Soil organic matter is a small part of most soils, but it drives much of what they do. It holds soil together, stores water, feeds soil life and releases nutrients to crops and grass. But a standard soil test only covers pH, phosphorus and potassium. Unless you add an organic matter test, you cannot tell if your fields are building organic matter or losing it.
Teagasc says Irish soils have good levels of soil organic matter overall. It also warns those reserves can be depleted through disturbance, compaction and bare ground. Testing shows you where each field stands, so you can protect what you have and rebuild what you have lost.
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What Is Soil Organic Matter?
Soil organic matter (SOM) is the remains of plants, animals and microorganisms at different stages of decay. It also includes the living part of the soil, its soil biology.
Over half of soil organic matter is carbon. That is why labs report soil organic carbon (SOC) alongside, or instead of, organic matter.
Teagasc separates mineral soils from organic soils. A soil counts as organic, or peaty, where it holds at least 20% soil organic carbon in the top 30 to 40 cm. That works out at about 30% to 40% organic matter.
Why Soil Organic Matter Is Good for Your Soil
Teagasc’s 2025 handbook, Farming for Soil Health, sets out the main benefits. Soil organic matter:
Builds structure. It glues soil particles into aggregates and gives soil a sponge-like quality. This helps it resist compaction and keep its pore space.
Stores water. It supplies and retains water, which helps soils cope with dry spells and heavy rain.
Holds nutrients. Clay and organic matter set a soil’s cation exchange capacity (CEC), its ability to hold potassium, calcium, magnesium and ammonium.
Releases nutrients. Soil biology breaks down organic matter and converts the nutrients bound inside into forms plants can use.
Feeds soil life. Organic matter is the food source at the base of the soil food web, from bacteria and fungi to earthworms.
Buffers pH. It helps resist swings in soil acidity.
Suppresses disease. It helps keep soil-borne pests and diseases in check.
Stores carbon. Teagasc estimates Irish grassland mineral soils hold about 1,800 million tonnes of CO₂ equivalent in the top metre.
Structure carries a direct cost when it fails. In one Irish study, compaction that cut pore space by 12% led to a drop of about 20% in barley yields. Teagasc notes that even small changes in organic matter can make a big difference to soil compaction.
What Depletes Soil Organic Matter?
Soil organic matter is not fixed. It falls when more carbon leaves the soil than enters it. According to Teagasc, the main pressures are:
- Repeated disturbance, such as frequent or deep tillage
- Compaction from heavy machinery or livestock on wet ground
- Bare soil left without living roots
- Harvest removals, where carbon taken off in crops, straw or silage is not replaced
Teagasc flags arable land and silage blocks as places where depletion is common. Drained peat grassland is the extreme case. It can release up to 20 tonnes of CO₂ per hectare each year as its organic matter breaks down.
Rebuilding is slow. Teagasc notes that it takes time, above all in soils that are rich in carbon to begin with.
Why a Standard Soil Test Will Not Show Soil Organic Matter
Most farmers test for pH, phosphorus (P) and potassium (K). These results drive lime and fertiliser plans, but they say nothing about organic matter.
Changes in soil organic matter are also hard to see in the short term. Teagasc notes this is most true in soils that are rich in organic matter. You cannot judge it by eye from one season to the next.
There is no single target that suits every field. Soil texture and climate shape how much organic matter a soil can hold. Teagasc notes that clay soils retain more nutrients, water and carbon than sandy soils. A sandy field and a heavy clay field need different benchmarks, which is why you need data from each one.
How Testing Improves Soil Organic Matter
A lab test turns soil organic matter from a guess into a number you can manage. It helps you:
- Set a baseline. Know the starting point for each field before you change management.
- Track change. Retest the same fields, using the same method, every few years. This shows whether your practices are working.
- Target organic manures. Teagasc advises directing slurry and farmyard manure to fields that need P and K or are low in organic matter, such as silage ground and arable land.
- Read nutrient-holding capacity. A CEC test shows how well a soil holds nutrients. Teagasc advises applying nutrients little and often on low-CEC soils.
- Measure carbon. Total organic carbon separates organic carbon from inorganic carbon. Inorganic carbon cannot be counted towards carbon credits.
Soil carbon is also moving up the policy agenda. The EU Soil Monitoring Law, Directive (EU) 2025/2360, entered into force on 16 December 2025. It lists soil organic carbon as one of the descriptors Member States must use to assess soil health. Ireland must transpose it by 16 December 2028. The directive places no new obligations on farmers, but national monitoring of soil carbon is on its way.
When to Test
With the slurry closed period now in force, winter is a practical window for sampling. Southern Scientific advises sampling before you spread lime or fertiliser, over winter or early spring. Most farmers retest every two to three years, or sooner if land use changes.
Test Your Soil Organic Matter With Southern Scientific
Soil organic matter underpins structure, water, nutrients and carbon on every farm. Knowing your levels lets you protect your best fields and target the ones that are slipping.
Southern Scientific runs Ireland’s largest soil testing laboratory, accredited by INAB to ISO 17025. Our agricultural soil testing suites include:
- Organic matter analysis, on its own or added to pH, P, K and lime suites
- CEC testing for nutrient-holding capacity
- A Soil Health Check suite for a wider view of soil condition
For carbon baselines, our soil carbon analysis measures total carbon, total organic carbon and total nitrogen. Results arrive within 10 working days, with a 3 to 5 day express service available.
Testing your soil organic matter is the first step in moving from reactive crisis management to proactive protection of your land.
