. To each flask, 5 ml of the extracted solution, 5 ml. of molybdate reagent are added and diluted to about 20 ml 1 ml of dilute SnCl2 solution is added to each flask. The resulting solution is shaken thoroughly and the volume is made up accurately to 25 ml marks. Reading for the blue colour developed is taken in a photoelectric calorimeter using 660 mμ red filter (Klett No-66). The meter readings are plotted against microgram P2O5 and the points are joined to get a straight line.
5 gm of soil is measured in conical flasks and 60 ml of Bray’s extractant No. 1 is added to each flask with the help of a multiple dispenser. The solution is shaken for 5 minutes and filtered.
5 ml of filtered Soil extract is taken by a bulb pipette in a 25 ml volumetric flask. 5 ml of Ammonium molybdate reagent is added to it by an automatic pipette. The solution is diluted to 20 ml with distilled water. It is shaken well and 1 ml of SnCl2 (dilute) is added to it by a bulb pipette. The volume is made upto 25 ml mark.
It is now thoroughly shaken, allowed to stand for 10 minutes for development of colour and a reading is taken in a phosphoelectric calorimeter using 660 mμ, red filter (Klett No – 66) after estimating the zero error. A blank reading is also taken using all the reagents as above but without the Plant Soil extract. The blank reading is subtracted from the original reading.
Find the P2O5 content of the filter from the standard curve and calculate in terms of lbs of P2O5 per acre (90743 kg/2,000,000 lbs) of Soil by multiplying the reading obtained from the curve in μg. If X μg is read from the curve.
(i) To convert lbs P2O5/acre to kg/ha multiply by 1.12.
(ii) To convert lbs P2O5/acre to kg per ha multiply by 0.44.
(i) Photoelectric Calorimeter.
(ii) Multiple dispenser or a 50 ml of automatic Pipette.
(iii) Automatic Shield Pipette.
(i) Stannous Chloride (SnCl2) Solution. Same as Bray’s Method No – 1.
(ii) Molybdate reagent – Same as Bray’s Method No.1 except that instead of 350 ml of 10 N HCl, 40 ml of 10 N HCl is added.
(iii) Activated Carbon – Darco G 60 or any other suitable decolouring carbon made free from soluble Phosphorus by repeated washing with bicarbonate extractant Solution.
(iv) Bicarbonate extractant – 840 gm of NaHCO3 is dissolved in 20 litre of distilled water and the pH is adjusted to 8.5 by adding dilute NaOH or HCl Solution. The solution may be filtered if necessary.
Preparation of the Standard Curve:
Same as in Bray’s Method No. 1.
2.5 gm Soil Sample is measured in 100 ml conical flasks and 50 ml of bicarbonate extractant is added to each flasks with the help of multiple dispenser. One gram of discolourised carbon is added by the measure. The resulting solution is shaken for 30 minutes on a mechanical shaker and filtered.
Same as described under the Bray’s Method No. 1. Calculate lbs of P2O5 per acre by multiplying the curve reading in μg (microgram) by 8.0.
The intensity of blue colour changes slightly with every batch of molybdate reagent and with other factors. Therefore, the standard curve should be checked every day by using 2-3 dilutions of standard phosphate solution. If the Standard Curve differs from the one prepared earlier, a new standard curve is to be prepared or a fresh molybdate reagent is prepared.
The available Potassium is determined by the following method. Flame Photometer Method.
(i) Flame Photometer.
(ii) Multiple dispenser or a 25 ml automatic Pipette.
(iii) Mechanical Shaker.
(i) Normal neutral Ammonium acetate solution – 1,640 gm of Ammonium acetate (NH4C2H3O2) is dissolved in 20 litres of water. The pH of the solution is tested by a pH meter and if it is not neutral, the pH is adjusted to 7.0 by the addition of either Ammonium hydroxide or Acetic acid.
(ii) Standard Potassium solution – 1,5851 gm of pure Potassium Chloride (KCl) is dissolved in one litre of distilled water. This solution is equivalent to one gram of K2O per ml. This is the standard solution of Potassium.
0, 5, 10, 15 and 20 ml of stock solution are separately taken with care and diluted to one litre with normal Ammonium acetate solution. Few drops of butyl alcohol are added to each to improve the spraying Properties of the solution. These solutions are equivalent to 0, 5, 10, 15 and 20 μg (microgram) K2O/mI respectively.
Preparation of Standard Curve:
The flame Photometer is set up by atomising 0 and 20 μg/ml K2O solutions alternately to 0 and 100 Scale reading respectively. A direct reading Perkin Elmer flame Photometer is used. Intermediate working standard solutions are atomised and the meter readings are noted. The meter readings are plotted against the respective potassium content and the points are joined to get a straight line.
5 gm of soil is measured and transferred to conical flasks. 25 ml. of Ammonium acetate extractant is added to soil by a multiple dispenser. The resulting solution is shaken for 5 minutes on a mechanical shaker and filtrate and potassium is determined by a flame photometer.
The Potassium content of the filtrate is found from the Standard Curve and the Curve readings are converted to lbs of K2O/acre by multiplying this figure by 10.
(i) lbs K2O/acre is converted to kg/ha by multiplying by 1.12.
(ii) lbs K2O/acre is converted to the kg/ha by multiplying by 0.83.
6. Lime Requirement:
(i) Glass electrode pH meter.
(ii) 20 ml. automatic pipette.
(iii) Mechanical Shaker.
Extractant buffer – 1.8 gm of nitrophenol, 2.5 ml. triethanolamine, 3.0 gm Potassium dichromate (K2Cr2O7), 2.0 gm Calcium acetate [Ca(C2 H3O2)2] and 53.1 gm Calcium chloride (CaCl2, 2H2O) are dissolved in a litre of water. The pH is adjusted to 7.5 with NaOH.
Lime is not required for soils having pH 6.4 or above. When the pH of the Soil is 6.3 or below, lime is required and recommendation is made according to the pH value. The lime requirement is calculated in accordance with the following procedure-
5 gm of Soil is measured in 50 ml beaker, 5 ml distilled water and 10 ml extractant buffer are added to it. The resulting mixture is shaken well for 10 minutes on a mechanical shaker or intermittently for 20 minutes. The pH of the Soil buffer suspension is read on the pH meter using a glass electrode. The lime requirement is found from the pH of the soil buffer suspension.
7. Gypsum Requirement:
(i) 50 ml burette.
(ii) Pipette – 5 ml and 100 ml.
(iii) Mechanical Shaker.
(i) Standard EDTA (Versenate) Solution of 0.01 N : 2.0 gm of Disodium dihydrogen ethylene diamine tetra acetate and 0.05 gm of MgCl26H2O are dissolved in distilled water and diluted to one litre. It is standardised against CaCl2 Solution.
(ii) Standard CaCl2 Solution – 0.01 N-0.5 gm Pure CaCO3 is dissolved in 10 ml dilute HCl and diluted to one litre.
(iii) Erichome Black T indicator – 0.5 gm of Erichome Black T and 4.5 gm of Hydroxyl amine hydrochloride (HAH) are dissolved in 100 ml of 95% ethanol.
(iv) NH4Cl + NH4 OH Buffer – 67.5 gm of NH4Cl are dissolved in 570 ml of NH4OH (S.G. 0.88) and diluted to one litre.
(v) Saturated CaSO4 Solution – 5 gm of CaSO4 are shaken with one litre of distilled water for 10 minutes on a mechanical shaker. The solution is then filtered.
5 gm of the soil is measured and transferred to a 250 ml. conical flask. 100 ml of saturated CaSO4 solution is added to it. The resulting mixture is shaken on a mechanical shaker for 5 minutes and filtered. 5 ml of the soil extract is pipetted out in a 100 ml. conical flask. It is diluted to about 25 ml with distilled water and 0.5 ml of NH4Cl + NH4OH buffer and 3-4 drops of Erichrome Black T indicator are added to it. It is titrated with Standard EDTA Solution until the colour changes from wine red to blue. A blank titration with 5 ml. saturated solution of CaSO4 is done separately.
Gypsum requirement in tons/acre (2,000,000 lbs of Soil)
= 344 N (A – B)
When A = ml. of EDTA used for blank titration.
B = ml of EDTA used for the soil extract titration.
N = Normality of the EDTA Solution.
Recommendation Based on Soil Test Data:
Based on Soil test data, use of the rating chart is made for recommendation of fertilizers etc. (Table 13.2).
After obtaining the soil test data and gaining a knowledge of the management practices of the Cultivation, the Cropping history and Crops to be grown, fairly accurate prediction can be made regarding the requirement of fertilizers and their response.
(Total Soluble Salts) (in millimhos)
Below 1 – Normal
1- 2 – Critical for germination
2- 3 – Critical for the growth of Salt Sensitive Crops
Above 3 – Injurious to most Crops.
8. Cation Exchange Capacity of Soil:
Numbers of method have been suggested to determine the cation exchange capacity of Soil. Most of the methods are based on the Principle that the Soil Colloids are saturated with a single species of cation like Calcium, Barium, Ammonium etc. and the amount of cation absorbed by the soil Colloid is determined.
(ii) Filter Paper (Whatman No – 42)
(iii) Distillation flask –400 ml
(i) Ammonium acetate
(ii) Ammonium chloride
(iii) Ethyl alcohol
(iv) Magnesium oxide
(v) Sulphuric acid
(vi) Methyl red indicator
(vii) Sodium hydroxide
10 gms of Soil weighed accurately and was transferred in a 250 cc beaker. 50 ml of Normal Ammonium acetate Solution was added to it and stirred thoroughly. Then the content of the beaker was heated at 70°C for about half an hour with occasional stirring. Then it was cooled and the liquid was decanted thorough Whatman No-42 filter paper and the soil is transferred quantitatively into the filter paper.
The soil was leached with neutral normal Ammonium acetate solution adding about 25 ml of solution in each lot. The leaching process was Continued until the leaches makes up the volume of 250 ml. In the last lot of Ammonium acetate solution added, a pinch of Ammonium chloride was added.
To remove the excess ammonium ions, the soil was washed with 40% Ethyl alcohol until the washing is free from chloride. Then the soil was transferred from the filter paper into distillation flask by repeated washing with distilled water (the volume of distillation flask will be 400 ml). A few glass bits was added into distillation flask and it was connected with the condenser and about 5 gm of ignited Magnesium Oxide (MgO) is added and the distillation was started collecting the ammonia released in 50 ml of N/10 Sulphuric acid (H2SO4) containing 3-4 drops of Methyl Red indicator.
The distillation was continued until 150-200 ml of distillate has been collected, the receiver was removed by washing taking the delivery end of the condenser into the receiver. The excess acid was back titrated with N/10 NaOH solution. The end point was indicated by the disappearance of colour.
It is also possible to make the soil tested at free of cost by sending the same to government soil testing laboratory situated at different place of India. For this, the bag is well labelled with the information sheet and is to be sent to the nearest soil testing laboratory through the K.P.S. or A.D.O. now A.D.A. of the Block for testing of the soil.
The sample information sheet is given below:
Soil Test interpretation and fertilizer recommendations.
Informations written in Soil Testing Report:
A = Acidic
Alk = Alkali
H = High
L = Low
M = Medium
M L = Medium low
N = Normal
S1 = Critical for germination
S2 = Critical for growth of salt sensitive crop.
S3 = Injurious to most crop
T. Alk = Tending to become alkali
V H = Very high
V L = Very low
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