How does soil carbon testing work?
Louise Edmonds, CEO. Published July 2023
In this blog post, we will explore the importance of soil carbon baseline testing, the methods used to obtain soil samples, and the laboratory processes involved in determining soil carbon percentages.
As we grapple with the urgent need to combat climate change, it has become increasingly evident that agriculture plays a crucial role in both mitigating and adapting to the environmental challenges we face. One promising avenue in sustainable agriculture is soil carbon sequestration, a process that involves capturing carbon dioxide from the atmosphere and storing it in the soil. To embark on this transformative journey, farmers need a solid understanding of their soil’s carbon content, which can be achieved through soil carbon testing.
The Importance of Soil Carbon Baseline Testing
Soil carbon baselining serves as the foundation for effective soil carbon management. It involves establishing a baseline measurement of the amount of carbon present in the soil at a particular location. This baseline measurement provides farmers with a reference point against which they can evaluate the effectiveness of their soil carbon sequestration efforts. By periodically testing their soil’s carbon content, farmers can monitor changes over time and assess the impact of various land management practices. This data-driven approach empowers farmers to make informed decisions and optimise their strategies for maximising carbon sequestration potential.
The Role of the Clean Energy Regulator
In Australia, new soil carbon farming projects must be registered by the Clean Energy Regulator (CER) and be undertaken using the 2021 soil carbon methodology, which has been approved by the CER. Soil carbon project proponents are required to prepare a baseline soil sampling plan and provide it to the Clean Energy Regulator.
According to the CER, the method relies on random allocation of sampling locations to be undertaken without bias to estimate changes in carbon stocks. The general steps involved in using soil samples to estimate changes in carbon stocks are:
- Develop a sampling plan for the project area;
- Sample collection;
- Sample preparation;
- Laboratory analysis;
- Calculation of the organic carbon stock from the soil samples; and
- Calculation of the change in soil organic carbon stocks over time within each carbon estimation area (CEA).
Soil Sampling Guidance
The CER provides a guidance document for soil carbon projects called “Sampling Guidance for Measurement-Based Soil Carbon Methods.” The document provides guidance for projects using measurement-based soil carbon methods under the Emissions Reduction Fund, specifically focusing on the Carbon Credits (Carbon Farming Initiative) methodologies. It aims to clarify and improve sampling assurance processes and controls for accurate soil carbon
measurement. The guidance covers various aspects of sampling plans and procedures for baseline and subsequent sampling rounds.
The guidance document outlines the purpose and importance of soil sampling for estimating changes in carbon stocks. It explains the general steps involved in sampling, including developing a sampling plan, sample collection, preparation, laboratory analysis, and calculation of organic carbon stock changes within carbon estimation areas (CEAs).
To meet the method requirements, participants are advised to prepare a geospatial map of the project area, including CEAs, exclusion zones, emissions accounting areas, and strata boundaries. The decision to divide the project area into multiple CEAs and stratify them depends on factors such as land use history, management actions, and soil carbon stock variations. Sampling density and random sampling approaches are also discussed, including default assurance and pre-approved assurance approaches for generating random sampling
The document emphasises the importance of preparing a comprehensive sampling plan that includes geospatial information and describes the process for random location assignment. Participants are required to notify the regulator of their intended sampling locations prior to sampling. The document also provides guidance on handling obstacles that may hinder sampling and requirements for qualified personnel undertaking the sampling.
Overall, the document aims to enhance the accuracy and reliability of soil carbon measurement by providing clear guidance on sampling plans, random location assignment, and compliance with method requirements. By following these guidelines, participants can ensure the integrity of their soil carbon projects and contribute to effective carbon stock estimation and reporting.
Obtaining Soil Samples
To accurately assess soil carbon content, representative soil samples must be collected from the target area. Soil sampling should account for variations within the field, such as soil types, topography, and land management practices. The sampling process typically involves the following steps:
- Stratification: Soil types and management zones are grouped together within a ‘carbon estimation area’ this is called stratification. Random sample points are generated within each stratum.
- Sampling equipment: Soil carbon baseline measurement requires the extraction of intact soil cores to a depth of 1- 1.2 meters. Vehicle-mounted hydraulic coring machines are the most common equipment used.
- Samples: The top 300mm is separated from the sample, and the top and bottom samples are independently tested in the lab.
Once the soil samples are collected, they are sent to a laboratory for analysis. In the lab, several techniques are employed to determine the percentage of carbon in the soil. The most common method used is known as loss-on-ignition (LOI), which involves the following steps:
- Drying: Soil samples are first dried to remove any moisture content. This step ensures accurate measurement of the organic matter and carbon present in the soil.
- Controlled heating: The dried soil samples are heated at a specific temperature (usually around 360°C or 680°F) in a furnace. This process causes the organic matter to combust, leaving behind the inorganic components of the soil.
- Weighing: After combustion, the remaining soil is weighed. The weight loss corresponds to the amount of organic matter that was present in the soil.
- Carbon calculation: Using conversion factors, the weight loss obtained is converted into an estimate of the soil’s organic carbon content. This measurement is then adjusted to account for the bulk density of the soil.
The Role of Soil Carbon Testing
Soil carbon testing plays a vital role in helping farmers understand the potential for soil carbon sequestration and implement effective land management practices. By establishing a baseline measurement of soil carbon content and regularly monitoring changes over time, farmers can make informed decisions to enhance carbon sequestration in their fields. Through the collection of representative soil samples and laboratory analysis, the percentage of carbon in
the soil can be determined, providing essential data for sustainable agricultural practices.
As we continue to recognise the importance of soil health in combating climate change, soil carbon testing remains a valuable tool in our collective journey toward a more resilient future.
If you’re curious about your farm’s soil carbon stocks, Carbon Sync is offering FREE Soil Carbon Baseline Sampling and Lab Testing, valued at $5000-$7000, in conjunction with FarmLab and The University of Queensland, to uncover the possible value beneath your feet. We’ll also include a FREE farm emissions baseline report.
Our soil carbon testing and farm baselining are no-cost and completely obligation-free. We’re here to help you explore the benefits with no strings attached.