Regrow's Soil Sample Models & ESM 

What is Equivalent Soil Mass (ESM)?

Equivalent Soil Mass (ESM) is a method for comparing soil organic carbon (SOC) stocks over time using a fixed mass of soil, rather than a fixed depth.

Why this matters

Soils are not static. Their physical structure can change over time in response to land management practices, climate, and biological activity. One of the key properties that changes is bulk density, which describes how much soil mass exists in a given volume.

Bulk density can change due to:

• Tillage management (such as reduced till or no till practices), which can cause soil disturbance, and soil aggregate formation and persistence
• Root activity (such as increased root growth), which creates pore space as roots grow and decay
• Changes to soil organic matter over time (such as increasing SOC), which influence bulk density due to its lower compositional density than mineral soil 
• Changes in practices that influence soil compaction, such as altered grazing intensity or machinery use
• Soil settling, which can occur naturally and accelerated by freeze/thaw cycles

Over the course of a carbon project, bulk density may change. Under many regenerative practices, we expect soils to become less compacted over time. In cases where bulk density decreases, soil samples taken at the same depth as the original sample will contain less total soil mass overall. The same is true conversely, in cases where bulk density increases.

If SOC stocks from soil samples were compared at baseline and true-up using a fixed depth (ie 0–30 cm) both times:

• A soil that has loosened (bulk density decreases) could appear to lose carbon or remain static simply because there is less soil mass in that depth layer
  
  
• Conversely, a compacted soil (bulk density increases) could appear to gain carbon even if the SOC concentration stayed the same, or decreased
  
  

This can lead to reporting artificial SOC gains or losses that are driven by physical changes in soil structure rather than real changes in SOC / carbon storage resulting from the adoption of regenerative ag practices through carbon programs.

How ESM solves this

The Equivalent Soil Mass approach fixes this problem by:

• Comparing SOC stocks using the same mass of soil (rather than the same depth of soil) at different points in time
• Allowing the depth associated with that mass to be calculated in correspondence with bulk density changes
• For example, let's say initial soil samples were taken to a depth of 30cm
  • At the time of true-up 5 years later the bulk density had decreased 
  • ESM allows for taking samples deeper than 30cm at true up, to ensure enough soil is collected to achieve an 'equivalent soil mass' to use for calculating SOC content

In practice, this means that the initial project sampling establishes a reference soil mass, and at re-sampling the SOC is calculated to the depth required to reach that same soil mass.

This ensures that reported SOC changes reflect true changes in carbon storage, not changes in soil compaction or loosening.

What do Verra's ESM sampling requirements mean for project developers taking samples & Regrow's soil stratification models?

Under VM0042, ESM applies to the reporting of SOC stock changes when SOC is quantified using direct measurements (soil samples). ESM is most critical at re-sampling (true-up), when bulk density may differ from initial soil measurements.

At the project start, Verra does not require SOC stocks to be calculated on an ESM-adjusted basis. The initial measurements establish the reference soil mass using measured bulk density and depth, for which ESM will apply at re-sampling.

Project stratification and soil sample design

Verra’s ESM requirements primarily affect how soil samples are collected in the field, not project stratification or where the samples are taken. 

Regrow's soil stratification model determines where samples should be taken. The model stratifies the project area in accordance with VM0042 requirements, ensuring a representative distribution of samples that minimize error and bias, with a minimum of three samples per stratum. ESM does not change how strata are defined or how sample locations are selected.

When taking the actual soil samples, it's up to the project developer to determine how the samples are taken - for example, determining the sampling depths and layers. 

VM0042 sample requirements

Depth requirements

• Samples must be taken to a minimum of 30 cm to ensure SOC is calculated to this minimum depth
  
  
  
  • Note: Regrow's predictive model (used to extrapolate measured soil properties to all remaining fields in the program) only uses one SOC value. If two sample layers are taken at different depths, it's the responsibility of the lab or project developer to provide a single SOC measurement either at the 0-30cm depth or combined across sampled layers (as allowed by Verra). Only soil mass must remain distinct to enable ESM calculations, at re-sampling.
    
• Verra strongly recommends sampling to 50 cm, typically as in two distinct layers of 0–30 cm and 30–50 cm.
  
  • The deeper layer is not explicitly required at initial sampling, but is suggested to act as a buffer layer that allows equivalent soil mass adjustments if bulk density changes at the time of re-sampling
    • Note: Regrow's predictive model at initialization only uses one bulk density measurement, from the 0-30 cm layer
  • A second sampling layer beyond the 0-30 cm layer is required at re-sampling (true-up), where Verra suggests using the same depth increments used in initial sampling, or to a depth of 30-50 cm if only 0-30 cm were taken initially

Using initial measured samples to initialize entire project area

At project initialization, Regrow's Predictive Soil Model extrapolates initial measured soil properties to all fields in the project. The model uses SOC concentration and bulk density from the 0–30 cm layer (unless SOC is provided by the lab for the entire depth sampled). 

The model does not perform an explicit ESM adjustment (as it's not necessary at the project start), or attempt to predict future bulk density-driven depth shifts to inform sampling depths needed at true-up. It’s up to the project developer to ensure the field sampling protocol enables ESM at re-sampling, for compatibility with VM0042 requirements.

A practical example of implementing an ESM approach at re-sampling (true-up)

At project start, a field is sampled at 0–30 cm under conventional management.

• Measured bulk density (0–30 cm): 1.40 g/cm³
  
• Soil mass in 0–30 cm (per hectare): 4,200 tonnes of soil
  
• This 4,200 t/ha becomes the reference soil mass for ESM
• SOC stocks at baseline are calculated for this soil mass
  
  

Over the five years of the program, a farmer adopts reduced tillage and cover crops. Over that time, it's expected that soil structure improves, aggregate soil stability increases and roots-in-the-ground increase pore space.

• At true-up when the project does re-sampling, bulk density is expected to be lower.
• When measured in the upper soil, bulk density: 1.25 g/cm³
  
  

Because the soil is now less compacted, taking a soil sample at the original 0–30 cm depth no longer contains the original mass of 4,200 t/ha of soil that was used to calculate SOC.

To account for this, a second depth layer is sampled at true-up to ensure there is enough soil mass to get an apples-to-apples SOC calculation. In practice, most projects will not take an initial sample to determine how much the bulk density changed. Instead, most will ensure the second sample layer depth is conservatively sufficient to ensure at least the same soil mass is taken.