Methods of test for soils, Part 22: Determination of organic matter

IS 2720 Part 22 — Scope & Key Specifications

This part covers the determination of organic matter content in soil using chemical oxidation.

Key Formula for Organic Matter Content:

[ V = 10.5 \times \left(1 - \frac{Y}{X}\right) \quad \text{(Volume of K}_2\text{Cr}_2\text{O}_7\text{ used, ml)} ]

[ % \text{Organic Matter} = \frac{W_1 - W_2}{W} \times 0.6 \times V ]

Where:

• (W_1) = Total weight of original soil sample (g)
• (W_2) = Weight of dry soil specimen used (g)
• (W) = Weight of soil passing 10-mm IS sieve (g)
• (X) = Volume of ferrous sulphate solution added to standardize potassium dichromate (ml)
• (Y) = Volume of ferrous sulphate solution used to oxidize excess potassium dichromate (ml)
• (V) = Volume of potassium dichromate solution used to oxidize organic matter (ml)

Test Data Recording Pro Forma (Appendix A):

This standard aligns with international practices and is specific to Indian soil testing.

flowchart TD
    A[Soil Sample] --> B[Weigh Soil Passing 10mm Sieve]
    B --> C[Prepare Dry Soil Specimen]
    C --> D[Add K2Cr2O7 Solution]
    D --> E[Add Ferrous Sulphate Solution (X)]
    E --> F[Measure Volume Used (Y)]
   

IS 2720 Part 22: Key Formulas & Tables for Organic Matter Content

1. Calculation of Organic Matter Content

Formula:

[ V = 10.5 \times \left(1 - \frac{Y}{X}\right) ]

[ \text{Percentage of Organic Matter} = \frac{W_1 \times W \times 0.6}{W \times V} ]

2. Pro Forma for Recording Test Results

• Includes sample weights, solution volumes, and calculated organic matter percentage.
• Ensures standardized data recording.

3. Reference & Contact Details

• BIS Regional Offices available nationwide for standard copies and clarifications.
• Substitution: IS: 266-1977 replaces IS: 266-1961 in related clauses.

This ensures accurate determination of organic matter in soils per IS 2720 Part 22.

IS 2720 Part 22 – Terminology & Key Formulas for Organic Matter Content

Key Formula for Organic Matter Content:

[ \text{Organic Matter %} = \frac{(W_1 - W) \times 1.6 \times V}{W \times V_1} \times 100 ]

Where:

• (W_1) = Total weight of original soil sample (g)
• (W) = Weight of dry soil specimen used (g)
• (V) = Volume of potassium dichromate solution used to oxidize organic matter (ml)
• (V_1) = Volume of ferrous sulphate solution added to standardize potassium dichromate (ml)

Test Data Recording Table (Pro Forma):

Notes:

• Rounding off results should follow IS 2-1960.
• Terminology aligns with international standards and Indian practices.

flowchart TD
    A[Soil Sample (W1)] --> B[Pass 10-mm Sieve (Wa)]
    B --> C[Dry Soil Specimen (W)]
    C --> D[Add K2Cr2O7 Solution (V)]
    D --> E[Add FeSO4 Solution (X)]
    E --> F[Measure FeSO4 used (Y)]
    F --> G[Calculate Organic Matter %]

This ensures consistent terminology and accurate calculation of organic matter in soils per IS 2720 Part 22.

IS 2720 Part 22: Apparatus and Reagents for Organic Matter Determination

Key Specifications:

• Reagents: Analytical quality only (Clause 3.1).
• Potassium Dichromate Solution: Normality (N) specified, typically 0.4 N.
• Ferrous Sulphate Solution: Used for titration.
• Sulphuric Acid: Concentrated, 20 ml added carefully.
• Orthophosphoric Acid: 10 ml added after oxidation.
• Indicator: 1 ml added, with additional 1 ml if absorbed by soil.
• Sodium Fluoride: 0.2 g added if ferric ions interfere (Note, Clause 6.2).

Procedure Summary (Clause 6.2):

1. Add 10 ml potassium dichromate to flask.
2. Add 20 ml concentrated sulphuric acid carefully.
3. Swirl 1 min, stand 30 min protected from draughts.
4. Add 200 ml distilled water, 10 ml orthophosphoric acid, 1 ml indicator.
5. Titrate with ferrous sulphate until color changes blue→green.
6. Add 0.5 ml potassium dichromate, then titrate again dropwise until blue→green endpoint.

Calculation Formula (Clause 8.1):

[ \text{Organic Matter %} = \frac{(W_1 - W_s) \times 0.6 \times V}{W_s \times V_1} ]

Where:

Pro Forma Table (Clause 8.1):

IS 2720 Part 22: Soil Sample Preparation Key Points

Sample Preparation (Clauses 5.1 & 12.1)

• Initial Preparation:

  • Air-dry soil sample as per IS 2720 Part 1.
  • Weigh and determine moisture content separately to find oven-dried equivalent weight ( W_1 ).
  • Sieve soil through 10-mm IS Sieve.
  • Crush particles >10 mm to pass 10-mm sieve.
  • Calculate oven-dry weight of material passing 10-mm sieve ( W_a ).

• Sub-sampling:

  • Quarter ~100 g from material passing 10-mm sieve.
  • Pulverize to pass 425-micron IS Sieve (0.425 mm).

• For Organic Matter Test:

  • Grind soil to pass 212-micron IS Sieve.
  • Use:
    • 0.25 g soil normally,
    • 0.05 g if peat,
    • 1.00 g if organic content <1%.

• Special Treatments:

  • For sulphides: Add dilute 2N sulphuric acid until no H₂S evolves.
  • For chlorides: Wash with distilled water until no turbidity with silver nitrate; or use concentrated sulphuric acid with silver sulphate.

Key Formula for Organic Matter Content (Clause 8.1)

[ \text{Organic Matter %} = \frac{W_1}{W_s} \times 6 \times V ]

Where:

Sample Data Recording Table (Appendix A)

IS 2720 Part 22: Test Procedures for Organic Matter Content in Soil

Key Formulas (Clause 8.1)

• Volume of potassium dichromate solution used:

  [ V = 10.5 \times \left(1 - \frac{Y}{X}\right) \quad \text{(ml)} ]

  Where:

  • (X) = Volume of ferrous sulphate solution added to standardize potassium dichromate (ml)
  • (Y) = Volume of ferrous sulphate solution used to oxidize excess potassium dichromate (ml)

• Percentage of Organic Matter:

  [ % \text{Organic Matter} = \frac{W_1 \times 0.6 \times V}{W_s} ]

  Where:

  • (W_1) = Weight of original soil sample (g)
  • (W_s) = Weight of dry soil specimen used (g)
  • (V) = Volume of potassium dichromate solution used (ml)

Test Data Recording Table (Appendix A)

Notes

• Final results should be rounded off per IS 2-1960.
• Preparation of dry soil specimen follows IS 2720 Part 1.
• Ensure soil passes 10-mm sieve before testing.

flowchart TD
    A[Original Soil Sample (W1)] --> B[Pass through 10-mm IS Sieve (Wa)]
    B --> C[Prepare Dry Soil Specimen (Ws)]
    C --> D[Add K2Cr2O7 Solution]

IS 2720 Part 22: Calculation of Organic Matter Content in Soil

Key Formula (Clause 7.2)

[ \text{OM %} = \frac{0.67 \times W \times V \times w}{W_1 \times W_2 \times W_3} ]

where:

• OM % = Organic Matter content by weight
• ( V ) = Total volume of potassium dichromate used to oxidize organic matter (Clause 7.1)
• ( W_1 ) = Oven-dry weight of total soil sample before sieving
• ( W_2 ) = Oven-dry weight of soil passing 10 mm IS sieve
• ( W_3 ) = Oven-dry weight of soil specimen used in test
• ( w ) = Weight factor (context-specific, often related to titration volume or weight)
• 0.67 = Factor accounting for 50% carbon in organic matter and 77% oxidation efficiency

Notes:

• The method estimates organic matter from organic carbon content.
• Assumes organic matter contains 50% carbon.
• Approximately 77% of carbon is oxidized during test.

Summary Table:

This formula provides reproducible, engineering-accurate organic matter content for soil samples.

IS 2720 Part 22: Reporting of Results for Organic Matter Content

Key Points & Formulas

• Rounding off: Final test values must be rounded as per IS 2-1960.
• Reporting precision: Organic matter content reported to the nearest 0.1% of original oven-dry soil.
• Recommended recording: Use the pro forma in Appendix A (Clause 8.1) for uniform data presentation.

Calculation of Organic Matter Content

Organic matter content (%):

[ \text{Organic Matter} = \frac{W_1 \times V \times 0.003}{W_2} ]

(Note: The constant 0.003 relates to the oxidation equivalent; verify exact factor from IS 2720 Part 22)

Pro Forma for Recording (Summary)

Summary

• Use Appendix A pro forma.
• Report organic matter to 0.1% accuracy.
• Follow IS 2-1960 for rounding.
• Calculate V and organic matter using volumes of ferrous sulphate and potassium dichromate.

flowchart TD
    A[Soil Sample] --> B[Weigh W₁]
    B --> C[Take dry specimen W₂]
    C --> D[Add K₂Cr₂O₇ solution]
    D --> E[Add ferrous sulphate (X ml) to standardize]
    E --> F[Measure ferrous sulphate used (Y ml)]
    F --> G[Calculate V = 10.5(1 - Y/X)]
    G --> H

IS 2720 Part 22: Precision and Accuracy - Key Points

Precision & Accuracy Context

• The code emphasizes strict measurement sensitivity:
  • Chemical balance sensitivity: 0.001 g (Clause 2.2).
• Rounding off numerical values must follow IS 2-1960 (Clause None, 0.5).

Key Formula for Organic Matter Content (Clause 8.1)

[ \text{Organic Matter %} = \frac{W_1 \times W \times 6 \times V}{W_a} ]

Where:

• (W_1) = Total weight of original soil sample (g)
• (W_a) = Weight of soil passing 10-mm IS Sieve (g)
• (W) = Weight of dry soil specimen used (g)
• (V) = Volume of potassium dichromate solution used (ml), calculated as:
  [ V = 10.5 \times \left(1 - \frac{Y}{X}\right) ]
• (X) = Volume of ferrous sulphate solution added to standardize potassium dichromate (ml)
• (Y) = Volume of ferrous sulphate solution used to oxidize excess potassium dichromate (ml)

Specifications Summary

• Use chemical balance with 0.001 g sensitivity for weighing.
• Follow IS 2-1960 for rounding off results.
• Record all weights and volumes precisely in the pro forma (Appendix A).

Rounding Rule (IS 2-1960)

• Round off final values to the nearest unit as specified.
• Ensure consistent significant figures based on measurement precision.

flowchart TD
    A[Soil Sample] --> B[Weigh Original Sample (W1)]
    B --> C[Pass Soil through 10-mm Sieve (Wa)]
    C --> D[Weigh Dry Soil Specimen (W)]
    D --> E[Add K2Cr2O7 Solution]
    E --> F[Add Ferrous Sulphate (X)]
    F --> G[Measure Ferrous Sulphate Used (Y)]
    G --> H[Calculate Volume V = 10.5(1 - Y/X)]
    H --> I[Calculate Organic Matter %]

IS 2720 Part 22 - Safety Precautions: Key Points

Though Clause 8.1 and Appendix A primarily focus on recording test results for organic matter content, general safety precautions for soil testing per IS 2720 Part 22 include:

• Use of Personal Protective Equipment (PPE): Gloves, goggles, and lab coats to avoid contact with chemicals like potassium dichromate and ferrous sulphate.
• Handling Chemicals: Potassium dichromate is toxic and an oxidizer; handle in a fume hood with care.
• Proper Disposal: Dispose of chemical wastes as per environmental guidelines.
• Avoid Inhalation: Work in well-ventilated areas to avoid inhaling fumes.
• Calibration and Standardization: Ensure reagents like ferrous sulphate and potassium dichromate are standardized before use to maintain accuracy.

Key Formula for Organic Matter Content:

[ \text{Organic Matter %} = \frac{W_1}{W} \times 0.6 \times V ]

Where:

Pro Forma Table for Recording:

Summary: Follow strict PPE use and chemical handling protocols, standardize reagents, and use the formula above

IS 2720 Part 22: Alternative Method for Determination of Organic Matter Content

Key Points:

• The alternative method uses chromic oxidation to estimate soil organic matter.
• It involves titration with potassium dichromate and standardization with ferrous sulphate.
• Results are recorded as per Appendix A (Clause 8.1) pro forma.

Important Formulas:

Organic Matter Content Calculation:

[ \text{Organic Matter %} = \frac{W1 \times V \times 0.003 \times 100}{W2} ]

(Note: The factor 0.003 corresponds to the equivalent weight of organic carbon oxidized per ml of dichromate; adjust as per standard.)

Specifications:

• Report organic matter content to nearest 0.1% of oven-dry soil.
• Use soil passing 10-mm IS sieve.
• Record all weights and volumes carefully as per pro forma in Appendix A.

flowchart TD
    A[Soil Sample] --> B[Weigh original sample (W1)]
    B --> C[Pass soil through 10-mm sieve]
    C --> D[Weigh dry soil specimen (W2)]
    D --> E[Add potassium dichromate]
    E --> F[Add ferrous sulphate (X ml) for standardization]
    F --> G[Titrate excess dichromate with ferrous sulphate (Y ml)]
    G --> H[Calculate volume V = 10.5(1 - Y/X)]
    H --> I[Calculate % Organic Matter]

This method provides a reliable alternative to dry combustion for engineering soil organic matter estimation.

IS 2720 Part 22: Key Notes, Annexures & Formulas for Organic Matter Content

Appendix A: Pro Forma for Recording Test Results

Key Formulas:

• Volume of potassium dichromate solution used (V):

  [ V = 10.5 \times \left(1 - \frac{Y}{X}\right) \quad \text{(ml)} ]

• Percentage of organic matter in soil:

  [ \text{Organic Matter %} = \frac{W_1 - W_2}{W_2} \times 0.6 \times V ]

Notes:

• Weigh soil passing the 10-mm sieve for consistency.
• Use ferrous sulphate to titrate excess potassium dichromate.
• 0.6 is a constant relating oxidizable organic carbon to organic matter.

Summary Diagram (Test Process Flow):

flowchart TD
    A[Collect Soil Sample] --> B[Weigh Total Sample (W1)]
    B --> C[Pass through 10-mm IS Sieve]
    C --> D[Weigh Passing Soil (Wa)]
    D --> E[Take Dry Soil Specimen (W2)]
    E --> F[Add Potassium Dichromate]
    F --> G[Add Ferrous Sulphate (X)]
    G --> H[Titrate Excess Dichromate with Ferrous Sulphate (Y)]
    H --> I[Calculate V and Organic Matter %]

This pro forma and formula ensure standardized, accurate determination of soil organic matter as per IS 2720 Part 22.