Soil Composition » Organic Content in Soil

What you'll learn this session

Study time: 30 minutes

The composition and importance of organic matter in soil
How humus forms and its role in soil fertility
The carbon cycle in soil ecosystems
Factors affecting organic content levels
How to measure and improve soil organic content
The environmental impacts of soil organic matter loss

Introduction to Organic Content in Soil

Soil is much more than just dirt! It's a complex mixture of minerals, air, water and organic matter that supports life on Earth. The organic content in soil is particularly important as it determines soil fertility and structure. Without it, our food systems would collapse!

Key Definitions:

Soil organic matter (SOM): The organic component of soil made up of plant and animal residues at various stages of decomposition.
Humus: The dark, stable form of organic matter that remains after most decomposition has occurred.
Soil biota: The living organisms in soil including bacteria, fungi, earthworms and insects.
Decomposers: Organisms that break down dead organic material into simpler substances.

🌾 What is Soil Organic Matter?

Soil organic matter (SOM) consists of all the organic substances in the soil, including:

Living organisms (about 15%)
Fresh residues from plants and animals
Partially decomposed residues
Humus - the stable end product of decomposition

Though it typically makes up only 2-10% of soil mass, organic matter is crucial for soil health!

💪 Why Organic Matter Matters

Organic content in soil is vital because it:

Improves soil structure and reduces erosion
Increases water retention capacity
Provides nutrients for plants
Supports biodiversity in the soil ecosystem
Acts as a carbon sink, helping to mitigate climate change

The Formation of Humus

When plants and animals die, their remains begin to decompose. This process transforms fresh organic material into the stable substance we call humus. Let's explore this fascinating journey!

The Decomposition Process

Decomposition happens in stages, with different organisms playing key roles:

🐛 Stage 1: Initial Breakdown

Larger soil animals like earthworms, beetles and millipedes physically break down organic matter into smaller pieces. This increases the surface area for smaller decomposers to work on.

🦋 Stage 2: Microbial Action

Bacteria and fungi continue the decomposition process by releasing enzymes that break complex molecules into simpler compounds. This releases nutrients like nitrogen, phosphorus and potassium.

🌲 Stage 3: Humus Formation

The remaining resistant materials combine with microbial products to form humus - a dark, spongy material that's resistant to further decomposition and can remain in soil for hundreds of years.

The Carbon Cycle in Soil

Soil organic matter is about 58% carbon, making soil the largest terrestrial carbon store on the planet. Understanding how carbon moves through soil is essential for addressing climate change.

🔑 Carbon Inputs

Carbon enters the soil system through:

Photosynthesis: Plants convert CO₂ into organic compounds
Plant residues: Leaves, roots, stems that die and decompose
Root exudates: Substances released by living roots
Animal waste and remains: Manure, dead insects, etc.

🔓 Carbon Outputs

Carbon leaves the soil through:

Respiration: Soil organisms release CO₂ when breaking down organic matter
Erosion: Organic matter can be washed or blown away
Leaching: Dissolved organic carbon moving downward with water
Harvesting: Removal of plants that contain carbon derived from soil

Factors Affecting Soil Organic Content

The amount of organic matter in soil varies widely depending on several factors:

Natural Factors

These natural elements influence how much organic matter accumulates in soil:

Climate: Decomposition happens faster in warm, moist conditions. Cold or dry areas often have higher organic matter because decomposition is slower.
Soil texture: Clay soils can protect organic matter from decomposition by forming clay-humus complexes. Sandy soils typically have lower organic content.
Topography: Lowlands and depressions tend to accumulate more organic matter than upland areas.
Vegetation type: Grasslands often develop higher organic matter than forests because more of their biomass is below ground in roots.
Soil organisms: The types and numbers of decomposers affect how quickly organic matter breaks down.

Human Impacts on Soil Organic Content

Human activities can dramatically increase or decrease soil organic matter:

👎 Practices that Reduce Organic Content

Intensive tillage (ploughing) that exposes organic matter to oxygen
Monoculture farming without crop rotation
Removing crop residues after harvest
Overgrazing that reduces plant growth
Deforestation and land conversion

👍 Practices that Increase Organic Content

Adding compost, manure or other organic amendments
Growing cover crops and green manures
Implementing crop rotation systems
Reducing tillage (no-till or minimum tillage farming)
Agroforestry and mixed farming systems

Measuring Soil Organic Content

Scientists and farmers need to measure organic content to assess soil health and make management decisions. Here are some common methods:

Loss on ignition: Soil is weighed, then heated to burn off organic matter, then weighed again. The difference represents organic content.
Walkley-Black method: Uses chemical oxidation to measure organic carbon.
Visual assessment: Darker soils generally have more organic matter (though this can be misleading).
Field tests: Simple tests like the "soil ribbon test" can give rough estimates of organic content.

Case Study Focus: The Rothamsted Experiments

The world's oldest agricultural research station, Rothamsted in Hertfordshire, UK, has been studying soil organic matter since 1843. Their "Broadbalk Experiment" has shown that plots receiving farmyard manure for over 175 years have more than doubled their soil organic carbon compared to unfertilized plots. This long-term study demonstrates how farming practices directly impact soil organic content over time and provides valuable data for sustainable agriculture.

Environmental Impacts of Soil Organic Matter Loss

When soils lose their organic content, several serious environmental problems can occur:

🌊 Water Quality

Soils with low organic matter have poor structure and are more prone to erosion. This leads to sediment pollution in waterways, along with agricultural chemicals that would otherwise be bound to organic matter.

🌐 Climate Change

When soil organic matter decomposes rapidly, carbon that was stored in the soil is released as CO₂, contributing to greenhouse gas emissions. Globally, soil degradation has released billions of tonnes of carbon into the atmosphere.

🌾 Food Security

Declining soil organic matter leads to reduced fertility, water retention and resilience to extreme weather events. This threatens agricultural productivity and food security, especially in vulnerable regions.

Sustainable Soil Management

Maintaining or increasing soil organic matter is a key goal of sustainable land management. Here are some practical approaches:

Conservation Agriculture

This approach is based on three principles:

Minimal soil disturbance: Reducing or eliminating tillage
Permanent soil cover: Using cover crops or mulch
Crop diversity: Rotating crops and using mixed cropping systems

These practices can increase soil organic matter by up to 0.5% per year in some soils.

Did You Know?

Just a 1% increase in soil organic matter helps soil hold about 20,000 more gallons of water per acre! This makes farms more resilient to both droughts and floods, which are becoming more common with climate change.

Understanding and managing soil organic content is crucial for sustainable agriculture, environmental protection and addressing climate change. By implementing practices that build soil organic matter, we can create healthier ecosystems while producing the food we need.

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