Partenariat mondial sur les sols

SOPs on organic carbon using titration and colorimetric methods

Standard operating procedures (SOPs) offer step-by-step instructions on how to perform laboratory analyses.

SOPs are a critical component in ensuring the replicability, credibility and traceability of data. They are crucial in case of measurement's errors and go hand-in-hand with the execution of internal and external quality control exercises.

Indeed, SOPs help lab staff to carry out routine operations by describing, in detail, how to perform a laboratory process or experiment safely and effectively. SOPs aim to achieve efficiency, quality of output and uniformity of performance, while reducing miscommunication and failure to comply with laboratory and international standards. The harmonization of laboratory standard operating procedures and methods is at the basis of harmonizing soil laboratory data.


Background information

GLOSOLAN aims to harmonize soil analysis methods and data so that soil information is comparable and interpretable across laboratories, countries and regions. Evidence-based decisions are critical to the achievement of Sustainable Soil Management (SSM), food security and nutrition, and Agenda 2030 for Sustainable Development. While the quantity and quality of soil data are fundamental, soil information must also be harmonized and globally consistent to have impact.

GLOSOLAN harmonizes SOPs for soil chemical, physical and biological parameters at a global scale and makes them available online - to all users - for FREE. All SOPs are compiled in the GLOSOLAN’s Best Practice Manual, a living document, available online and structured as a flexible compendium of volumes.

Moreover, thanks to the support of some network members (in this case the Bureau of Soils and Water Management of the Philippines and the Land Development Department of Thailand), GLOSOLAN released two videos to support soil laboratories in the adoption of this SOP and guide them in all the steps of the SOP on soil organic carbon using Walkley-Black (titration and colorimetric) method. The videos are available in English with subtitles in different languages.

GLOSOLAN – Standard operating procedure for soil organic carbon

Soil carbon is probably the most important component in soils as it affects almost all soil properties. Determining its quantity in soils allows to obtain a lot of information, including soil fertility, soil biodiversity, soil erosion, and soil buffer capacity for hazardous chemicals. GLOSOLAN started from the harmonization of SOP on the measurement of soil organic carbon (SOC) following the Walkley and Black titration and colorimetric method in 2019. 

Walkley and Black colorimetric method

Watch the video on Walkley and Black colorimetric method: HERE

This SOP is based on the Walkley & Black colorimetric method. The organic carbon can be calculated from the amount of chromic ion (Cr3+) formed,using a colorimetric procedure measuring absorbance at 588 nm (after Sims and Haby 1971). An advantage of this procedure over the titrimetric method is that accurate standardisation of theCr2O7 2- solution is not required.

This video, realized in collaboration with the Bureau of Soils and Water Management - Laboratory Services Division (BSWM-LSD) of the Philippines, aims to support soil laboratories in the adoption of this SOP.

In this video, Ms Gina P. Nilo, Chief of the Laboratory Services Division serving also as the chairperson of the FAO Asian Laboratory Network (SEALNET), together with her soil chemistry team – composed of Mr Bergil Bernaldo (co-author), Ms Florfina Sancez, and Ms Joerdette Jimenez - present the Soil Organic Carbon Walkley and Black Method using colorimetric method in three simple steps. They also explain in simple words how and why it is so important for countries (and in this case for the Philippines) the SOP harmonization work performed by the national laboratories in the framework of GLOSOLAN and SEALNET.

Walkley and Black titration method

Watch the video on Walkley and Black titration method: HERE

This SOP is based on the Walkley & Black chromic acid wet oxidation method.  Basically, oxidizable organic carbon in the soil is oxidised by a 0.167 M potassium dichromate solution in concentrated sulfuric acid. The temperature needed to induce substantial oxidation is raised by the heat of reaction. The dichromate (Cr+6) reduced during the reaction with soil is proportional to the oxidisable organic C present in the sample. The organic carbon can, then, be estimated by measuring the remaining unreduced dichromate by back-titrating with ferrous sulphate or ammonium ferrous sulphate using diphenylamine or o-phenanthroline-ferrous complex as an indicator.

This video on the titration method was made in collaboration with the Office of Science for Land Development (OSLD) – Land Development Department (LDD) of the Ministry of Agriculture and Cooperatives of Thailand, and aims to support soil laboratories in the adoption of this SOP.

In this video, Ms Sumitra Wattana, OSLD Director, show how to quantify soil organic carbon, using Walkley-Black titration method. In particular, this protocol allows to determine the oxidizable organic carbon content in the soil, calculated from the amount of chromic ion (Cr3+) formed during the reaction.


Standard operating procedure for soil organic carbon. Walkley-Black method: titration and colorimetric method

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