Intertwined roots in rich, dark soil, highlighting the vital role of humic substances in soil health.

Unlocking Earth's Secrets: How Humic Substances Impact Soil Health

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Kai Mendoza in Climate & Environment August 2025 • 4 min read.

"Dive into the groundbreaking research that explores the vital role of humic substances in maintaining soil health and preventing subsidence, particularly in diverse environments across Brazil."


Soil, the very ground beneath our feet, is far more than just dirt. It's a dynamic system teeming with life and constantly shaped by a multitude of factors. Climate, the types of clay present, and human activities like agriculture and drainage all play crucial roles in determining the health and stability of this precious resource. These factors influence the rates at which organic matter is added to the soil and how it transforms over time, affecting the evolution of the organic compounds that are so essential for soil fertility.

At the heart of this intricate system lie humic substances (HSs), complex compounds that give soil its characteristic dark color. Formed through the breakdown and transformation of organic materials, HSs are categorized based on their solubility: humins (insoluble in both acid and alkali), humic acids (soluble in alkali but not acid), and fulvic acids (soluble in both acid and alkali). These substances differ from the original biomolecules in plants and animals due to their complex structure and long-term persistence in the soil.

Among the various soil types, Organosols stand out due to their high organic carbon content, exceeding 80 g kg-1. These soils, often found in wetlands and peatlands, are particularly vulnerable to a process called subsidence, where the soil surface sinks due to the decomposition of organic matter accelerated by drainage. Understanding the composition and behavior of HSs in Organosols is crucial for managing these soils sustainably and preventing environmental degradation.

The Brazilian Study: Humic Substances and Subsidence

Intertwined roots in rich, dark soil, highlighting the vital role of humic substances in soil health.

A recent Brazilian study delved into the intricate relationship between humic substances and the degree of subsidence in Organosols across different formation environments. Researchers collected soil samples from 11 locations, encompassing 32 horizons, to analyze the distribution of humic fractions and their connection to subsidence processes. The study focused on quantifying carbon content within these fractions, revealing key insights into soil dynamics.

The analysis revealed a clear trend: in soils from mountainous and plateau regions, humic acid was the dominant carbon fraction. This prevalence is likely due to the cooler climate and higher fertility in these areas, which favor the formation of humic acids over fulvic acids and humins. However, this also means that Organosols in these environments are more susceptible to subsidence, particularly when used for agriculture.

  • Mountainous and plateau regions favored humic acid formation due to cooler climates and higher fertility.
  • Floodplains and coastal plains showed greater humin formation, the most decomposition-resistant fraction.
  • The ratio of alkaline extract to humin (EA/HUM) can differentiate Organosol classes by formation environment.
  • Humic fractions accounted for about 89% of total carbon.
Conversely, in floodplain and coastal plain environments, humin was the dominant carbon fraction. Humin is the most resistant to decomposition, helping to preserve the soil structure in these areas. The study also found that the ratio of alkaline extract to humin (EA/HUM) could effectively differentiate Organosol classes based on their formation environment, suggesting its potential as a diagnostic tool in soil classification systems.

Implications for Soil Management

This research underscores the importance of understanding humic substances and their behavior in different environments. By recognizing the factors that influence the formation and decomposition of these compounds, we can develop more sustainable soil management practices that protect soil health and prevent subsidence. The EA/HUM ratio, in particular, offers a promising avenue for classifying Organosols and tailoring management strategies to specific soil types.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1590/s1806-66902013000200003, Alternate LINK

Title: Substâncias Húmicas E Suas Relações Com O Grau De Subsidência Em Organossolos De Diferentes Ambientes De Formação No Brasil

Subject: Horticulture

Journal: Revista Ciência Agronômica

Publisher: FapUNIFESP (SciELO)

Authors: Adierson Gilvani Ebeling, Lúcia Helena Cunha Dos Anjos, Marcos Gervasio Pereira, Gustavo Souza Valladares, Daniel Vidal Pérez

Published: 2013-06-01

Everything You Need To Know

1

What are humic substances, and why are they important for soil?

Humic substances are complex organic compounds formed through the decomposition of plant and animal matter. They are categorized into three main fractions based on their solubility: humins, which are insoluble in both acid and alkali; humic acids, which are soluble in alkali but not in acid; and fulvic acids, which are soluble in both acid and alkali. These substances are vital for soil fertility and structure.

2

What are Organosols, and why are they particularly prone to subsidence?

Organosols, characterized by a high organic carbon content exceeding 80 g kg-1, are particularly vulnerable to subsidence. Subsidence occurs when the organic matter in the soil decomposes, often accelerated by drainage, causing the soil surface to sink. Managing the composition and behavior of humic substances in Organosols is crucial for sustainable soil management and preventing environmental degradation.

3

How does the composition of humic substances vary in different Brazilian environments, and what are the implications?

The Brazilian study found that in mountainous and plateau regions, humic acid was the dominant carbon fraction due to cooler climates and higher fertility, making these soils more susceptible to subsidence when used for agriculture. Conversely, in floodplain and coastal plain environments, humin was the dominant carbon fraction, providing greater resistance to decomposition and helping preserve soil structure.

4

What is the EA/HUM ratio, and how can it be used in soil management?

The ratio of alkaline extract to humin (EA/HUM) serves as a diagnostic tool to classify Organosols based on their formation environment. This ratio helps differentiate soil types and tailor management strategies to specific soil characteristics, supporting sustainable soil practices. By understanding this ratio, targeted approaches to soil management can be implemented to maintain soil health and prevent subsidence.

5

Based on the Brazilian study, what sustainable soil management practices can be implemented to protect soil health and prevent subsidence?

Sustainable soil management practices involve understanding and managing humic substances to protect soil health and prevent subsidence. This includes recognizing the factors influencing their formation and decomposition in different environments. Utilizing the EA/HUM ratio to classify Organosols allows for tailored management strategies. Additional strategies involve optimizing drainage to minimize organic matter decomposition and promoting agricultural practices that enhance soil organic matter content, ensuring long-term soil fertility and stability.

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