In agricultural production and ecological research, the importance of soil health and fertility has become increasingly evident. Soil organic matter, humus, and humic acid are fundamental components in enhancing soil quality and supporting crop growth. This article provides a comprehensive introduction to soil organic matter, humus, humic acid, and their derivatives, exploring their formation, functions, and applications, as well as their value and utility in modern agriculture.
I. Soil Organic Matter: The Foundation of Fertile Soil
1. What is Soil Organic Matter?
Soil organic matter (SOM) encompasses all organic components in the soil, including plant and animal residues, animal excreta, and microbial metabolites. These organic materials continuously decompose within the soil, releasing essential nutrients for plant growth. Additionally, SOM enhances the soil’s physical structure and promotes microbial activity within the soil ecosystem.
2. Sources and Importance of Soil Organic Matter
Soil organic matter is derived from various sources, primarily plant residues (leaves, roots, branches), animal waste, and microbial byproducts. The organic matter content varies significantly across ecosystems.
For example, forest soils typically contain high levels of organic matter due to the accumulation of leaves and woody debris, while agricultural soils tend to have lower organic matter levels. However, practices such as returning crop residues to the soil and applying organic fertilizers can effectively increase SOM levels in farmland.
The presence of organic matter is crucial for enhancing soil fertility and is key to maintaining soil health and crop productivity.
3. Composition and Decomposition Process of Organic Matter
The composition of soil organic matter is complex, consisting of carbohydrates, proteins, lipids, and other compounds. Through microbial activity, this organic matter gradually breaks down into smaller molecules, such as amino acids and sugars. Eventually, these smaller molecules undergo polymerization to form more stable, high-molecular-weight compounds known as humus.
4. Functions of Soil Organic Matter
Soil organic matter serves as a vital source of nutrients for plant growth and plays a significant role in improving soil structure, water retention, and nutrient-holding capacity. The SOM content directly impacts soil fertility, making it a crucial metric for assessing soil quality.
II. Humus: Stable and Efficient Soil Organic Matter
1. Definition and Origin of Humus
Humus is a stable form of organic matter in the soil, formed through the aggregation of decomposed organic matter. It consists of large organic macromolecules with high stability, allowing it to persist in the soil for decades or even centuries. Humus also serves as a significant reservoir of soil organic carbon.
2. Formation of Humus
The process of humus formation, known as humification, involves the gradual restructuring and polymerization of plant and animal residues and microbial metabolites within the soil environment. This transformation results in humus, which includes various humic substances, such as humic acid, fulvic acid, ulmic acid, and humin.
3. Classification of Humus
Humus can be divided into different components based on its structure and chemical properties, including humin, humic acid, fulvic acid, and ulmic acid. Here are the main distinctions among them:
Component | Solubility | Molecular Weight | Key Functions |
Humin | Insoluble in both acidic and alkaline conditions | High | Provides long-term stability in soil organic matter, enhances soil structure and water retention over time |
Humic Acid | Soluble in alkaline solutions, insoluble in water and acidic conditions | Relatively high | Improves soil fertility, enhances nutrient retention, promotes water retention and soil aggregation |
Fulvic Acid | Soluble in water across all pH ranges (acidic, neutral, alkaline) | Low | Increases nutrient absorption efficiency (especially trace elements), offers immediate benefits to plant growth and microbial activity |
Ulmic Acid | Partially soluble in water | Moderate | Enhances soil structure, improves nutrient availability, supports soil microbial life with balanced effects |
4. Functions of Humus
With its stable structure, humus acts as a long-term, slow-release nutrient source for the soil. It improves soil water retention and promotes microbial activity, which is crucial for sustainable increases in soil fertility. Humus forms the foundation of healthy and productive soils.
III. Humic Substances: The Active Core of Humus
Humic substances are the primary active components within Humic Acid, Fulvic Acid and Humates. These compounds exhibit significant bioactivity and play a critical role in enhancing soil structure and facilitating nutrient uptake, making them a staple in modern agricultural practices.
1. Humic Acid
Definition: Humic acid is a high molecular weight organic acid within humus, soluble in alkaline environments but insoluble in acidic conditions.
Functions: Humic acid improves both the physical and chemical properties of soil, aiding plants in nutrient uptake while promoting growth and regulating soil pH.
Sources: Abundant in soils, peat, and especially in oxidized lignite, which can contain up to 70% humic acid, making it a premium source.
2. Fulvic Acid
Definition: Fulvic acid is a low molecular weight humic substance with a lighter color, soluble in both acidic and alkaline environments.
Functions: Known for its high bioactivity, fulvic acid is readily absorbed by plants, enhancing nutrient uptake and stress resistance.
Applications: Commonly used to supplement plant nutrition in foliar sprays and irrigation systems, fulvic acid is especially effective under drought or nutrient-poor conditions to stimulate growth.
3. Humates
Definition: Humates are salts of humic acid, formed by bonding with potassium, sodium, or ammonium ions, making humic acid more accessible for agricultural use.
Functions: Humates have excellent water solubility, which facilitates plant absorption and enables the rapid release of humic acid’s active components.
Applications: As soil conditioners and fertilizer additives, humates improve nutrient uptake efficiency and enhance soil structure, making them widely used in liquid fertilizers and soil amendments.
The effects of humic substances on soil are primarily reflected in the following aspects:
Improving Soil Structure
Humin promote the aggregation of soil particles, forming a stable soil aggregate structure. This structure enhances soil aeration, water infiltration, and water retention, making the soil both breathable and capable of holding adequate moisture, which supports healthy root growth in plants.
Enhancing Soil Water Retention
Humic acid has strong water absorption properties, which can increase the soil’s water-holding capacity. This is particularly beneficial in sandy soils or under drought conditions, as it effectively reduces water loss. This water-retention characteristic is crucial for improving crop drought resistance.
Increasing Soil Fertility
Humic acid can bind nutrients in the soil through chelation, reducing the loss of nitrogen, phosphorus, potassium, and other nutrients, thus improving fertilizer efficiency. They can also bind with trace elements (such as iron, zinc, copper), enhancing plant absorption and ultimately boosting the overall fertility of the soil.
Regulating Soil pH
Humic acid has a buffering capacity, which helps regulate soil pH towards neutrality. This is particularly important for acidic or alkaline soils, creating a more suitable growth environment for plants.
Promoting Soil Microbial Activity
Humic acid serves as a nutrient source for soil microbes, helping to promote the proliferation of beneficial microorganisms in the soil. An active microbial community can decompose organic matter, further releasing nutrients and improving the health and stability of the soil ecosystem.
Enhancing Soil Resilience
Humic acid enhances the soil’s resistance to external environmental stress, helping it remain relatively stable under adverse conditions such as drought, salinity, and heavy metal pollution. By chelating heavy metal ions and reducing the biological toxicity of harmful substances, humic acid protects plant roots from damage.
Boosting Plant Disease Resistance
Humic acid can indirectly increase crop disease resistance by promoting root growth and enhancing plant vitality. A healthy soil and root environment helps plants better resist pests, diseases, and environmental stresses.
As a natural soil conditioner, humic acid substances significantly improve soil structure, water retention, fertility, and ecological balance, providing strong support for sustainable agriculture.
IV. Leonardite and Lignite: Key Sources of Humic Acid
Leonardite and lignite are two types of coal with high humic acid content, making them primary sources for humic acid extraction. Due to their rich humic acid composition, they are valuable in agricultural applications, yet differ in formation, distribution, and humic acid levels. Below is a detailed overview:
1. Lignite
Definition: Lignite is a low-grade coal that falls between peat and bituminous coal in its composition. It has a high content of organic matter and moisture, with low carbonization. Due to its relatively short burial time and low humification, lignite retains a substantial amount of plant organic components, giving it a moderately high humic acid content.
Source and Formation: Lignite originates from the accumulation of ancient plants, such as ferns and mosses, in moist environments. Over time, these plant remains decomposed under microbial activity, eventually forming lignite. This process took place primarily during the Tertiary (65 to 2.5 million years ago) and Jurassic (200 to 145 million years ago) geological periods, when large vegetation covered the earth. As these plants decomposed, they were gradually buried by sediment layers, transforming under specific temperature and pressure conditions.
Distribution: Lignite is widely distributed globally, with significant reserves in China, Germany, Russia, and the United States, particularly in Inner Mongolia, Xinjiang, and Shanxi provinces in China.
Humic Acid Content: The humic acid content in lignite typically ranges from 20% to 50%, varying with burial depth, degree of humification, and specific geological conditions. The humic acid extracted from lignite has good bioactivity and is suitable for producing humic acid-based fertilizers.
Applications: Humic acid derived from lignite is commonly used in the production of potassium and sodium humates, soil conditioners, and fertilizers. Due to its abundant reserves and relatively low extraction cost, lignite serves as a primary raw material for humic acid products in agricultural production.
2. Leonardite
Definition: Leonardite refers to low-rank coal, such as lignite, that has undergone natural weathering. During the weathering process, surface coal is exposed over long periods to air, water, and microbial action, which results in oxidation and degradation, leading to higher and more stable humic acid content.
Source and Formation: Leonardite is formed when low-grade coal, including lignite and sub-bituminous coal, is exposed on the earth’s surface over extended periods. Under the influence of weathering agents such as oxygen, water, and microbial activity, organic matter undergoes further decomposition and humification, significantly increasing the humic acid content and bioavailability in the coal layer.
Distribution: Leonardite is primarily found in open-pit mines or shallow coal deposits, with major reserves in Inner Mongolia and Xinjiang in China. The thick surface layers of coal in these regions undergo exposure to environmental conditions, resulting in high-humic-acid-content Leonardite.
Humic Acid Content: Leonardite may contain up to 70% humic acid, significantly higher than that of untreated lignite. This higher humic acid concentration and increased bioactivity make Leonardite an excellent source for humic acid extraction.
Applications: Leonardite’s high humic acid content makes it ideal for producing concentrated humic acid products, including potassium and sodium humates, as well as other humic acid fertilizers. Leonardite-based humic acid extracts are widely used in agriculture to improve soil structure, enhance crop resilience, and increase nutrient retention, making it a valuable resource in modern agricultural practices.
Feature | Lignite | Leonardite |
Definition | Low-grade coal with high organic content and low carbonization | Weathered low-rank coal with higher humic acid content |
Formation | Ancient plant deposits transformed through microbial action | Prolonged surface exposure leads to further decomposition |
Distribution | Globally distributed, mainly in China, Germany, Russia, and the U.S. | Found in open-pit mines, mainly in Inner Mongolia and Xinjiang |
Humic Acid Content | 20%-50% | Up to 70%, with higher bioactivity |
Applications | Used for basic humic acid fertilizers, granular fertilizers, etc., ideal for general soil improvement and base fertilization | Best for concentrated humic acid products, effective in soil conditioners, foliar sprays, and liquid fertilizers |
V. Potassium Humate, Sodium Humate, and Mineral Potassium Fulvic Acid: Efficient Applications of Humic Derivatives
With the growing demand for soil enhancement, potassium and sodium salts of humic substances have been widely adopted in agriculture. These products leverage the natural activity of humic substances while supplying essential elements like potassium and sodium to support plant growth.
1. Potassium Humate
Definition: Potassium humate is the potassium salt of humic acid.
Characteristics: Potassium humate provides an essential potassium source for plants, enhances soil fertility, and improves soil structure. It also boosts crop drought tolerance and disease resistance.
Applications: Suitable for foliar spraying, drip irrigation, and base fertilization. It is especially beneficial for potassium-demanding crops, such as fruit trees and root vegetables.
2. Sodium Humate
Definition: Sodium humate is the sodium salt of humic acid.
Characteristics: Sodium humate improves soil structure, particularly effective for saline-alkaline soils and sandy soil improvement. It helps reduce toxicity from heavy metals and other harmful ions.
Applications: Widely used for saline-alkaline soil remediation, aquaculture, and environmental pollution control, showing marked effects in areas with severe soil salinity.
3. Mineral Potassium Fulvic Acid (Potassium Fulvate)
Definition: Mineral potassium fulvic acid, also known as potassium fulvate, is derived from fulvic acid combined with potassium ions, providing high bioactivity.
Characteristics: With a smaller molecular weight, potassium fulvic acid is quickly absorbed by plants. It provides potassium while enhancing crop stress resistance, making it ideal for nutrient-poor soils.
Applications: Used in foliar sprays and drip irrigation systems, it is widely applied in fruit trees, vegetables, and high-value crops for nutrient supplementation, particularly beneficial in poor or challenging soil conditions.
VI. Additional Applications of Humic Substances
1. Environmental Applications
Humic substances are effective in adsorbing heavy metals and organic pollutants from water, making them valuable for water purification and pollution remediation. Due to its high adsorption capacity, sodium humate is particularly suited for wastewater treatment and soil remediation projects.
2. Aquaculture and Animal Husbandry
Humates serve as feed additives to improve gut health and promote growth in livestock. Sodium humate is also used in aquaculture to regulate water quality and reduce harmful substances in the water, enhancing fish health and growth.
3. Cosmetics and Personal Care Due to its mild antioxidant and anti-inflammatory properties, humic acid is utilized in skincare products within the cosmetics industry. It has soothing and restorative effects on the skin, supporting cell regeneration and reducing inflammation.
Summary
Soil Organic Matter refers to all organic components within the soil, including plant and animal residues and their decomposition products.
Humus is the stable portion of organic matter in the soil, containing active components such as humic and fulvic acids that play a vital role in sustaining long-term soil fertility.
Humic Substances are the active elements within humus, including humic acid and fulvic acid, which significantly enhance soil structure and plant nutrition.
Humic Derivatives (e.g., potassium humate, sodium humate, mineral potassium fulvic acid) are advanced soil conditioners developed from humic acids, combined with potassium and sodium ions to improve nutrient uptake and soil health.
Conclusion
Soil organic matter, humus, humic acid, and their derivatives are indispensable resources for maintaining soil health and agricultural productivity. The transformation of organic matter from plant and animal residues into humus and the extraction of humic substances reveal the intricate decomposition and synthesis processes within soils. This cycle reflects the essential nutrient relationship between plants and soil. Humic substances and their potassium and sodium derivatives play a critical role in modern agriculture, promoting a more sustainable, efficient, and environmentally-friendly approach to crop production.
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