The short answer. There are four common ways to test soil carbon in the UK: loss on ignition, dry combustion, spectral probes, and satellite mapping tied to soil samples. Loss on ignition, found in many standard soil analysis packages, estimates organic matter from the weight a sample loses when it is heated and converts it to carbon with an assumed factor. Dry combustion measures the carbon a sample releases directly, and it is the reference laboratory method under ISO 10694. Probes and spectral readings are quick, but only as good as their calibration. None of them measures a field on its own: that needs enough samples, the bulk density of the soil and a way to cover the variation between sampling points. Ecometric anchors every result to dry combustion in a laboratory accredited to ISO/IEC 17025 and extends it with satellite imagery at 100 mapped values a hectare.
What a soil carbon test is for
Before choosing a method, be clear about the question. A one-off check of a sample tells you the concentration of carbon in that soil on that day. Knowing whether a field is gaining or losing carbon needs the stock, the tonnes of carbon held per hectare, measured the same way at the same point in the year, every year. The first is a snapshot. The second is what shows whether your management is working, and it is what a buyer, a lender or an auditor will ask to see.
The methods compared
| Method | What it measures | Best used for | Main limitation |
|---|---|---|---|
| Loss on ignition | Weight lost when a sample is heated, read as organic matter | A rough organic matter figure alongside a nutrient test | Converts to carbon with an assumed factor; clay and carbonates distort it |
| Dry combustion | Carbon released when the sample is combusted, measured directly | Accurate soil organic carbon for a sample | Measures a sample, not a field, without a sampling design |
| Wet oxidation | Carbon oxidised by a chemical reagent | Older laboratory records | Incomplete recovery needs a correction factor; hazardous chemicals |
| Probes and spectral readings | Light or sensor response, converted to carbon by a model | Fast readings at many points | Only as accurate as their calibration to laboratory results |
| Satellite mapping tied to samples | Variation across a field, anchored to laboratory results | A value every 10 metres and an accurate field total | Must be trained on real samples and checked against samples it never saw |
Loss on ignition
A sample is dried, weighed, heated until the organic matter burns off, and weighed again. The weight lost is reported as organic matter, and a conversion factor turns it into an estimate of organic carbon. It is inexpensive and common, which is why it appears in many routine soil analysis packages. The weakness is the conversion: the share of carbon in organic matter varies from soil to soil, and clay can lose water on heating that is counted as organic matter. It is a useful guide to the trend on one sample, but it is not precise enough to measure a change in carbon stock or to stand behind a report to a buyer.
Dry combustion
The sample is combusted at high temperature in oxygen and the carbon dioxide released is measured by an elemental analyser. That measures carbon directly rather than inferring it, and it is the reference method set out in ISO 10694. In soils that contain carbonates, such as chalk and limestone soils, the inorganic carbon is removed or corrected for so that the result is organic carbon only. This is the method we use: every sample we take is analysed by dry combustion in a laboratory accredited to ISO/IEC 17025, and that laboratory value is the anchor for everything that follows.
Wet oxidation
Older methods such as Walkley-Black oxidise the carbon with a chemical reagent. They do not recover all of the carbon, so the result depends on a correction factor, and they use hazardous chemicals. You will still find them in historic soil records, which matters if you are comparing today’s results with an old report.
Probes and spectral readings
Near and mid infrared spectroscopy, and some in-field probes, read how the soil responds to light or a sensor and use a model to convert that into carbon. They are quick and can take many readings. Their accuracy depends on how well they are calibrated against laboratory results for soils like yours, and moisture, texture and stones all affect the reading. Used without that calibration, a fast number can be a confident wrong one.
Satellite mapping tied to samples
A satellite cannot see carbon below the surface. What it can see is how a field varies, and an AI model trained on real laboratory samples can use that variation to estimate carbon between the sampling points. Done properly, some samples are held back and the model never sees them, so its error can be measured on your soil in that year and deducted in full. That is how we give a value every 10 metres and a field total with its error stated, rather than a list of sample results, while every figure stays tethered to the laboratory. The step by step detail is in how soil carbon is measured, and the error handling is in accuracy stated.
From a sample to a field
Whichever laboratory method you use, three things turn a sample result into a field result you can trust.
- Stock, not just concentration. Stock is calculated from the concentration, the bulk density of the soil and the depth sampled, so bulk density has to be measured too.
- Enough samples in the right places. Carbon varies across every field, so the sampling design has to capture that variation rather than rely on one average.
- The same method at the same time of year. Soil carbon has to be sampled at the same point in the year for a change to be real, which is why we measure once a year, in the same window.
Which test do you need?
If you want a rough organic matter figure to sit alongside this season’s nutrient plan, a standard soil analysis with loss on ignition does that job. If you want to know whether your soil carbon is rising or falling, to see which practices are working, or to give a buyer or lender evidence rather than an assumption, you need measured stock change: dry combustion, bulk density, a sampling design that covers each field, the same window each year and the accuracy stated. That is what an annual round with us delivers, and what sets the price per hectare explains the three things that move the cost.
Questions we are asked about soil carbon testing
What is the most accurate way to test soil carbon?
Dry combustion in an accredited laboratory is the most accurate way to measure the carbon in a sample. For a whole field, accuracy also depends on the number and placement of samples, the bulk density measurement and how the error between sampling points is measured and deducted.
Does a standard soil test measure soil carbon?
A standard soil test reports pH, phosphorus, potassium and magnesium, and sometimes organic matter by loss on ignition. It does not measure soil organic carbon stock, so it cannot show whether a field is gaining or losing carbon.
Can I test soil carbon myself?
A home kit gives a rough organic matter reading for the sample it tests. Measuring a change in soil carbon stock needs laboratory analysis, the bulk density of the soil and enough samples to cover each field.
How much does soil carbon testing cost?
It depends on the size of the holding, the number of fields and the tier you choose, and larger holdings cost materially less per hectare. Send us your SBI number and hectares and we will send a written quote within two working days.
How do I get my fields tested?
Send us your SBI number and your hectares. We agree field boundaries and sampling access on a short call, sample in the autumn-winter window, and report the result field by field with the accuracy stated.
Part of soil carbon measurement for farms: the complete guide. Related: how soil carbon is measured · what soil organic carbon is · soil health testing in the UK · soil sampling for soil carbon.
