Soil profile: the organic matter content, the backbone of living soil
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Time to read 5 min
Summary
Introduction Why does the organic matter content matter so much? How does organic matter get into the soil? The "right" organic matter content: how to determine it How to measure and monitor organic matter? Role, mechanisms, and long-term impact Economic and regulatory issues How to measure and monitor organic matter? Here are the main tools for monitoring organic matter: What is the impact of tillage? Why is the soil profile useful? Practical tips for maintaining or increasing organic matter Frequently asked questions (FAQ) Why trust Pouryère for your soil analysis? To learn more
Introduction
The composition of the soil tells the story of the earth's life. It reveals its layers, from the surface layer of plant debris down to the bedrock. We can read its structure, moisture, color… and above all, its organic matter (OM), this mixture of decomposing plant and animal remains and living organisms. Understanding OM means understanding the fertility, resilience, and capacity of a soil to sustainably nourish plants.
Why does the organic matter content matter so much?
Organic matter acts everywhere at once. It stabilizes soil aggregates (improving porosity), retains water (reducing water stress), nourishes soil life (microbes, fungi, earthworms), and serves as a reservoir of nutrients released gradually (mineralization). It also plays a climatic role by storing carbon in the surface horizons. In France, the 0–30 cm layer of soils stores several billion tons of organic carbon. Soil rich in organic matter is more tolerant of extremes: heavy rainfall, drought, trampling, and repeated tilling.
How organic matter arrives in the soil
It all starts with inputs: dead leaves, roots, mulch, green manure, compost, and manure. Fauna and microflora decompose these materials. Some mineralize (making nutrients available), while some transform into humus (stable organic matter) which accumulates mainly in the topsoil. For a garden, simple practices like home composting and mulching regularly replenish nutrients and organic matter.
The "right" organic matter content: how to approach it
There is no single organic matter content that is universally applicable. Climate, soil texture ( clay / silt / sand ), land use, and management history all influence what are considered "normal" levels. A robust approach is to consider soil organic carbon (SOC) and aim for at least one level that supports productivity. A global meta-analysis suggests that yields tend to increase with SOC up to approximately 2% SOC (or about 3.4% organic matter when applying the usual conversion factor). This value is not an absolute, but a useful benchmark for temperate garden and crop soils.
How to measure and monitor organic matter?
Here are the simple steps to monitor organic matter:
Take a soil sample (e.g., 0–30 cm).
Send to laboratory to measure the SOC (TOC) and the C/N ratio.
Observe the soil profile: color, structure (clumps), life (worms, roots).
Repeat the measurements every 2 to 3 years.
Role, mechanisms and long-term impact
Throughout the seasons, organic matter helps the soil maintain its balance. It makes the soil more supple, retains moisture, limits nutrient loss, and prevents it from deteriorating too quickly. These effects are visible in the soil profile: the top layer is darker, more crumbly, and full of small roots or worm burrows. It is this richness that we seek to protect, as it is what keeps the soil alive and productive.
The economic and regulatory challenges
Managing heavy metals is expensive: analysis, removal, treatment. But ignoring pollution leads to far greater costs:
Legal proceedings.
Health risks for local residents.
Land devaluation of polluted land.
From a regulatory standpoint, priority is given to recovery rather than storage (Environmental Code, Article L.541-1). BRGM guides are references for determining the number of analyses, thresholds, and appropriate disposal channels.
How to measure and monitor organic matter?
Here are the simple steps to monitor organic matter:
Take a soil sample (e.g., 0–30 cm).
Send to laboratory to measure the SOC (TOC) and the C/N ratio.
Observe the soil profile: color, structure (clumps), life (worms, roots).
Repeat the measurements every 2 to 3 years.
Here are the main tools for monitoring organic matter:
Here is a table detailing the purpose of each indicator:
| Indicator | What this reveals | Suggested frequency |
|---|---|---|
| SOC / COT (lab analysis) | Size of the carbon reserve / quantity of organic carbon | Every 2–3 years |
| C/N ratio | Decomposition equilibrium | At the same time as SOC |
| observed soil profile | Structure, life, depth (color, structure, visible roots) | With each major change |
| Soil life (worms, microbes) | Soil biological vitality (insects, worms, fungi) | 1–2 times a year |
What is the impact of soil cultivation?
Deep plowing accelerates the loss of organic matter . It exposes humus to oxygen, causing it to disappear more quickly. In contrast, no-till or reduced-till methods better preserve surface organic matter. They promote plant cover and reduce erosion. This also helps to store more carbon.
Why soil profiling is useful
The soil profile shows you where organic matter is concentrated. It's often denser near the surface (dark color, clumps, roots). This allows you to target improvements: avoid disturbing the surface too much, add compost in the right place, and preserve this fertile layer.
Practical tips for maintaining or increasing organic matter
Three simple actions are very effective:
- Compost and mulch to nourish the soil and limit evaporation.
- Plant cover crops or green manure between crops to nourish the soil.
- Reduce soil tillage , favour surface tillage.
These simple practices increase organic matter in just a few years.
The author of the article: Stanislas FAHY
Stanislas Fahy spent his entire youth in the Dordogne region, exploring its fields, forests, and vineyards. Passionate about nature, he quickly understood that our world must be cherished to preserve its beauty. After studying business, he specialized in sustainable development. Today, as co-founder of Pouryère, he dedicates himself daily to making soil knowledge accessible to everyone.
Frequently Asked Questions (FAQ)
What is a good organic matter content?
As a general rule, aiming for ~2% SOC (≈ 3.4% OM) gives good results for temperate soils.
What is the C/N ratio and why is it important?
C/N = %C / %N. A ratio around 10–12 indicates good decomposition and good nitrogen release for plants.
How often should organic matter be analyzed?
Every 2–3 years in the laboratory. In between, you can observe the soil profile with the naked eye.
What impact does tilling have on organic matter?
Deep plowing accelerates the loss of organic matter. No-till or reduced tillage better preserves the soil.
Why trust Pouryère for your soil analysis?
Comprehensive support, from sampling to interpretation
Pouryère supports you throughout the entire soil analysis process. Our sampling kits come with a comprehensive guide to walk you through the process. Once you've completed the analysis, simply send us your samples for full analysis and interpretation, which takes approximately ten days.
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Each soil analysis kit is specialized and pursues a specific purpose:
