Overview of NY/T85-1988 Soil Organic Matter Determination Method

NY/T85-1988 is a classic method for soil organic matter determination in China's agricultural industry standards, widely used in agriculture, forestry, and environmental monitoring. This standard uses the potassium dichromate volumetric method (external heating method), determining soil organic carbon content through redox titration, then converting it to organic matter. This method is simple to operate, has good reproducibility, and is suitable for batch soil sample analysis.

This article is aimed at engineering technicians and procurement personnel, systematically organizing the principles, steps, calculations, and key control points of this standard to help you quickly master and correctly apply it.

Method Principle and Scope of Application

Determination Principle

Under heating conditions, excess potassium dichromate-sulfuric acid solution is used to oxidize organic carbon in the soil, and the remaining potassium dichromate is titrated with ferrous sulfate standard solution. The organic carbon content is calculated based on the amount of potassium dichromate consumed, then multiplied by the conversion coefficient to obtain the organic matter content.

The reaction formula is simplified as: 2K2Cr2O7 + 3C + 8H2SO4 → 2K2SO4 + 2Cr2(SO4)3 + 3CO2 + 8H2O.

Scope of Application

  • Suitable for samples with soil organic matter content in the range of 0.5%~15%.
  • Not suitable for soils containing large amounts of reducing substances (such as sulfides, ferrous iron) or high chlorides.
  • The standard method is the external heating method, and oil bath or electric heating plate heating can also be used.

Main Instruments and Reagents

Instruments and Equipment

  • Hard glass test tubes: 20mm×200mm
  • Oil bath pot or electric heating plate: controllable temperature up to 170~180℃
  • Burette: 25mL acid type
  • Analytical balance: sensitivity 0.0001g
  • Thermometer: 0~200℃

Reagent Preparation

  • 0.4mol/L potassium dichromate-sulfuric acid solution: Weigh 40g K2Cr2O7 and dissolve in 500mL water, slowly add 500mL concentrated sulfuric acid, cool and dilute to 1L.
  • 0.1mol/L ferrous sulfate standard solution: Weigh 28g FeSO4·7H2O and dissolve in water, add 5mL concentrated sulfuric acid, dilute to 1L, requires standardization.
  • Indicator: o-phenanthroline indicator or sodium diphenylamine sulfonate.

Detailed Operational Steps

Sample Pretreatment

  1. Grind air-dried soil sample through a 0.25mm sieve and mix well.
  2. Weigh 0.1~0.5g (accurate to 0.0001g) of soil sample and place it in a hard glass test tube.
  3. Add 5mL potassium dichromate-sulfuric acid solution and shake well.

Heating Oxidation

  1. Insert the test tube into the oil bath pot, controlling the temperature at 170~180℃.
  2. After the liquid in the test tube boils, continue heating for 5 minutes (timing from the start of boiling).
  3. Remove the test tube and cool to room temperature.

Titration

  1. Transfer the solution in the test tube to a 250mL conical flask, rinse the test tube several times with water, controlling the total volume to 60~80mL.
  2. Add 2~3 drops of o-phenanthroline indicator.
  3. Titrate with ferrous sulfate standard solution until the solution changes from orange-yellow to blue-green, then to brown-red as the endpoint.
  4. Perform a blank test simultaneously (using quartz sand instead of soil sample).

Result Calculation and Data Recording

Calculation Formula

Soil organic matter content (g/kg) is calculated as follows:

Organic matter = (V0 - V) × c × 3.0 × 1.724 × 1000 / m

  • V0: Volume of ferrous sulfate consumed in blank titration (mL)
  • V: Volume of ferrous sulfate consumed in sample titration (mL)
  • c: Concentration of ferrous sulfate standard solution (mol/L)
  • 3.0: Molar mass of 1/4 carbon atom (g/mol)
  • 1.724: Conversion coefficient from organic carbon to organic matter
  • m: Mass of soil sample (g)

Data Recording Table

Item Blank Sample 1 Sample 2
Soil sample mass (g) — 0.2500 0.3000
Ferrous sulfate consumption (mL) 25.00 18.50 16.20
Organic matter (g/kg) — Calculated value Calculated value

Precautions and Common Issues

Key Control Points

  • Heating time must be timed from the start of boiling, strictly controlled for 5 minutes, otherwise oxidation is incomplete or excessive.
  • The oil bath temperature must be stable at 170~180℃. Too low a temperature leads to insufficient oxidation, while too high causes decomposition of potassium dichromate.
  • The ferrous sulfate solution must be standardized immediately before use to avoid oxidation failure.
  • The soil sample weighing amount should be adjusted according to organic matter content; reduce the sample amount when content is high.

Interference Elimination

  • Chlorides in soil can interfere with the determination; a small amount of silver sulfate can be added to eliminate this.
  • Soils containing reducing substances need to be air-dried and oxidized first or use other methods.

Standard Original Text and Download

The following is the original PDF of the standard for engineering technicians and procurement personnel to consult the complete specification.

If you need to purchase related instruments and reagents, please refer to the equipment and reagent specifications listed in the standard.