Fact-checked by Grok 2 weeks ago

Humus

Humus is the stable, dark-colored organic fraction of soil formed by the microbial decomposition of plant and animal residues.^[1]^[2] This amorphous substance arises through humification, a process involving the breakdown of organic detritus by bacteria, fungi, and soil fauna, followed by chemical transformations that yield complex, recalcitrant polymers rich in carbon.^[3]^[4] Humus constitutes a significant portion of soil organic matter, typically comprising up to 85% in stabilized forms, and exhibits colloidal properties with charged surfaces that facilitate cation exchange and nutrient retention.^[5] In soil profiles, it predominantly accumulates in the surface horizons, contributing to aggregate stability and porosity.^[6] Its presence markedly enhances soil fertility by increasing water-holding capacity, promoting microbial activity, and buffering against nutrient leaching and erosion.^[7]^[5] Humus formation is influenced by climatic factors, vegetation type, and soil management practices, with higher levels observed in temperate forests compared to arid regions.^[8]

Definition and Historical Context

Traditional Definition and Properties

In classical soil science, humus is defined as the dark, amorphous organic material formed through the microbial decomposition of plant and animal residues in soil, representing a stable end-product that resists further breakdown. This fraction, often comprising 40-60% of soil organic matter in fertile soils, arises from the transformation of fresh organic inputs via humification processes, yielding a substance rich in carbon compounds.^[9] Key physical properties of humus include its colloidal nature, which enhances soil aggregation, tilth, and porosity, thereby improving water infiltration and retention capacities—humus can hold up to 6 times its weight in water. It contributes to soil's dark coloration and plasticity, with particle sizes typically in the clay range (<2 μm), fostering aeration and root penetration in agricultural contexts. Chemically, humus exhibits high cation exchange capacity (CEC), often exceeding 200 cmol/kg, due to its polyphenolic and carboxylic acid groups, enabling nutrient retention and buffering against pH extremes, typically ranging from 4-7 in humus-rich soils. Biologically, it supports microbial activity by providing energy substrates and habitats, though its stability limits rapid mineralization rates.^[10]^[5]^[9]

Historical Development of the Concept

The term humus, derived from Latin meaning "earth" or "soil," entered scientific discourse in the mid-18th century to denote the dark, organic component of topsoil formed by the decay of plant and animal residues.^[11] Swedish chemist Johan Gottschalk Wallerius employed the term in 1753 to describe the surface organic horizon, distinguishing it from mineral soil layers and recognizing its role in fertility through decomposition processes.^[12] This early conceptualization built on empirical observations of manuring practices dating back centuries, viewing decayed matter as essential for crop productivity without a formalized nutritional mechanism. Albrecht Daniel Thaer advanced these ideas into a coherent framework with his "humus theory" outlined in Principles of Rational Agriculture (1809–1812), asserting that plants derived their carbon and primary nutrients directly from humus, which he quantified as requiring 100 parts humus to yield 20–30 parts plant dry matter.^[13] ^[14] Thaer's model, grounded in field experiments and economic assessments, promoted organic recycling via manure and crop residues to replenish soil humus, influencing agricultural practices across Europe for roughly 30–50 years and emphasizing soil organic matter's centrality to fertility.^[15] ^[16] The theory's dominance waned in the 1840s as Justus von Liebig's experimental work refuted humus as the chief carbon source, demonstrating instead that plants assimilate CO₂ from air and absorb mineral elements (nitrogen, phosphorus, potassium, etc.) from soil solutions.^[17] ^[18] Liebig's mineral nutrition paradigm, detailed in publications like Organic Chemistry in Its Application to Agriculture and Physiology (1840), shifted focus to inorganic fertilizers and the "law of the minimum," where yield limits arise from the scarcest nutrient rather than organic bulk, though it acknowledged humus's indirect benefits for soil structure.^[13] This transition marginalized humus theory by the mid-19th century, fostering chemical agriculture while prompting later refinements incorporating microbial decomposition and stable organic fractions.^[17]

Chemical Composition

Components of Humic Substances

Humic substances, the primary organic constituents of humus, are operationally classified into three fractions based on solubility in aqueous solutions: humin, humic acids, and fulvic acids.^[19] This classification arises from standard extraction procedures involving alkali solubilization followed by acidification, reflecting differences in molecular size, polarity, and chemical functionality rather than distinct chemical identities.^[20] These fractions collectively represent heterogeneous assemblages of polydisperse macromolecules derived from the biochemical transformation of plant, animal, and microbial residues, comprising aromatic cores linked to aliphatic chains and bearing oxygen-containing functional groups such as carboxyl and phenolic hydroxyls.^[20] Humin constitutes the insoluble residue remaining after exhaustive extraction with dilute acid and alkali, accounting for 40-60% of total humic substances in most soils.^[19] It exhibits high molecular weights exceeding 1,000,000 Da and is characterized by strong associations with soil mineral surfaces, rendering it recalcitrant to biodegradation.^[20] Chemically, humin features a predominance of aromatic structures with lower contents of polar functional groups compared to soluble fractions, contributing to its dark coloration and stability in the soil matrix.^[20] Humic acids, precipitated from alkaline extracts upon acidification to pH 1-2, form the dark brown to black fraction soluble only at neutral to alkaline pH.^[19] Their molecular weights range from 2,000 to over 1,000,000 Da, with typical values of 50,000-100,000 Da, and elemental compositions averaging 50-60% carbon, 3.5-4.8% hydrogen, 0.7-5.1% nitrogen, and 31.6-45.5% oxygen.^[20] These substances are enriched in aromatic carbon skeletons, including quinone and phenolic moieties, alongside carboxyl groups that confer ion-exchange capacity and metal chelation properties essential for nutrient retention in soils.^[20] Fulvic acids, the lightest fraction, remain soluble across all pH levels and are typically isolated via resin adsorption from acidified alkaline extracts.^[19] With molecular weights of 500-10,000 Da, they display higher oxygen content (up to 50%) and a greater proportion of aliphatic chains derived from polysaccharides and low-molecular-weight precursors, alongside abundant carboxyl and hydroxyl functionalities.^[20] This composition enhances their mobility in soil solutions and reactivity with cations, influencing trace element transport and microbial activity.^[20]

Molecular Structure and Analysis Methods

Humic substances, the primary components of humus, exhibit a complex, heterogeneous molecular architecture rather than a uniform polymeric structure. Traditionally conceptualized as rigid, high-molecular-weight macromolecules formed through condensation reactions of precursor biomolecules, contemporary evidence supports a supramolecular model wherein humic substances comprise dynamic associations of diverse, low-molecular-mass (<1000 Da) organic molecules, including plant- and microbial-derived fragments, stabilized by non-covalent interactions such as hydrophobic forces (van der Waals, π–π, CH–π), hydrogen bonding, and charge-transfer complexes.^[21] ^[22] This shift in understanding, articulated in analyses from 2005 onward, arises from advanced spectroscopic data revealing flexible, adaptive assemblies rather than covalent macromolecules, with aromatic cores (e.g., benzene-like rings from lignin degradation) linked to aliphatic chains, carboxyl (-COOH), phenolic (-OH), and carbonyl groups, alongside nitrogen- and sulfur-containing functionalities.^[23] Elemental compositions typically range from 45-60% carbon, 3-6% hydrogen, 30-45% oxygen, 1-5% nitrogen, and trace sulfur, varying by source material and environmental conditions.^[24] Operational fractionation distinguishes humic acids (HA; alkali-soluble, acid-insoluble, higher molecular weight ~2000-5000 Da), fulvic acids (FA; soluble in both alkali and acid, lower weight ~500-2000 Da), and humin (insoluble residue bound to minerals).^[24] HA and FA feature greater aromaticity and oxygen content in FA due to its mobility and oxidation exposure, while humin integrates mineral-associated organics with similar but more recalcitrant structures. Structural heterogeneity precludes a singular formula; instead, models depict polyphenolic networks with quinone-like moieties facilitating redox activity.^[25] Analysis of humic molecular structure relies on non-destructive spectroscopic techniques to probe functional groups and carbon skeletons without altering native associations. Fourier-transform infrared (FTIR) spectroscopy identifies vibrational modes of key moieties, such as O-H stretches (3400 cm⁻¹) for hydroxyls, C=O (1700 cm⁻¹) for carboxyls/ketones, and aromatic C=C (1600 cm⁻¹), enabling quantification of aromatic vs. aliphatic content when calibrated quantitatively.^[26] ^[27] Solid-state ¹³C nuclear magnetic resonance (NMR), particularly cross-polarization magic-angle spinning (CP/MAS), delineates carbon types: alkyl (0-50 ppm, 20-40%), O-alkyl (50-110 ppm, polysaccharides), aromatic (110-160 ppm, 20-30%), and carboxyl/carbonyl (160-220 ppm, 10-20%), revealing supramolecular dynamics through signal broadening and comparison to solution-state spectra.^[28] ^[29] Complementary methods include UV-Vis spectroscopy for aromaticity via E4/E6 ratios (absorbance at 465 nm / 665 nm, lower values indicating higher condensation), fluorescence for molecular associations, and two-dimensional NMR variants like ¹³C-¹⁵N heteronuclear correlation for N-proximate carbons in enriched samples.^[21]^[30] Elemental analysis (CHNS) and pyrolysis-gas chromatography-mass spectrometry provide bulk composition and volatile fragment profiles, respectively, though destructive methods like alkaline extraction risk artifactual alterations. These techniques, often integrated, confirm the adaptive, non-polymeric nature of humic structures across soil types.^[31]^[27]

Formation and Processes

Precursors and Inputs

The primary precursors to humus formation consist of organic residues derived from plant materials, including aboveground litter such as leaves, stems, and floral structures, as well as belowground inputs from root turnover and exudates.^[32]^[33] These plant-derived compounds, particularly lignocellulosic materials rich in lignin, cellulose, and hemicellulose, serve as the dominant carbon sources entering the humification process, with lignin historically viewed as a selective preservative due to its resistance to rapid decomposition.^[34] Root inputs contribute disproportionately to stable humus pools compared to surface litter, as they interact more closely with soil minerals and microbial communities in the rhizosphere.^[35] Microbial biomass and necromass represent critical secondary precursors, where dead microbial cells and their metabolites—such as polysaccharides and low-molecular-weight dissolved organic matter (DOM)—undergo transformation into humic substances during decomposition.^[36]^[37] Fungi and bacteria initially break down complex plant polymers into reactive monomers, which then polymerize abiotically or biotically to form humus; microbial-derived inputs can account for up to 50-80% of stabilized soil organic matter in some ecosystems.^[38]^[39] Animal-derived inputs, including manure and carcass residues, augment humus precursors by providing readily decomposable organic matter like proteins and lipids, which enhance microbial activity and subsequent humification rates.^[40]^[41] In agricultural systems, external amendments such as compost or manure can increase humus formation by 20-30% through elevated carbon inputs, though their efficacy depends on soil conditions and application rates.^[42] Overall, the balance of these inputs determines humus accumulation, with ecosystems featuring high root biomass and microbial diversity exhibiting greater persistence of precursors in mineral-associated forms.^[43]

Humification Mechanisms

Humification encompasses the biochemical and physicochemical transformations of organic residues into stable humic substances, primarily through microbial degradation followed by secondary synthesis reactions. This process begins with the partial decomposition of lignocellulosic materials, such as plant litter, where heterotrophic microorganisms break down easily degradable components like carbohydrates and proteins, leaving behind more recalcitrant fractions rich in lignin and polyphenols.^[44] The resulting monomers, including phenolic compounds and amino acids, then undergo oxidation and condensation to form complex associations.^[38] Microbial activity drives the initial stages via extracellular enzymes such as lignin peroxidases, laccases, and hydrolases, which catalyze the hydrolysis and oxidation of polymers into reactive intermediates. Fungi, particularly white-rot species, excel in lignin depolymerization, producing quinone-like structures that serve as precursors, while bacteria contribute to nitrogen incorporation through ammonification.^[45] These enzymatic processes generate functional groups like carboxyl and hydroxyl, facilitating subsequent bonding, with studies showing enhanced humification when microbial consortia increase amide and aromatic content by up to 9.46%.^[45] Anaerobic conditions can slow decomposition but favor humic acid formation through reduced quinone polymerization.^[44] Chemically, humification involves non-biological reactions like the Maillard condensation between reducing sugars and amino compounds, yielding melanoidin-like structures, alongside phenol-protein interactions where oxidized polyphenols bind nitrogenous substances.^[38] Traditional theories emphasize covalent polymerization: the lignin theory posits humus derivation from degraded lignin cores, while the polyphenol theory highlights quinone polymerization with or without amino acids.^[38] These yield dark, amorphous materials with high aromaticity, as evidenced by FTIR spectra showing increased C=O and O-H peaks during synthesis.^[45] Contemporary models challenge polymeric views, proposing humic substances as supramolecular aggregates of small molecules (<700 Da) stabilized by hydrophobic interactions, hydrogen bonding, and mineral sorption rather than inherent recalcitrance.^[46] Lehmann and Kleber (2015) argue that alkali-extracted "humic substances" artifactually represent soil organic matter continua, not distinct polymers, supported by NMR and mass spectrometry revealing dynamic, plant-derived fragments over stable humics.^[46] This shift underscores microbial priming and environmental interactions as key to persistence, rather than chemical uniqueness.^[44]

Stability and Dynamics

Factors Affecting Persistence

The persistence of humus, the stable fraction of soil organic matter, is governed by interacting mechanisms of stabilization and destabilization, primarily involving protection from microbial attack and inherent molecular resistance. Stabilization occurs through chemical recalcitrance, where aromatic and polyphenolic structures resist enzymatic breakdown; physical inaccessibility within microaggregates formed by fungal hyphae and polysaccharides; and organo-mineral interactions, such as sorption to clay minerals, iron oxides, or aluminum complexes that encapsulate humic substances.^[47] These processes can extend humus residence times to centuries, contrasting with labile organic inputs that decompose rapidly. Destabilization arises from depolymerization of recalcitrant compounds, desorption from minerals under changing pH or ionic strength, and aggregate disruption, which exposes humus to decomposers.^[47] Environmental factors exert strong control over humus dynamics, with temperature and moisture modulating microbial decomposition rates. Elevated temperatures accelerate enzymatic activity, reducing persistence by 20-50% per 10°C rise in many soils, while excessive moisture promotes anaerobic conditions that slow oxidation but can enhance certain fungal degradation.^[47] Soil pH influences mineral interactions; acidic conditions (pH <5.5) favor aluminum-humic complexation for greater stability, whereas neutral to alkaline pH may promote desorption. Topography and elevation proxy climatic effects, as higher altitudes correlate with thicker humus layers and increased organic matter stocks due to cooler temperatures limiting decomposition, observed in Mediterranean Scots pine stands where humus buildup rises from 1100 to 1600 m.a.s.l.^[48] Soil properties, including texture and mineralogy, mediate persistence via protective associations. Clay-rich soils enhance humus stability through adsorption and aggregate formation, with fine-textured fractions retaining up to twice the carbon of sandy soils under comparable inputs. Iron and aluminum oxides in variable-charge minerals like allophane further bind humic acids, as seen in Andisols where such complexes resist microbial access. Calcium content inversely affects buildup by stimulating biological activity and litter decomposition.^[47]^[48] Biotic influences, particularly microbial communities, determine carbon use efficiency and necromass incorporation into humus. Fungi-dominated assemblages produce stable aggregates and contribute recalcitrant residues, enhancing persistence compared to bacteria, which favor labile substrates. Plant litter quality, such as high lignin-to-nitrogen ratios, yields more persistent humic precursors, while soil fauna like earthworms can accelerate turnover by fragmenting aggregates. Management practices, including conservation tillage, preserve organo-mineral associations and aggregates, increasing humus stability by 10-30% over conventional plowing in long-term studies.^[47]

Decomposition and Turnover Rates

Decomposition of humus proceeds slowly through microbial oxidation of its polyphenolic and aromatic structures, primarily by white-rot fungi and bacteria specialized in lignin-like degradation, resulting in mineralization to CO₂ or further transformation into simpler compounds. Unlike labile organic matter, humus resists rapid breakdown due to its heterogeneous, cross-linked nature, with annual decomposition rates typically below 0.5-1% in temperate soils.^[32] Turnover rates for humus are expressed as mean residence time (MRT), the average period before carbon is lost via decomposition, often estimated via radiocarbon or stable isotope analysis. The stable humus pool exhibits MRTs ranging from several centuries to millennia, far exceeding the global average of 32 years for total soil organic carbon, as humus constitutes the passive fraction protected against enzymatic attack. For example, mineral-associated humic substances in mature soils show MRTs of 1,000 years or more, reflecting long-term sequestration.^[49] ^[32] ^[50] Key factors modulating humus turnover include climatic variables, with elevated temperatures increasing decomposition kinetics by 2-3 times per 10°C rise under optimal moisture, while excessive dryness or waterlogging suppresses microbial activity. Soil mineralogy plays a critical role, as higher clay contents (e.g., >30%) extend MRTs by sorbing humic molecules onto surfaces and incorporating them into aggregates, reducing accessibility to decomposers. Vegetation type and land use further influence rates; grasslands maintain longer MRTs (up to 29 years for topsoil organic carbon, longer for humus) compared to croplands due to persistent root inputs favoring stable forms, whereas disturbances like tillage accelerate turnover by exposing protected humus.^[49] ^[51] ^[52]

Occurrence in Soils

Distribution Across Soil Horizons

Humus, the stable organic fraction of soil organic matter, exhibits a pronounced vertical gradient, with concentrations highest in the uppermost horizons and declining exponentially with depth. In the O horizon, composed largely of partially decomposed plant residues and microbial biomass, humus can constitute over 20-30% of the layer by dry weight, serving as the primary reservoir of fresh organic inputs.^[53] This layer's thickness and humus content vary by vegetation type and climate, ranging from thin litter mats in arid regions to thick mor humus in boreal forests.^[54] The A horizon, or topsoil, integrates humus with mineral particles through bioturbation and root activity, typically holding 1-6% organic carbon, which correlates closely with humus stability.^[55] Here, humus enhances soil structure via aggregation, but its abundance decreases subsurface within this horizon due to oxidative decomposition and leaching. Studies of soil profiles consistently show soil organic carbon—a key component of humus—peaking in the 0-20 cm layer and dropping by 50-90% by 50 cm depth across diverse ecosystems.^[56] Deeper into the B horizon, humus levels fall below 1%, limited by reduced microbial activity, lower oxygen availability, and minimal organic inputs beyond root exudates or dissolved organic matter translocation.^[57] The C horizon, representing weathered parent material, contains trace humus (<0.5%), primarily as relic molecules adsorbed to minerals rather than active humic substances. This distribution reflects first-order controls like litter quality, microbial processing rates, and physicochemical stabilization, with empirical profiles from global datasets confirming near-surface dominance of humus stocks—often 70-90% of total profile organic matter confined to the top 30 cm. Variations occur in soils with histic epipedons or deep-rooted systems, but the general pattern underscores humus as a surface-enriched feature.^[58]

Interactions with Soil Minerals and Environment

Humic substances form stable organo-mineral complexes through adsorption onto clay minerals such as montmorillonite, kaolinite, and illite, primarily via ligand exchange, cation bridging involving divalent cations like Ca²⁺ and Fe²⁺, and hydrophobic interactions, which enhance the persistence of soil organic carbon by protecting it from microbial decomposition.^[59]^[60] These associations occur preferentially on mineral surfaces with high specific surface area, where humic acids exhibit adsorption maxima influenced by solution pH; for instance, adsorption increases at lower pH due to protonation of functional groups, facilitating stronger electrostatic bonds.^[61]^[62] Clay-humus complexes contribute to soil aggregation by bridging mineral particles with organic coatings, improving structural stability and porosity, with humic substances promoting aggregation more effectively than clays alone owing to their amphiphilic nature that facilitates flocculation.^[63] In iron oxide-rich soils, humic matter interacts via surface complexation and precipitation, sorbing heavy metals and nutrients, which elevates cation exchange capacity (CEC) beyond that of minerals alone; studies show humus-clay systems yield higher base saturation and cation activities compared to pure clays.^[64]^[65] These interactions also modulate mineral weathering: humic acids chelate Al³⁺ and Fe³⁺, potentially inhibiting clay dissolution by organic acids while accelerating silicate breakdown in acidic conditions through proton promotion and metal mobilization.^[66] Environmental factors like pH, ionic strength, and redox potential govern these dynamics; at alkaline pH (>7), humic insolubility rises via organo-metal bridging, reducing mobility, whereas reducing conditions enhance Fe-humic complexation, altering phosphorus sorption.^[67]^[68] Microbial activity influences interactions indirectly by producing extracellular polymeric substances that compete for mineral surfaces, though abiotic controls dominate in mineral-rich horizons.^[69] In polluted soils, humus-mineral sorption reduces bioavailability of contaminants like mercury, with clays coated by humus showing lower Hg release under varying conditions.^[70] Overall, these interactions underpin soil's carbon sequestration capacity, with mineral-associated humus comprising up to 70-80% of stabilized organic matter in temperate soils.^[71]

Functions and Roles

Contributions to Soil Fertility

Humus significantly enhances soil fertility by acting as a stable reservoir for essential plant nutrients, releasing them gradually through mineralization processes that sustain availability over extended periods. Unlike readily soluble fertilizers, humus-bound nutrients, including nitrogen, phosphorus, and micronutrients, decompose slowly due to its recalcitrant chemical structure, reducing losses from leaching and volatilization while supporting consistent crop uptake.^[72] This slow-release mechanism is particularly valuable in maintaining long-term productivity, as evidenced by field studies showing sustained yield improvements in humus-amended soils compared to inorganic inputs alone.^[73] A primary mechanism of humus's fertility contribution is its exceptionally high cation exchange capacity (CEC), which enables the soil to retain positively charged ions such as calcium, magnesium, potassium, and ammonium against downward migration in rainfall or irrigation. Humus colloids exhibit CEC values typically ranging from 150 to 500 cmol/kg, two to five times higher than montmorillonite clay and up to 30 times greater than kaolinite, allowing it to bind and exchange nutrients efficiently at soil pH levels common in agricultural settings.^[74] This property not only buffers soil acidity but also facilitates nutrient availability to plant roots, with research demonstrating that soils enriched with humus maintain higher base saturation and reduce fertilizer requirements by 20-30% in nutrient-poor profiles.^[75] Humus further bolsters fertility by fostering microbial activity, which drives organic matter decomposition and nutrient cycling. Its polyphenolic and carboxylic compounds provide carbon substrates that support diverse bacterial and fungal populations, increasing enzyme activities like phosphatase and urease that liberate bound phosphorus and nitrogen.^[76] Studies indicate that humus incorporation elevates soil microbial biomass by up to 50% and correlates with enhanced nitrogen fixation and organic phosphorus solubilization, indirectly amplifying plant nutrient acquisition.^[5] Direct biostimulatory effects of humus on plants include promotion of root elongation, lateral branching, and overall biomass accumulation, attributed to hormone-like auxins and cytokinins within humic substances. Experimental applications of humic extracts have shown 10-25% increases in shoot and root dry weights across crops like maize and wheat, alongside improved nutrient uptake efficiency under stress conditions such as drought or salinity.^[77] These physiological responses underscore humus's role in elevating inherent soil productivity beyond mere nutrient holding, with meta-analyses confirming yield gains of 5-15% in low-fertility soils amended with stable organic matter fractions akin to humus.^[78]

Effects on Water Retention and Nutrient Cycling

Humus enhances soil water retention through its colloidal structure, which promotes aggregation and increases porosity, allowing soils to store more water against gravity. Organic matter derived from humus can hold three to five times its weight in water, significantly boosting available water content and mitigating drought impacts in arid conditions.^[79] Empirical studies confirm that elevating soil organic carbon—closely tied to humus levels—improves water balance by raising field capacity while reducing evaporation losses, with effects varying by depth and climate.^[80] For example, incorporating humus-rich compost into sandy soils has been shown to increase water-holding capacity by up to 2.5 times compared to unamended controls.^[81] In nutrient cycling, humus acts as a stable matrix that buffers nutrient availability, binding elements like nitrogen, phosphorus, and sulfur in organic forms for gradual mineralization by decomposer microbes. This process sustains long-term fertility by preventing rapid leaching and synchronizing nutrient release with plant uptake demands.^[82] Humus's high cation exchange capacity (CEC), typically 250–400 meq/100 g due to abundant negatively charged sites on humic molecules, excels at retaining essential cations (e.g., K⁺, Ca²⁺, Mg²⁺) far beyond that of mineral clays, which often range below 100 meq/100 g.^[74] ^[83] This retention minimizes losses during heavy rainfall, while associated microbial activity accelerates transformations such as nitrification and phosphorus solubilization, optimizing cycling efficiency.^[84] The interplay between water retention and nutrient dynamics is amplified by humus, as retained moisture supports microbial proliferation essential for decomposition and nutrient mobilization. In humus-enriched soils, this synergy reduces fertilizer needs by 20–30% in some cropping systems, as documented in field trials, though outcomes depend on initial soil pH and texture.^[85] Disruptions like tillage can diminish these benefits by accelerating humus turnover, underscoring the need for conservation practices to maintain cycling integrity.^[32]

Controversies and Scientific Debates

Challenges to the Existence of Humus

In the early 21st century, soil scientists increasingly questioned the traditional notion of humus as a distinct, stable, amorphous class of organic polymers formed through humification processes in soil.^[86] Critics argued that humic substances, operationally defined by alkali extraction methods pioneered by Achard in 1786, represent artifacts generated under laboratory conditions rather than naturally occurring entities in soil.^[87] Specifically, extraction at high pH (around 13) solubilizes and alters organic compounds indiscriminately, producing melanoidin-like structures from non-humic precursors, with no comparable humic material identifiable in untreated soil via advanced techniques such as solid-state NMR or Fourier-transform ion cyclotron resonance mass spectrometry.^[87] Empirical evidence from molecular-level analyses further undermined the existence of humus as a chemically recalcitrant pool. A 2011 study by Schmidt et al., published in Nature, analyzed global datasets and isotopic labeling experiments, finding that soil organic matter (SOM) persistence correlates with ecosystem properties like mineral content and aggregation rather than inherent molecular stability of humic forms.^[86] For instance, adsorption to clay minerals or occlusion within microaggregates (typically <250 μm) protects fresh plant litter and microbial necromass from decomposition, explaining long residence times (up to millennia in some subsoils) without invoking humic polymerization.^[86] Lehmann and Kleber reinforced this in a 2015 Nature Geoscience review, asserting that available spectroscopic and pyrolytic data show no support for the secondary synthesis of large-molecular-weight humic substances (>1,000 Da) in natural soils; instead, SOM comprises a continuum of identifiable biomolecules from selective preservation of inputs. They cited experiments where supposed humic structures disassembled upon mild dissolution, indicating supramolecular associations rather than covalent polymers. These critiques extended to functional claims, positing that attributes ascribed to humus—such as cation exchange capacity or water retention—stem from mineral-organic associations or microbial byproducts, not a unique humic phase.^[86] Kleber and Lehmann's 2019 analysis in the Journal of Environmental Quality reviewed over 50 studies, concluding that alkali-extracted "humics" fail as proxies for SOM dynamics, as they overestimate stability and ignore spatial heterogeneity in soils where 70-80% of carbon resides in mineral-bound fractions averaging 10-50 nm in size.^[87] Proponents of abandoning the term "humus" argued it perpetuates outdated paradigms, hindering precise modeling of carbon turnover; for example, Schmidt et al. estimated that misconceptions of recalcitrant humus inflate projected SOM responses to climate change by 20-50% in Earth system models.^[86] Despite these challenges, some researchers defended residual humic-like properties observable via milder extractions, though consensus leans toward redefining SOM without invoking humus as a foundational entity.^[88]

Alternative Models of Soil Organic Matter

The traditional concept of humus as a stable, chemically recalcitrant pool of amorphous polymers has faced scrutiny due to advances in analytical techniques revealing that soil organic matter (SOM) primarily consists of recognizable biomolecules, microbial residues, and plant fragments rather than discrete humic substances.^[89] Alternative models emphasize dynamic processes such as physical protection, microbial physiology, and mineral interactions over inherent molecular stability. These frameworks better align with empirical observations from spectroscopy and isotopic studies showing rapid turnover and site-specific stabilization mechanisms.^[90] One prominent alternative is the aggregate hierarchy model, which posits that SOM persistence arises from nested structures of soil aggregates that physically limit microbial and enzymatic access to organic substrates. Proposed by Tisdall and Oades in 1982, this model describes microaggregates (<250 μm) forming within larger macroaggregates through binding agents like roots, fungal hyphae, and transient microbial products, creating compartments where carbon is occluded for decades to centuries.^[91] Evidence from aggregate disruption experiments demonstrates that dispersing these structures accelerates decomposition rates by up to 10-fold, underscoring physical inaccessibility as a primary stabilizer rather than chemical resistance.^[92] This hierarchy extends to submicron scales, as confirmed in volcanic soils where nano-scale associations enhance long-term carbon storage.^[93] The soil continuum model (SCM) further challenges humus-centric views by framing SOM as a gradient of accessibility to decomposers, from fresh plant inputs to highly processed forms protected by adsorption or encapsulation, without invoking stable humic polymers. In this paradigm, carbon stabilization depends on spatial and temporal barriers to microbial processing, supported by data showing that 50-70% of SOM in mineral soils is plant-derived material shielded within aggregates or sorbed to clays.^[94] Critiques of traditional humus extraction methods, which produce alkali-soluble "humic substances" as artifacts rather than native entities, bolster the SCM, as in-situ analyses via NMR reveal diverse, identifiable compounds persisting due to protection rather than recalcitrance. Microbial efficiency models integrate physiological traits, positing that low carbon use efficiency (CUE) in soil microbes—typically 0.1-0.4—drives net carbon accumulation by partitioning more substrate into biomass and necromass, which then stabilizes via mineral sorption or aggregation. The Microbial Efficiency-Matrix Stabilization (MEMS) framework unifies this with matrix interactions, where microbial residues contribute 50-80% of persistent SOM through selective preservation and organo-mineral associations.^[96] Global modeling incorporating variable CUE explains observed soil carbon stocks better than fixed-pool humus models, with a 2023 study attributing 20-30% of variation in storage to microbial traits under warming scenarios.^[97] These alternatives collectively shift focus from static humus to process-based dynamics, informing more accurate predictions of carbon persistence amid environmental change.^[98]

Recent Research and Applications

Advances in Understanding SOM Dynamics (2020–2025)

Recent studies have refined the conceptualization of soil organic matter (SOM) dynamics by emphasizing the partitioning of SOM into particulate organic matter (POM), which cycles rapidly, and mineral-associated organic matter (MAOM), which persists longer due to physicochemical protection. A 2023 analysis of long-term field experiments demonstrated that common agricultural practices, such as manure application, preferentially increase POM stocks, while mineral fertilization enhances MAOM stabilization, altering overall SOM turnover rates by up to 20% in arable soils.^[99] Similarly, a 2024 global synthesis using 8,341 observations mapped turnover times, revealing MAOM persistence exceeding 1,000 years in tropical soils versus under 100 years in temperate regions, driven by mineral sorption and microbial processing.^[100] Microbial mechanisms have gained prominence in explaining SOM stabilization, with models incorporating microbial necromass as a dominant contributor to persistent carbon pools. A 2024 review highlighted how advancements in microbial-explicit models simulate SOM decomposition by accounting for enzyme kinetics and substrate quality, improving predictions of carbon persistence under varying climates by 15-30% compared to earlier pool-based approaches.^[101] Empirical work in 2021 showed that SOM turnover rates adapt to increased substrate inputs, maintaining steady-state carbon stocks despite elevated decomposition, challenging assumptions of linear priming effects.^[102] Climate-substrate interactions have been modeled to predict SOM responses to warming, with a 2023 framework indicating that moisture limitations, rather than temperature alone, constrain decomposition in drylands, potentially stabilizing 10-20% more carbon than previously estimated.^[103] A 2025 profile-scale model integrated vertical transport and protection factors, estimating that organo-mineral associations extend mean residence times by factors of 5-10 across soil depths.^[104] Landscape-scale perspectives, advanced in 2025, underscore how topography and erosion redistribute SOM, with upslope inputs fueling downslope stabilization, informing broader carbon sequestration forecasts beyond plot-level data.^[105] These developments collectively shift focus from bulk SOM to dynamic, fraction-specific processes, enhancing accuracy in projections for agriculture and climate mitigation.^[106]

Practical Implications for Agriculture and Carbon Management

Humus accumulation in agricultural soils supports enhanced fertility through improved nutrient retention and microbial activity, which facilitate efficient nutrient cycling and reduce leaching losses. Long-term field experiments demonstrate that practices such as organic matter inputs, including straw returns and compost, increase humus content, thereby boosting crop yields by 10-20% in nutrient-poor soils while stabilizing soil aggregates against erosion.^[6] Additionally, humus improves water retention capacity by up to 20% in amended soils, mitigating drought stress and enhancing resilience in variable climates, as evidenced by studies on conservation tillage systems that preserve humic structures.^[107] These benefits arise from humus's chelating properties, which bind cations like calcium and magnesium, promoting root proliferation and overall soil tilth without relying on synthetic inputs.^[5] In carbon management, humus serves as a recalcitrant pool within soil organic matter, enabling long-term sequestration of atmospheric CO2 at rates of 0.15-0.6 tons per hectare annually under optimized practices like reduced tillage and cover cropping. Recent analyses from 2020-2025 indicate that integrating humus-building strategies, such as carbonized straw applications, not only elevates stable carbon stocks by enhancing aggregate protection but also correlates with sustained yield increases, countering potential trade-offs from intensified farming.^[6] ^[108] However, sequestration efficacy is constrained by factors including initial soil quality and plant productivity, with subsoil humus formation offering deeper, more persistent storage but requiring targeted deep-rooted crops to minimize saturation limits observed in some trials.^[109] Agricultural policies promoting humus programs, as piloted in Europe since 2022, incentivize farmers via carbon credits, yet empirical data underscore that gains are site-specific and reversible under poor management, emphasizing continuous organic inputs for durability.^[110] ^[111]

References

[1]
Humus - National Geographic Education
Oct 19, 2023 · Humus is dark, organic material that forms in soil when plant and animal matter decays. When plants drop leaves, twigs, and other material to the ground, it ...
[2]
Humus - an overview | ScienceDirect Topics
Humus is defined as the most abundant natural organic material in the environment, formed from the decomposition of plant materials by microorganisms, ...
[3]
Humus: Why Is Humus Important? How Do You Increase Soil ...
Mar 9, 2023 · Humus is the final stage in the degradation of soil organic matter. It forms when plant (leaves and straw) and animal matter (mostly worms and insects) ...
[4]
Humus: What is it and How is it Formed? - EcoFarming Daily
Humus formation is carried out in two steps. First, the organic substances and minerals in the soil disintegrate. Next, totally new combinations of these broken ...Missing: science | Show results with:science
[5]
The essential role of humified organic matter in preserving soil health
Feb 17, 2025 · In fact, humus stabilizes the soil structure, improving its porosity and therefore the diffusion and availability of water, thus facilitating ...
[6]
Soil Organic Carbon and Humus Characteristics - MDPI
Oct 17, 2024 · Humus carbon strengthens the stability of soil aggregates through its role in soil microstructure [22], and the addition of organic materials ...
[7]
Five Benefits of Soil Organic Matter | Mosaic Crop Nutrition
Humus improves soil fertility by acting as a reservoir for nutrients, increasing the water holding capacity of the soil, improving soil structure and ...
[8]
Humus Accumulation - Oz Soils 4 - UNE
Fresh plant residues are attacked by soil microorganisms and rapidly (within weeks to months) are converted to a more stable material called humus, which can ...
[9]
[PDF] ORIGIN, CHEMICAL COMPOSITION, AND IMPORTANCE IN NATURE
Humus characterizes the soil, since differences in the origin, abundance, and chemical nature of humus result in the formation of distinct soil types. Humus ...
[10]
Humus - an overview | ScienceDirect Topics
Soil humus and clay minerals play a major role in regulating the physical and chemical properties of soils (Fig. 2.4). Both have high surface areas due to their ...
[11]
Humus is Dead (Long Live Humus)
The humification model describes humus as being composed of distinct fractions of organic matter that have undergone many biological and chemical changes over ...
[12]
https://rogitex.com/blogs/soil-for-humanity/history-of-soil-science
[13]
Historical evolution of soil organic matter concepts and their ...
In 1809 Thaër proposed a “Humus Theory” that remained very influential for 30 years, as well as a quantified assessment of the agro-ecological and economic ...
[14]
”The principles of rational agriculture” by Albrecht Daniel Thaer ...
Dec 10, 2003 · Thaer's approach was used with success during half a century as it combined numerous empirical findings on soils and fertilization with organic ...
[15]
The principles of rational agriculture” by Albrecht Daniel Thaer ...
Unfortunately and despite effective practical applications, the scientific foundations of Thaer's “Humus Theory” proved definitively false as soon as 1840 ...
[16]
HUMUS AND PLANT (The Direct Humus Effect) - jstor
Historical Review. Albrecht Thaer (1800), the founder of the " humus theory " o plant nutrition, was convinced that humus was the only direct nutrient source ...
[17]
On the Origin of the Theory of Mineral Nutrition of Plants ... - ACSESS
Sep 1, 1999 · Albrecht Thaer (1752–1828), Sprengel's mentor, was one of the most well-known advocates of the humus theory (Wendt, 1950).
[18]
Law of the Minimum - Soil and Environmental Sciences
Mar 6, 2000 · Liebig essentially debunked the humus theory and made a scientific case for plant requirements for mineral elements from the soil, carbon from ...
[19]
What are humic substances | IHSS
Humic substances in soils and sediments can be divided into three main fractions: humic acids (HA or HAs), fulvic acids (FA or FAs) and humin. The HA and FA ...
[20]
Humic Substances as a Versatile Intermediary - PMC - NIH
Mar 23, 2023 · Humic substances are organic ubiquitous components arising in the process of chemical and microbiological oxidation, generally called ...5. The Effect Of Humic... · 6. The Impact Of Humic... · Table 3
[21]
Molecular Structure in Soil Humic Substances: The New View
Two-dimensional double cross polarization (DCP) MAS 13C−15N NMR spectroscopy has been used on 15N-enriched humic materials to detect 13C nuclei close to 15N ...<|separator|>
[22]
THE SUPRAMOLECULAR STRUCTURE OF HUMIC SUBSTANCES ...
Hydrophobic (van der Waals, π–π, CH–π) and hydrogen bonds are responsible for the apparent large molecular size of humic substances, the former becoming more ...
[23]
Molecular structure in soil humic substances: the new view - PubMed
According to the new view, humic substances are collections of diverse, relatively low molecular mass components forming dynamic associations stabilized by ...
[24]
Chemical Structure and Biological Activity of Humic Substances ...
The structure of HS is operationally defined in (i) humic acids (HA), which are the fraction soluble in alkali, but insoluble during subsequent acidification, ...
[25]
[PDF] Molecular Structure in Soil Humic Substances: The New View
According to the new view, humic substances are collections of diverse, relatively low molecular mass components forming dynamic associations stabilized by ...
[26]
Quantitative Fourier Transform Infrared spectroscopic investigation ...
This new technique extends the previous qualitative IR spectroscopic studies of humic substances to a quantitative method for the investigation of humic ...
[27]
Characterization of different humic materials by various analytical ...
NMR and IR can be used as complementary methods for the characterization of humic materials. Solid state 13C direct polarization/magic angle spinning (DP/MAS) ...
[28]
NMR spectroscopy study of freshwater humic material in light of ...
NMR spectroscopy study of freshwater humic material in light of supramolecular assembly.Missing: methods | Show results with:methods
[29]
Spectroscopic characterization of humic and fulvic acids in soil ... - NIH
Jun 6, 2020 · The aim of this study was to characterize humic and fulvic acids using infrared spectroscopy and nuclear magnetic resonance (NMR) in aggregates.
[30]
Spectral characterization of selected humic substances
The application of non-destructive analytical methods such as UV-VIS, FTIR, 13C NMR, and fluorescence spectroscopy help us to provide main characteristics of ...
[31]
[PDF] Chemical and spectroscopic characterization of humic acids ...
Aug 10, 2010 · Analysis was carried out applying a combination of chemical and spectroscopic techniques including SEM, CHNO-S analysis, TG, and UV-Vis, FT-IR, ...
[32]
The nature and dynamics of soil organic matter: Plant inputs ...
This review covers historical perspectives, the role of plant inputs, and the nature and dynamics of soil organic matter (SOM), often known as humus.
[33]
[PDF] Soil Biology & Biochemistry - Jackson Lab - Stanford University
In the soileplant system, carbon (C) stabilized into soil organic matter has two distinct origins: aboveground inputs (leaves, stems, and floral structures) and ...<|control11|><|separator|>
[34]
[PDF] Understanding and Measuring Organic Matter in Soil - Municipal One
Lignin was considered a major precursor to soil humus because it was considered to be difficult to decompose and therefore selectively preserved (Kögel-Knabner ...
[35]
[PDF] Fast-decaying plant litter enhances soil carbon in temperate forests ...
Roots and root-associated microbes are likely to drive patterns in SOC dynamics. ... Plant litter: Decomposition, humus formation, carbon sequestration ...
[36]
The role of soil microbes in the global carbon cycle - NIH
Microbial necromass and metabolites are the precursors for stable soil organic matter, while extracellular microbial carbon may also influence the stability of ...
[37]
[PDF] Soil organic matter formation, persistence, and functioning
Multiple studies in contrasting ecosystems are confirming that low molecular weight C inputs as DOM are efficient precursors of SOM. (e.g., Lynch et al., 2018; ...
[38]
Humus Formation - an overview | ScienceDirect Topics
Humus formation is defined as a two-stage process involving the degradation of organic matter to produce reactive monomers, which then spontaneously polymerize ...
[39]
The role of microorganisms in the creation of humus
Humification is essentially a reorganization of organic debris due to the activities of microorganisms: beginning with fungi, which break up the solid carbon ...
[40]
The role of cattle manure-driven polysaccharide precursors in ...
Jul 18, 2024 · In this study, the addition of cattle manure promoted the formation of humus components as well as accelerated the synthesis and decomposition ...
[41]
Soil organic matter in cropping systems
When organic materials, such as residues (leaves and roots) or organic amendments (manures and composts), are added to the soil, they provide a source of active ...
[42]
The influence of organic and inorganic nutrient inputs on soil ...
Aug 28, 2021 · Abakumov et al. (2018) suggest that humification processes supplemented by organic inputs application boosts SOM.
[43]
Ch 3. Amount of Organic Matter in Soils - SARE
In stockThe amount of organic matter in a soil is the result of all the additions and losses of organic materials that have occurred over the years.
[44]
The (Bio)Chemistry of Soil Humus and Humic Substances - Frontiers
Mar 5, 2019 · Three years ago, a novel “soil continuum model” was proposed, in which soil organic matter was suggested to be of heterogeneous composition ...
[45]
Molecular mechanisms of humus formation mediated by new ...
Mar 1, 2024 · The new ammonifying microorganism cultures (NAMC) promoted humus formation. · NAMC facilitated the three-dimensional polymerization of humus.
[46]
[PDF] The contentious nature of soil organic matter
Nov 23, 2015 · Early research based on an extraction method assumed that a 'humification' process creates recalcitrant (resistant to decomposition) and large ' ...
[47]
[PDF] • Stabilization and destabilization of soil organic matter: mechanisms ...
All processes are influenced by biotic controls, such as abundance of microbial and plant species, and environmental controls, such as temperature, moisture, ...
[48]
Factors controlling the buildup of humus and particulate organic ...
Nov 1, 2021 · Studies have demonstrated that the humus layer is related to climate, soil properties, topography and other environmental factors. However, the ...
[49]
Potential responses of soil organic carbon to global environmental ...
The average global turnover time for soil organic carbon (to 1-m depth) was estimated as 32 years by Raich and Schlesinger (34), who divided the total C stock ...
[50]
A review on carbon pools and sequestration as influenced by long ...
Sep 28, 2021 · Meanwhile, the stable C pool has mean residence times ranging from centuries to millennia because of its strong resistance to any change.
[51]
Soil organic matter turnover as a function of the soil clay content
It is concluded that the stabilising effect of clay is much stronger for SMB than for humus. This is in contrast to the DAISY clay modifier assuming the same ...
[52]
Roots are key to increasing the mean residence time of organic ...
On average, OC entering German agricultural topsoils had an MRT of 21.5 ± 11.6 years, with grasslands (29.0 ± 11.2 years, n = 465) having significantly higher ...
[53]
O - Horizons - NeSoil
O horizons: are soil layers with a high percentage of organic matter. ... humus (Oa). Field criteria: Greater than 20-30% organic matter (less if high clay ...
[54]
(PDF) Vertical distribution and soil organic matter composition in a ...
Aug 10, 2025 · The vertical distribution of SOC within soil has strong implications on the composition, stabilization and turnover of the soil organic matter ( ...
[55]
[PDF] Characteristics of Master Soil Horizons
In VA, OM content in A ranges from 1-5%. Soll structure is usually granular or crumb. (like crushed cookies/crackers). In VA, the A horizon is usually thin or ...
[56]
Vertical distribution characteristics of soil organic carbon and ...
Jan 4, 2024 · Results showed that the SOC content was the highest in 0–20 cm surface soil and gradually decreased with the deepening of the soil layer. It ...Missing: humus peer
[57]
Examining mineral-associated soil organic matter pools through ...
Nov 19, 2018 · The objective of this research was to describe and characterize organic matter-mineral interactions through depth in horizons of soils of contrasting stand age.
[58]
[PDF] Illustrated Guide to Soil Taxonomy
A diagnostic horizon constitutes a continuous horizon in the soil profile. ... A salic (high content of salts) horizon within a depth of. 100 cm ...
[59]
Significance of humic matters-soil mineral interactions for ...
This paper provides a comprehensive introduction and summary of the interaction mechanisms between humic matters and typical soil minerals
[60]
Organo–organic and organo–mineral interfaces in soil at ... - Nature
Nov 30, 2020 · The capacity of soil as a carbon (C) sink is mediated by interactions between organic matter and mineral phases.
[61]
Adsorption of Soil-Derived Humic Acid by Seven Clay Minerals
Oct 1, 2016 · Clay surfaces affected HA adsorption directly due to structural differences and indirectly by altering solution pH. The following order of HA ...
[62]
Interaction of fulvic acid with soil organo-mineral nano-aggregates ...
Interaction of fulvic acid with soil organo-mineral ... Adsorption of a soil humic acid at the surface of goethite and its competitive interaction with phosphate.
[63]
Effect of humic substances and clay minerals on the hydrosorption ...
Sep 8, 2015 · Humic substances promote the aggregation and stability of soil to a greater extent than clay minerals. The reason is their amphiphilic and ...
[64]
The Effect of Humus on Cationic Interactions in a Beidellite Clay
This relatively higher base saturation in the clay-humus systems resulted in higher activities of the cations in all cases than in the clay alone. In spite of ...
[65]
Adsorption of minerals enhances the stabilization of organic ...
The results of the linear fitting indicate that minerals exhibit adsorption preferences for different humus components depending on the phase of their addition.
[66]
Effect of partial removal of adsorbed humus on kinetics of potassium ...
This observation indicates that humus adsorbed on clay plays an important role in preventing clay dissolution by organic acids.
[67]
Solubility characteristics of soil humic substances as a function of pH
Apr 9, 2025 · These results would indicate that HS insolubility arises via organo-metal and organo-mineral interactions at alkaline pH, along with HApH 6 ...<|separator|>
[68]
Nanoscale Interactions of Humic Acid and Minerals Reveal ...
Dec 16, 2022 · We investigated the effect of calcium phosphate mineralization on humic acid (HA) fixation in simulated soil solutions, either with or without clay mineral ...Missing: substances | Show results with:substances
[69]
Abiotic and Biotic Controls on Soil Organo–Mineral Interactions ...
Sep 1, 2019 · This chapter aims to tackle a part of this problem through generating recommendations for improved model structure in the representation of SOM cycling.
[70]
Potential bioavailability of mercury in humus-coated clay minerals
It is well-known that both clay and organic matter in soils play a key role in mercury biogeochemistry, while their combined effect is less studied.
[71]
HUMUS-SOIL MINERAL STUDIES DIFFERENT ASPECTS - A review
(2003) Mean residence time of soil organic matter associated with kaolinite and smectite. European J Soil Sci. 54:269-278. Wiseman and Puttmann W., (2005) ...
[72]
[PDF] Soil Fertility Management for Organic Crops
Humus is the most resistant and mature fraction of soil organic matter. It is very slow to decompose and may last for hundreds of years. Plant residues that are ...
[73]
(PDF) Influence of Biohumus Application for Enhancing Crop Yield ...
Aug 4, 2025 · Numerous studies have demonstrated that biohumus promotes enhanced vegetative growth and increased yields due to its nutrient-rich composition, ...
[74]
Fact Sheets Cations and Cation Exchange Capacity - Tas - Soil Quality
Humus has a CEC two to five times greater than montmorillonite clay and up to 30 times greater than kaolinite clay, so is very important in improving soil ...
[75]
Soil Defined | Nutrient Management - Mosaic Crop Nutrition
Soil organisms feed and reproduce within humus. Because humus is a colloid, it increases the cation exchange capacity of the soil. Because of its critical role ...
[76]
Effects of humic acid fertilizer on the growth and microbial network ...
Aug 1, 2024 · Research has demonstrated its ability to stimulate plant growth, boost the availability of nutrients in soil, increase the activity of soil ...
[77]
Humic substances biological activity at the plant-soil interface - NIH
In this review article, we are giving an overview of available data concerning molecular structures and biological activities of humic substances, with special ...Missing: peer | Show results with:peer
[78]
How humus influences our soil climate - Sektion für Landwirtschaft
May 6, 2025 · In the soil, humus significantly determines the physical, chemical and biological soil fertility. Among other things, humus promotes plant ...
[79]
Water capacity of different soils - HORSCH
Mar 7, 2024 · In general, the following applies: humus can store three to five times its own weight in water.
[80]
Effects of improved water retention by increased soil organic matter ...
Dec 25, 2023 · We showed that both the improved water retention by SOC and its vertical distribution affect the soil water balance in a complex manner.
[81]
Compost can increase the water holding capacity in droughty soils
Nov 13, 2024 · A 1994 study by A. Maynard found that a 3 inch layer of leaf compost rototilled to a 6 inch depth increased water holding capacity 2.5 times that of a native ...Missing: scientific | Show results with:scientific
[82]
Systematic Review on the Role of Microbial Activities on Nutrient ...
Sep 4, 2024 · Humus is essential for soil structure and acts as a reservoir ... This review explores the role of soil microorganisms in nutrient cycling ...
[83]
[PDF] Cation Exchange Capacity and Base - Technical Information
Feb 5, 2024 · Cation exchange capacity (CEC) is a soil's ability to adsorb positively charged cations with negatively charged particles like clay and organic ...
[84]
Effect of humic substances on nitrogen cycling in soil-plant ecosystems
The addition of exogenous humic substances (HS) improves the soil habitat. · The interaction between HS and microorganisms co-mediates the soil nitrogen cycle.
[85]
Humic Substances: Bridging Ecology and Agriculture for a Greener ...
Humic substances (HSs) are emerging as multifunctional natural catalysts in sustainable agriculture, offering novel opportunities to enhance soil health.
[86]
Persistence of soil organic matter as an ecosystem property - Nature
Oct 5, 2011 · Globally, soil organic matter (SOM) contains more than three times as much carbon as either the atmosphere or terrestrial vegetation.
[87]
Humic Substances Extracted by Alkali Are Invalid Proxies ... - ACSESS
Mar 1, 2019 · The science and management of soil rely on an understanding of the composition of soil organic matter to evaluate and predict moisture retention ...
[88]
Concepts and Misconceptions of Humic Substances as the Stable ...
May 17, 2018 · For a long time, humic substances were considered as the stable part of soil organic matter with a yellow to dark brown color.5. Black Carbon In Soil · 6. Humic Substances In Soils... · 6.2. 2. Humic...
[89]
[PDF] The Humic Substances Paradigm - OSTI.GOV
Mar 8, 2019 · The conceptual rigor of using organic materials extracted from soil by alkali, called humic substances, as proxies for soil organic.
[90]
Current controversies on mechanisms controlling soil carbon storage
Feb 6, 2023 · We review and attempt to reconcile conflicting views on the mechanisms controlling organic carbon dynamics in soil.<|separator|>
[91]
A history of research on the link between (micro)aggregates, soil ...
Based on this concept, Tisdall and Oades [J. Soil Sci. 62 (1982) 141] coined the aggregate hierarchy concept describing a spatial scale dependence of mechanisms ...
[92]
Evidence of aggregate hierarchy at micro- to submicron scales in an ...
Mar 1, 2014 · This work corroborates the hierarchical conceptual model for soil aggregate structure presented by Tisdall and Oades (1982), extends it to ...<|separator|>
[93]
[PDF] Soil organic matter and aggregate hierarchy revisied: a case study ...
Aggregate hierarchy concept (Tidall and Oades, 1982) has been verified for most temperate and tropical soil types except for Andisols developed from tephra and ...
[94]
[PDF] The contentious nature of soil organic matter - Desert Blooms
Traditional. 'humification' concepts limit observations of soil organic matter to its solubility in alkaline extracts, unlike the emergent view of organic ...
[95]
Unifying soil organic matter formation and persistence frameworks
Our new model (MEMS v1.0) is developed from the Microbial Efficiency-Matrix Stabilization framework, which emphasizes the importance of linking the chemistry of ...<|control11|><|separator|>
[96]
Microbial carbon use efficiency promotes global soil carbon storage
May 24, 2023 · Microbial models with minimal mineral protection can explain long-term soil organic carbon persistence. Sci. Rep. 9, 6522 (2019). Article ...
[97]
A Soil-Science Revolution Upends Plans to Fight Climate Change
Jul 27, 2021 · Instead, they basically divided soil carbon into short-term and long-term pools, in accordance with the humus paradigm. More recent generations ...
[98]
Soil organic carbon sequestration in agricultural long-term field ...
In this study, we analyzed changes in fast-cycling particulate organic matter (POM) and slow-cycling mineral-associated organic matter (MAOM) induced by common ...
[99]
Global turnover of soil mineral-associated and particulate organic ...
Jun 22, 2024 · Here, we generate the global MAOC and POC maps using 8341 observations and then infer the turnover times of MAOC and POC by a data-model integration approach.
[100]
Modern Development of Soil Organic Matter Dynamics Models ...
Dec 18, 2024 · This review examines the achievements of the last decade in modeling the role of microorganisms in the stabilization of soil organic matter.
[101]
Soil organic matter turnover rates increase to match increased ... - NIH
Aug 27, 2021 · Model results indicated that decomposition rates of fast cycling C (0.1 to 0.2 year−1) increased to nearly offset increases in inputs. With ...Missing: humus | Show results with:humus
[102]
Research advances in mechanisms of climate change impacts on ...
Oct 1, 2023 · This review provides a concise SOC turnover framework to understand the impact of climate change on SOC dynamics.<|separator|>
[103]
A simple model of the turnover of organic carbon in a soil profile
Oct 1, 2025 · We describe here a simple model of SOC turnover at the soil profile scale that accounts for two key processes determining SOC persistence (i.e. ...
[104]
[PDF] A landscape-scale view of soil organic matter dynamics
Jan 7, 2025 · Effects of soil process formalisms and forcing factors on simulated organic carbon depth-distributions in soils. Sci. Total. Environ. 652 ...<|separator|>
[105]
A landscape-scale view of soil organic matter dynamics
Jan 7, 2025 · In this Perspective, we outline how understanding soil formation processes and complexity at the landscape scale can inform predictions of soil organic matter ...Missing: 2020-2025 | Show results with:2020-2025
[106]
Soil humus and aluminum—iron interactions enhance carbon ...
Soil humus regulates Al–Fe interactions, enhancing SOC sequestration and yield sustainability. NPK-R and OM-R were most effective treatments for improving SOC ...
[107]
Agricultural soils in climate change mitigation: comparing action ...
Aug 12, 2024 · Farmers sequester CO2 from the atmosphere by fostering humus in agricultural soils, thereby also improving the productivity and resilience of ...
[108]
Agricultural limitations to soil carbon sequestration: Plant growth ...
Jun 15, 2024 · Soil C sequestration can be limited by plant productivity and soil quality. We evaluated limiting factors for 20 farms testing C sequestration practices.
[109]
Farmers' Willingness to Participate in a Carbon Sequestration Program
Mar 21, 2024 · Farmers can counteract global warming by drawing carbon dioxide from the air into agricultural soils by building up humus. Humus programs were ...
[110]
Dynamic Stability of Soil Carbon: Reassessing the “Permanence” of ...
Long residence times of rapidly decomposable soil organic matter: application of a multi-phase, multi-component, and vertically resolved model (BAMS1) to ...