This guide is available to download as a PDF below.
The world’s soils store approximately three times as much carbon as the atmosphere, but these stores have been depleted by land clearing and intensive farming. Improving how soils are managed has the potential to remove more than a billion tonnes of carbon dioxide from the atmosphere every year, using land that is already being farmed and practices that are well understood.
In the past, soil methodologies have faced scrutiny for inadequate measurement and monitoring, leaving buyers unsure whether the removals actually happened. High-quality soil carbon certificates depend on the answers to three questions: did the removal happen, will the carbon stay out of the atmosphere, and would it have been stored anyway?
Isometric’s Improved Soil Management (ISM) Protocol answers those questions, and is already being used by project developers around the world.
This guide covers what soil carbon is, how projects work, and how removals are certified.
The basics
What is soil carbon?
Soils are the largest terrestrial carbon store on Earth, holding nearly three trillion tonnes. Healthy soils naturally accumulate organic carbon as plants capture carbon dioxide from the air and then transfer it to the soil through their roots or residues. Microbes help break down these residues, enabling the carbon to bind to clay and silt in the soil, where it can remain stable for decades.
Disturbing the soil, through land clearing or intensive farming, interrupts the accumulation of carbon and releases it into the atmosphere.
By changing how cropland and grasslands are managed, soils could remove 1.57 billion tonnes of carbon dioxide from the atmosphere each year, around four times the UK’s annual greenhouse gas emissions.
Is soil carbon a removal or a reduction?
ISM projects are carbon removal projects. Plants naturally capture and remove carbon dioxide through photosynthesis and transfer some of it into the soil through their roots and as they decompose. ISM projects increase how much carbon enters the soil and reduce how much is lost to the atmosphere, increasing the overall amount removed and stored.
This is different from a reduction, or avoided emission, where a project prevents greenhouse gases from reaching the atmosphere or being produced in the first place.
Some approaches to ISM can reduce emissions, such as by reducing fuel use. Isometric quantifies and certifies these reductions separately. Under Isometric’s ISM Protocol, projects cannot generate reduction certificates alone; they must also increase soil carbon.
How do soil carbon projects work?
There are many ways to improve how land is managed so that it stores more carbon. The right approach depends on the type of land, and most projects combine several.
For croplands, approaches include:
- Cover cropping: Plants are grown on fields between harvest and replanting, so living vegetation keeps adding carbon to soil that would otherwise be bare. Covering the soil also makes it more stable, increasing its resilience to erosion.
- Reduced tillage: Soils are plowed or turned less frequently, so that less stored carbon is lost to the atmosphere and more can accumulate.
- Inoculation: Beneficial live microorganisms, like bacteria and fungi, are added to seeds or soils to stimulate plant growth and root activity.
- Organic amendments: Compost, manure, or other organic material is applied to fields to increase the amount of carbon stored in the soil.
For grazing lands, including grasslands, rangeland, and pasture, approaches include:
- Rotational or adaptive grazing: Livestock are moved between pastures, allowing plants to recover and regrow between grazing, so more carbon enters the soil.
- Improved forage species: Grasses with deep roots or legumes are planted in grazing lands, increasing the amount of carbon delivered into the soil.
- Water management: Increased irrigation enables plants to grow on previously degraded soils or extends the growing season, so soils are bare for less of the year.
- Brush management: Managing the spread of trees or woody shrubs onto grasslands using controlled fires or machinery, to restore the health of grasslands and stimulate roots.
What are the benefits of soil carbon projects?
Alongside removing carbon, ISM projects can improve agricultural productivity, reduce reliance on synthetic fertilizers, increase biodiversity above and below ground, and reduce nutrient runoff into waterways. Healthier soils can absorb and store more water, protecting crops against droughts and floods, and can also make crops less vulnerable to pests and disease.
For farming communities, ISM projects can provide a valuable additional income stream, while also making farms more productive and resilient. These benefits matter because they help keep farmers engaged in projects for the long term.
How much does soil carbon cost?
At the time of publication, soil carbon certificates cost between $20 and $80 per tonne, making it one of the most cost-effective carbon removal pathways available at scale today. Cost can vary by approach, region, and durability.
How it’s certified
How is soil carbon measured?
Under Isometric’s ISM Protocol, soil carbon must be measured directly, by taking samples from the project area and analyzing them in an accredited laboratory. Each project must establish a baseline by taking samples before they improve the management of the soil, and then re-sample the same locations to measure how the amount of carbon stored has changed.
The protocol allows two approaches. Under measure and re-measure, every certificate issuance is based on new samples. Under measure and model, changes between sampling events are estimated by a model that must be calibrated and validated against project data. Modeled estimates are always anchored to physical samples from the field.
Soils are naturally variable, so measurement always carries some uncertainty. The protocol requires that uncertainty to be quantified and the removal estimate discounted accordingly, so certificates are conservative, not optimistic.
How durable is soil carbon?
If improved management approaches stop, the carbon can return to the atmosphere. Soil carbon projects certifying with Isometric must commit to maintaining soil carbon for between 40 and 100 years. The durability of each certificate is set at half that length, so a 100-year project generates certificates with a 50-year durability. Throughout, a project must monitor the soil and keep the amount of carbon stored at or above the level it has been certified for.
Isometric’s protocol includes protections against reversals. Projects must provide evidence of secure land tenure as well as a financial plan for maintaining soil management and monitoring. Each project also contributes certificates to a shared buffer pool, in proportion with its assessed reversal risk. This pool acts as insurance across all Isometric-certified soil carbon projects. If stored carbon is lost, certificates from the pool are canceled so buyers are protected.
What makes a soil carbon project additional?
Additionality is the test of whether carbon removed by a project would have been stored anyway. This is especially important for soil carbon projects because many of the practices involved can be used by farmers regardless of the carbon removal benefit. Isometric’s protocol tests additionality in three ways, and projects must pass all of them:
- Practices must be new: A project must prove that its practices were not in use on each enrolled field for at least five years before enrollment, so a farm cannot be paid for something it was already doing.
- Practices must not be widespread locally: A practice must not be widely adopted on comparable local land that is not receiving carbon finance. The default threshold is 20%, with higher thresholds permitted where adoption is flat or falling.
- Carbon finance must be needed: A project must show that its practice receives little or no funding from other sources, such as subsidies. Where it does, those payments and any cost savings must add up to no more than half the cost of adopting the practice.
Each project is also compared against a counterfactual: what would have happened to the soil carbon without the project. This is determined with control plots on comparable land or modeling, and is updated over time.
What safeguards do soil carbon projects need?
ISM projects take place on land that communities depend on, so every project must support the interests of the people who live and work there. Under Isometric’s protocol, projects working with smallholder farmers must share at least 20% of certificate revenue with those farmers, recognizing their role in storing and maintaining the carbon in their soil.
Projects must demonstrate that they maintain agricultural productivity and local food systems, and that they protect soil health, local water quality and availability, and existing habitats. Projects cannot convert native ecosystems, wetlands, or other high-conservation-value land.
Next steps
Every soil carbon certificate issued by Isometric is published on the Isometric Registry, alongside the evidence behind it.
If you’re a project developer exploring Improved Soil Management, or a buyer evaluating soil carbon certificates, get in touch to find out more.
